Bauxite description of the mineral. The state of the raw material base of the Russian aluminum industry. Industrial importance of aluminum
BOXITES [called area of Les Baux in the south of France, where bauxite deposits were first discovered], bauxite, consisting mainly of aluminum hydroxides (aluminum gel, gibbsite, boehmite, diaspores, etc.), oxides and hydroxides of iron and clay minerals. The color is red in various shades, brownish-brown, less often white, yellow, gray (to black). They are found in the form of dense (rocky) or porous formations, as well as in the form of loose earthy and clay-like masses. Based on their structure, they are classified as clastic (pelite, sandstone, gravelite, conglomerate) and concretionary (oolitic, pisolite, leguminous); by texture - homogeneous, layered and other bauxites. Density varies from 1800 kg/m 3 (loose) to 3200 kg/m 3 (rocky). According to the predominant mineral composition, bauxites are distinguished: monohydroxide (diaspore, boehmite), trihydroxide (gibbsite) and mixed composition (diaspore-boehmite, boehmite-gibbsite, chamosite-boehmite, chamosite-gibbsite, gibbsite-kaolinite, goethite-chamosite-boehmite, etc.). ).
Bauxites are formed during deep chemical transformations (lateritization) of aluminosilicate rocks in a humid tropical climate (lateritic or residual bauxites) or during the transfer of lateritic weathering products and their redeposition (sedimentary bauxites). As a result of the superposition of these processes, bauxites of a mixed (polygenic) type are formed. The deposits are sheet-like, lens-shaped or irregularly shaped (karst pockets). The quality of lateritic bauxites is usually high (50% $\ce(Al_2O_3)$ and higher), sedimentary bauxites can range from high-grade (55–75% $\ce(Al_2O_3)$) to substandard (less than 37% $\ce (Al_2O_3)$ ). In Russia, the quality requirements for mined (commercial) bauxite are determined by GOST, as well as contractual terms between suppliers and consumers. Depending on the ratio (by weight) of alumina and silica content (the so-called silicon module), bauxite is divided into 8 grades. For the lowest grade (B-6, 2nd grade), the silicon module should be over 2 with an alumina content of at least 37%; for high-grade bauxites (B-0, B-00) the silicon module should be over 10 with an alumina content of 50% and more. In foreign classifications, bauxite with a silicon modulus of over 7 is classified as high-quality.
Bauxite deposits according to reserves are divided into large (over 50 million tons), medium (5–50 million tons) and small (up to 5 million tons). The reserves of the world's largest deposit, Boke (Guinea), are estimated at 2.5 billion tons. 83.7% of reserves are concentrated in lateritic deposits, 9.5% in polygenic deposits and 6.8% in sedimentary deposits.
Bauxite deposits have been explored in more than 50 countries around the world. Total bauxite reserves are estimated at 29.3 billion tons, confirmed reserves at 18.5 billion tons (2nd half of the 2000s). The largest confirmed reserves are in: Guinea (7.4 billion tons; over 40% of world reserves), Jamaica (2 billion tons; 10.8%), Brazil (1.9 billion tons; 10.3%) , Australia (1.8 billion tons; 9.7%), India (0.77 billion tons; 4.2%), Guyana (0.7 billion tons; 3.8%), Greece (0. 6 billion tons; 3.2%), Suriname (0.58 billion tons; 3.1%), China (0.53 billion tons; 2.8%). The largest bauxite province in the world is the West African bauxite province (or Guinea).
In Russia, the total reserves of bauxite are over 1.4 billion tons, proven reserves are over 1.1 billion tons (beginning of 2013). There are 57 deposits (including 4 large and 7 medium). The main bauxite reserves are concentrated in the Sverdlovsk region (about 1/3 of the reserves of the Russian Federation; sedimentary deposits of the North-Ural bauxite-bearing region - large Cheremukhovskoye, medium - Krasnaya Shapochka, Kalinskoye, Novokalinskoye), the Komi Republic (26% of the reserves of the Russian Federation; polygenic deposits of the Vorykvinsky group of the Timan bauxite-bearing zones - large Vezhayu-Vorykvinskoye, medium - Verkhneshugorskoye, Vostochnoye), Arkhangelsk region (18% of reserves of the Russian Federation; large Iksinsky sedimentary deposit), Belgorod region (about 16% of reserves of the Russian Federation; large Vislovskoye laterite deposit, medium - Melikhovo-Shebekinskoye). Bauxite reserves have also been identified in the Krasnoyarsk and Altai territories, the Kemerovo region, the Republic of Bashkortostan, and the Leningrad region. Ore Russian deposits Compared to foreign analogues, they are of lower quality and more complex development conditions. The richest ores ($\ce(Al_2O_3)$ 56%) are in the deposits of the Northern Urals; The largest (about 18% of the reserves of the Russian Federation) Iksinsky deposit is composed of low-quality bauxites.
World bauxite production exceeded 196 million tons/year (2nd half of the 2000s). Main producing countries: Australia (62.6 million tons/year), China (27 million tons/year), Brazil (22.8 million tons/year), Guinea (18.2 million tons/year), Jamaica (14.9 million tons/year), India (13.9 million tons/year). In Russia, the extraction of bauxite from the subsoil in 2012 amounted to 5.14 million tons; 9 deposits were developed, 6 of them in the Sverdlovsk region.
Alumina and aluminum are extracted from bauxite. Bauxite is also used in the production of paints, artificial abrasives (electrocorundum), as fluxes in ferrous metallurgy, and sorbents for purifying petroleum products from various impurities; low-iron bauxites - for producing high-alumina refractories, quick-hardening cements, etc. Bauxites are complex raw materials; in addition to aluminum and iron, they contain gallium, as well as titanium, chromium, zirconium, niobium, and rare earth elements.
The name of aluminum stone comes from the name of the region where it was discovered in 1821 by the French geologist Pierre Berthier. This happened during his vacation in the settlement of Le Beau. Walking on the outskirts of the village, Pierre saw a rock that consisted of an unusual stone. Thanks to his many years of experience, the geologist studied the composition of the rock sample taken from the rock. It turned out that most of the mineral’s composition is aluminum compounds, the rest is silicon oxide and other impurities. “Nothing special,” thought Pierre. He could not have known then that a couple of decades later bauxite would be used as the most sought-after raw material in the industrial industry.
Main features of the mineral
Aluminum ore contains varying amounts of:
- Aluminum hydroxide
- Iron oxides
- Silicon oxides
- Alumina hydrate
- Types of mineral components.
The main chemical components of the ore are alumina and silica. Their percentage component varies from 28 to 80 percent of the total mass of the material under study. It is the ratio of alumina and silica content that determines the quality of bauxite. The percentage of the amount of the valuable component depends on the deposit.
The permanent constituent chemical part of the mineral is iron oxide. The remaining components are not constant. So, for example: if the ore contains one or more oxides, then other types may not be present. The physical properties of the stone depend on this.
Cleavage, color, density, hardness, luster, fracture, and transparency of a mineral characterize its physical properties. Bauxite comes in red and gray in many of their shades, depending, as mentioned above, on its chemical composition. It is a clay-like rock. Outwardly, it is more reminiscent of clay, but their physical and chemical characteristics are completely different. So bauxite does not dissolve in water. Hardness depends on its “breakability” (ease of alumina extraction). The denser the mineral, the higher its hardness according to the Mohs mineralogical scale. The hardest ore reaches number 6. The iron oxide content affects the density of the mineral, which varies from 2900 to 3500 kg/m3. The structure of bauxite is dense and porous. It's all about the break. Earthy fracture is a dense ore, cellular fracture is porous. According to transparency indicators, bauxite is not transparent.
The cleavage of bauxite is perfect, that is, when split, its parts are characterized by smooth, shiny plates with planes in three directions, which allows craftsmen to make souvenirs and, to a small extent, jewelry produce quite expensive products.
The cubic system of bauxite is the highest category, which indicates the highest symmetry of its crystals. Bauxite has several axes of the second order and four of the third. The cubic system is found in bauxite more often as a cube, but can be represented by a tetrahedron, rhombic dodecahedron, pentagon-dodecae and other complex geometric figures.
Birth of bauxite
Deposits of the vast majority of minerals of a given type, where it predominates in tropical and subtropical or subtropical climates. Aluminum ore predominates mainly where alumina precipitation, weathering of acidic, alkaline or mafic rock takes place. This process is influenced only by climate. The most favorable climatic conditions are characteristic of the island of Guinea and Australia. In total, about twenty-seven billion tons of bauxite lie there. Different minerals contribute to the mineral formation of the ore called bauxite, so it is divided into three groups:
There are much fewer monomineral bauxite ores than mixed ones, that is, gibbsite-boehmite or diaspore-boehmite. Aluminum ore is formed mainly where alumina precipitation, weathering of acidic, alkaline or basic rock takes place. Genetic signs of mineral formation are divided into two types:
- Platform. Continental sediments are formed in a horizontal plane.
- Geosynclinal. Coastal marine type of sediments.
Areas of application of the mineral
The possibility of extracting aluminum from bauxite is of primary interest to industrial sectors, but other areas, in particular ferrous metallurgy, use its composition as a flux. The chemical industry purchases the mineral for the manufacture of varnishes and paints as a necessary filler, and also as a sorbent, which helps purify petroleum products from unnecessary additives. If the ore is subjected to the smelting process in an electric furnace, it is converted into electrocorundum, from which artificial abrasive materials are produced.
Alumina is the main chemical constituent of bauxite. By extracting it, construction impurities are obtained. The astringent properties of alumina cement, derived from alumina, cause it to harden quickly. This ability has increased the material's effectiveness by orders of magnitude in low-temperature construction. Completing emergency work with limited deadlines has become much easier.
Bauxite, which contains little iron, is resistant to high temperatures, so it is used for the production of high-alumina bricks, fireclay, etc.
Since bauxite contains about a hundred substances representing the periodic table, the technological point of view divides into three groups:
This section is rather arbitrary, since not all qualities of the mineral are taken into account here. Different production conditions have different effects on the cleavage, density and hardness of bauxite, so the response is not always the same. For example: when processed using the Bayer method, it turns into a harmful impurity, but thanks to the sintering method, it becomes a useful component.
The mineral is not used for the production of jewelry, as it has no special value. Authors of jewelry self made It is used more often to make souvenir products, because, having high cleavage, bauxite can be split with even shiny parts. For example: to make a beautiful polished ball mounted on a forged stand.
In modern industry, aluminum ore is the most popular raw material. The rapid development of science and technology has made it possible to expand the scope of its application. What aluminum ore is and where it is mined is described in this article.
Industrial importance of aluminum
Aluminum is considered the most common metal. It ranks third in terms of the number of deposits in the earth's crust. Aluminum is also known to everyone as an element in the periodic table, which belongs to light metals.
Aluminum ore is a natural raw material from which it is mainly mined from bauxite, which contains aluminum oxides (alumina) in the largest amount - from 28 to 80%. Other rocks - alunite, nepheline and nepheline-apatite are also used as raw materials for the production of aluminum, but they are of poorer quality and contain significantly less alumina.
Aluminum ranks first in non-ferrous metallurgy. The fact is that due to its characteristics it is used in many industries. Thus, this metal is used in transport engineering, packaging production, construction, and for the manufacture of various consumer goods. Aluminum is also widely used in electrical engineering.
To understand the importance of aluminum for humanity, it is enough to take a closer look at the household things that we use every day. Many household items are made of aluminum: these are parts for electrical appliances (refrigerator, washing machine etc.), dishes, sports equipment, souvenirs, interior elements. Aluminum is often used for production different types containers and packaging. For example, cans or disposable foil containers.
Types of aluminum ores
Aluminum is found in more than 250 minerals. Of these, the most valuable for industry are bauxite, nepheline and alunite. Let's look at them in more detail.
Bauxite ore
Aluminum does not occur in nature in its pure form. It is mainly obtained from aluminum ore - bauxite. It is a mineral that mostly consists of aluminum hydroxides, as well as iron and silicon oxides. Due to the high alumina content (40 to 60%), bauxite is used as a raw material for the production of aluminum.
Physical properties aluminum ore:
- opaque mineral of red and gray colors of various shades;
- the hardness of the strongest samples is 6 on the mineralogical scale;
- The density of bauxite, depending on the chemical composition, ranges from 2900-3500 kg/m³.
Bauxite ore deposits are concentrated in the equatorial and tropical zones of the earth. More ancient deposits are located in Russia.
How is bauxite aluminum ore formed?
Bauxite is formed from alumina monohydrate, boehmite and diaspore, trihydrate hydrargillite and associated minerals hydroxide and iron oxide.
Depending on the composition of nature-forming elements, three groups of bauxite ores are distinguished:
- Monohydrate bauxite - contains alumina in monohydrate form.
- Trihydrate - such minerals consist of alumina in trihydrate form.
- Mixed - this group includes the previous aluminum ores in combination.
Deposits of raw materials are formed due to the weathering of acidic, alkaline, and sometimes basic rocks or as a result of the gradual deposition of large quantities of alumina on the sea and lake beds.
Alunite ores
This type of deposit contains up to 40% aluminum oxide. Alunite ore is formed in water basins and coastal zones under conditions of intense hydrothermal and volcanic activity. An example of such deposits is Lake Zaglinskoye in the Lesser Caucasus.
The rock is porous. Mainly consists of kaolinites and hydromicas. Ore with an alunite content of more than 50% is of industrial interest.
Nepheline
This is an aluminum ore of igneous origin. It is a fully crystalline alkaline rock. Depending on the composition and technological features processing produces several grades of nepheline ore:
- first grade - 60-90% nepheline; it contains more than 25% alumina; processing is carried out by sintering;
- second grade - 40-60% nepheline, the amount of alumina is slightly lower - 22-25%; enrichment is required during processing;
- the third grade is nepheline minerals, which are of no industrial value.
World production of aluminum ores
Aluminum ore was first mined in the first half of the 19th century in the southeast of France, near the town of Box. This is where the name bauxite comes from. At first it developed at a slow pace. But when humanity appreciated which aluminum ore was useful for production, the scope of aluminum application expanded significantly. Many countries have begun searching for deposits on their territories. Thus, the world production of aluminum ores began to gradually increase. The numbers confirm this fact. Thus, if in 1913 the global volume of ore mined was 540 thousand tons, then in 2014 it was more than 180 million tons.
The number of countries mining aluminum ore also gradually increased. Today there are about 30 of them. But over the past 100 years, leading countries and regions have constantly changed. Thus, at the beginning of the 20th century, the world leaders in the extraction of aluminum ore and its production were North America and Western Europe. These two regions accounted for about 98% of global production. A few decades later, Latin America and the Soviet Union became the leaders in quantitative indicators of the aluminum industry. And already in the 1950-1960s, Latin America became the leader in terms of production. And in the 1980-1990s. there was a rapid breakthrough in aluminum and Africa. In the current global trend, the main leading countries in aluminum production are Australia, Brazil, China, Guinea, Jamaica, India, Russia, Suriname, Venezuela and Greece.
Ore deposits in Russia
In terms of aluminum ore production, Russia ranks seventh in the world ranking. Although aluminum ore deposits in Russia provide the country with metal in large quantities, it is not enough to fully supply the industry. Therefore, the state is forced to buy bauxite from other countries.
In total, there are 50 ore deposits in Russia. This number includes both places where the mineral is being mined and deposits that have not yet been developed.
Most of the ore reserves are located in the European part of the country. Here they are located in the Sverdlovsk, Arkhangelsk, Belgorod regions, in the Komi Republic. All these regions contain 70% of the country's total proven ore reserves.
Aluminum ores in Russia are still mined from old bauxite deposits. Such areas include the Radynskoye field in the Leningrad region. Also, due to a shortage of raw materials, Russia uses other aluminum ores, the deposits of which differ worst quality mineral deposits. But they are still suitable for industrial purposes. Thus, in Russia, nepheline ores are mined in large quantities, which also make it possible to obtain aluminum.
In 1821, the French chemist Verny first examined and described a rock found near the city of Les Baux, in the south of France, containing 52% Al2O3, 27.6% F 2 0 3 and 20.4% H2O, and called its location by bauxite ( bauxite).
Currently, bauxite is the most important aluminum ore, on which, with few exceptions, almost the entire world aluminum industry is based.
Bauxite is a complex rock, which includes: aluminum oxide hydrates, forming the main ore mass; iron in the form of oxide hydrates, oxides and silicates; silicon, in the form of quartz, opal and kaolinite; titanium, in the form of rutile and other compounds; calcium and magnesium carbonate, as well as small amounts of compounds of sodium, potassium, zirconium, chromium, phosphorus, vanadium, gallium and other elements; Often an admixture of pyrite is also found in bauxite.
The chemical composition of bauxite, depending on the mineralogical form of aluminum hydroxide and the amount of impurities, varies widely. The quality of bauxite as an aluminum ore is determined primarily by the content of alumina and silica in them: the lower the content of SiO 2 and the more Al2Oz, the higher the quality of the ore, other things being equal. Of great importance is the so-called “breakability” of bauxite, i.e., the ease of extraction from alumina. The physical properties of bauxite are very diverse, and external differences are so variable that identifying bauxite by eye is very difficult. This causes great difficulties in searching for bauxite. Characterized by an extremely large dispersion of bauxite components. Therefore, under a conventional microscope, only isolated, well-crystallized precipitates and impurities can be distinguished in bauxite.
By appearance bauxites (are clay-like, and often stony rock; in general, their structure is very diverse. Bauxites (can be dense, with an earthy fracture, or porous, with a rough cellular fracture; often the bulk of bauxite contains rounded bodies, giving the oolitic structure of the ore. These the bodies are formed by iron oxides and sometimes alumina.
The color of bauxite is as varied as its structure. Bauxites are found in all sorts of shades - from white to dark red, but most often they are brown or brick-red in color. The specific gravity of bauxite varies widely. For light porous bauxites with low silica and iron contents, it is approximately 1.2; Dense, highly ferruginous, stony bauxites have a specific gravity of approximately 2.8. The hardness of bauxite on the Mohs scale varies from 2 to 7. Although sometimes resembling clay in appearance, bauxite has nothing in common, however, it has nothing in common with it. Characteristic hallmark Bauxite is that with water, unlike clays, it does not produce a plastic mass.
The mineralogical difference between bauxite and clay, as mentioned above, is that in the former, aluminum is in the form of hydroxides, and in the latter, in the form of kaolinite. Depending on the mineralogical form of boehmite hydroxide and diaspore AlOOH or hydrargillite Al(OH)3, in the form of which aluminum is found in bauxite, the types of bauxite are distinguished accordingly: boehmite, diaspore, hydrargillite and mixed.
To study the mineral composition of bauxite, it is very convenient to use thermal analysis to obtain heating curves.
In Fig. 1 shows the results of thermal analysis of various samples of Tikhvin bauxites, performed by academician. N. S. Kurnakov and G. G. Urazov. A number of endothermic sections (stops) are visible on the heating curves, which correspond to the dehydration of hydrargillite, diaspore (boehmite), and kaolinite. The thermal stop, corresponding to the dehydration of hydrargillite, lies in the range of 202-205°, diaspora - 509-555° and kaolinite - 558-605°
rice. 1. CurvesheatingTikhvin bauxites (according to N. KurnAkova AndG. Urazov)
a, b, V- endothermic stops corresponding to the release of water from hydrargillite -AI2 ABOUT3 * ZN2 Oh diaspora -AI2 ABOUT3 * N2 O and kaolinite -AI2 ABOUT3 *2 SiO2 * ZN2 ABOUT; G-spontaneous heating, characteristic of molecular transformation in dehydrated kaolin in the region of 960-1000°
In Fig. 2. Similar heating curves for bauxites of the Northern Urals (the “Red Riding Hood” deposit) are presented, indicating their diasporic (boehmite) nature.
rice. 2. Heating curves of bauxite from the Krasnaya depositwApochka" (according to Maldavantsev)
Horizontal sections on curves (uhndothermicnostops) answerallocation water from diaspora (bemAndta)— AI2 ABOUT3 * N2 ABOUT
By thermal analysis, the mineralogical form in which alumina is present in bauxite, and therefore the type of the latter, can easily be established.
Even though bauxite has been known for over 100 years; and in recent decades it has attracted exceptional attention as the most valuable aluminum ore. their genesis (origin) cannot yet be considered clarified. There is no unity of opinion among geologists and geochemists on this issue. There is, however, a lot of data indicating that the formation of bauxite is generally associated with different processes and therefore cannot be uniform for all deposits. There are three major hypotheses regarding the genesis of bauxites:
1) bauxite is the residue after the dissolution (leaching) of limestones by the so-called terro rossa (red earth), which consists of a mixture of various hydrous aluminosilicates (Fox);
2) bauxites are a product of weathering of ancient crust with subsequent mechanical movement and redeposition of the residual product, which is in a colloidal state (S. F. Malyavkin);
3) bauxite is a chemical precipitate formed during the decomposition of solutions of aluminum, iron and titanium salts (obtained by leaching of igneous rocks with natural waters) at the moment they enter reservoirs - seas and lakes (Academician A. D. Arkhangelsky).
The main types of bauxite, at least in the CIS, were formed precisely the last way Academician A.E. Fersman gives the following scheme for the precipitation of aluminum oxide hydrates from solutions of aluminum salts at different pH values, illustrating the possibility of hydrochemical formation of aluminum accumulations (in the form of hydrates):
From this diagram it can be seen that aluminum dissolves only at very high and at very low pH. The first rarely occurs in the earth's crust; much more important is the second group of solutions - acidic ones, in the form of which aluminum migrates (carried out) very easily. The places where such solutions are formed are, for example, areas of sulfide oxidation. These solutions can lead to the precipitation of aluminum hydroxide when the pH increases, which occurs if they enter the ocean environment of a sea or lake with a pH of 6-8.
Due to this, the distribution of bauxites should be primarily associated with coastal sediments of ancient (Paleozoic and Mesozoic) seas or lakes. In the CIS, such areas are both slopes of the Ural ridge and the territory of Central Asia.
Bauxite is quite widespread rock. The world's proven reserves are estimated at approximately one billion tons, with Europe taking first place in terms of reserves, Africa second, America third, Asia fourth and, finally, Australia fifth.
The development of bauxite for industrial purposes began relatively recently. Bauxite was first mined in France in the 70s of the last century. In 1890, bauxite development began in England and the USA, in 1/907 in Italy, in 1908 in India and Dutch Guiana.
The world's largest bauxite deposits are concentrated in the southeast of France, in the Var department, near the city of Beau. French bauxite is considered the best in the world. The bauxites of the Var department belong to the boehmite-diasporic type. Of the other Western European countries, the most important bauxite deposits are located in Hungary, Yugoslavia (in Dalmatia), Italy (on the Istrian Peninsula) and Greece. England, Switzerland and Norway do not have their own bauxites and import them from other countries.
In the USA, the most important deposits are located in Arkansas. Distinctive feature These bauxites are characterized by their belonging to the three-type hydrargillite type and low iron content.
In South America, significant bauxite deposits are located in British and Dutch Guiana. In 1916, bauxite deposits were discovered in Africa in the Gold Coast region. A special feature of these bauxites is that they contain small amounts of gold and silver. In India, bauxite deposits are located in inaccessible areas, and their industrial significance is still small.
Table 3 shows chemical composition bauxite from the most famous deposits in various countries.
| Components % | France (Var department) | Hungary | Yugoslavia | Greece | Italy | USA (Arkansas) | Dutch Guiana | British Guiana |
| Al2O3 | 57-62 | 57-62 | 48-54 | 56-59 | 54-58 | 57-60 | 60-61 | 59-60 |
| SiO2 | 3-5 | 2-7 | 1-4 | 3-7 | 2-4 | 4-7 | 2-2,5 | 1,5-2 |
| Fe2O3 | 18-26 | 12-20 | 20-24 | 16-21 | 22-26 | 2-7 | 2,5-3 | 5-6 |
| TiO2 | 3-4 | 2,5-3,5 | 2,5-3,5 | 2-2,5 | 2-3 | 2,5-3,5 | 2,5-3 | 2-2,5 |
| H2O | 10-12 | 14-16 | 18-24 | 13-16 | 12-15 | 28-30 | 29-31 | 29-30 |
The main industrial deposits of bauxite in our country are concentrated in two areas - the Tikhvin district of the Leningrad region and in the Urals.
Bauxite deposits in the Tikhvin region were discovered in 1916. Their formation dates back to the Carboniferous period. Tikhvin bauxites occupy a narrow strip 6-12 km wide. They usually occur in the form of irregularly shaped nests (lenses) and are covered on top with sandy and clayey rocks of glacial origin. In appearance, Tikhvin bauxites are extremely diverse: their color goes through all shades - from white to red and purple; their specific gravity and chemical composition are also variable.
The chemical composition of Tikhvin bauxites varies from such rocks, the ratio between the content of alumina and silica in which corresponds to clays, and to such ores, where the amount of alumina reaches 70%, a) the SiO2 content drops to 2-2.5%. The amount of chemically bound water in the bulk of bauxite lies in the range of 12-14%, but there are also bauxites that contain up to 20% H 2 0. The TiO2 content usually does not exceed 2.5-3.0%. As for Fe2O3, its amount varies greatly: from 3-5% in white bauxites to 30% in highly ferruginous (usually powdery) bauxites. Some varieties of Tikhvin bauxites contain CaO, as well as chromium compounds, the content of which reaches 0.2%.
The approximate average chemical composition of bauxite throughout the Tikhvin deposit is characterized by the following figures:
47.7% Al2O3; 17.2 Fe2O3; 13.2% SiO2; 2.6% TiO2; 3.9% CaO and 15.4% H 2 0.
Aluminum oxide hydrate is found in them mainly in the form of boehmite (and in a much smaller amount - in the form of hydrargillite. In addition to iron and silica oxides, the most important impurities in Tikhvin bauxites are kaolinite and calcite. Titanium oxide is present in them in the form of small crystals of the mineral rutile.
The increased silica content in Tikhvin bauxites reduces the quality of their aluminum ore.
The most important Ural bauxite deposits are concentrated in the Northern Urals in the area of T. Serov, in the Middle Urals in the area of Kamensk and in the Southern Urals in the Satka region Chelyabinsk region and Maloyazovsky district of the Bashkir Republic.
In the Northern Urals, bauxites, discovered in 1931, include a number of deposits, the most explored of which are “Little Red Riding Hood”, Bogoslovskoye and Ivdelskoye. The formation of the North Ural bauxites dates back to Paleozoic times. They occur among limestones, and the main mass of them is dense rock brown-red color of oolitic structure; less common is the flagstone variety of bauxite, which in appearance resembles jasper.
The layered nature of the deposits and the presence of coral skeletons in them suggest (Academician A.D. Arkhangelsky) that the bauxites of the Northern Urals were formed by chemical precipitation of hydrates from aqueous solutions salts to the bottom of the ancient sea. Due to the high content of Al2O3 and a small amount of SiO2 impurity, these bauxites can be equated to the best varieties French bauxite. The bauxites from the “Red Riding Hood” deposit are of especially good quality. On average, the chemical composition of bauxite from this deposit can be characterized by the following figures:
56% Al2O3; 25 Fe2O3; 3.5% SiO2; 2.2% TiO2 and 11% H 2 0.
Mineralogically, the bauxites of the Northern Urals are a rock of the diasporic-boehmite type. Iron is present in them mainly in the form of anhydrous hematite Fe2O3; silica is partly in a free state in the form of quartz and gel (opal), and partly in a bound form, in the form of chamoisite (3H 2 0 * 3FeO * 3Al2O3 * 2SiO2, finally, titanium - in the form of rutile crystals, as well as in the form of a gel.
According to geologist N. A. Arkhangelsky, the mineralogical composition of bauxites from the “Red Cap” deposit can be presented as follows (in%):
Bauxites of the “Red Cap” deposit occur in the form of an inclined layer with a dip angle of 25-30°. The ore body consists of dense rocks that require blasting during mining.
Several bauxite deposits are known in the Middle Urals region. The most studied is the Sokolovskoye deposit (Kamensky district), discovered and explored in 1932-1933. The deposit is a sheet-like, almost horizontal bauxite deposit, covered with a layer of sediment up to 5 m thick. The formation of Sokolov bauxite dates back to Mesozoic time. Depending on the SiO2 content, Sokolovsky bauxites can be divided into two most important varieties, more or less closely mixed in the ore strata: stony bauxite, containing silica up to 3.7%, and earthy (loose) bauxite - up to 9%. The average chemical composition of Sokolovsky bauxites is presented as follows:
31.7% Al2O3; 38.3 Fe2O3; 5.8% SiO2; 4.5% TiO2; and 18, 19% H 2 0.
The mineralogical composition of Sokolovsky bauxites (according to N.A. Arkhangelsky) can be characterized approximately as follows (in%):
The fact that alumina in Sokolovsky bauxites is present in the form of hydrargillite is their positive feature, since the latter is more chemically active than diaspore or boehmite.
This circumstance, as we will see below, facilitates the task of extracting alumina from such bauxite. However, the relatively low content of Al 2 0 3 and the high content in these bauxites make them less valuable compared to the bauxites of the Northern Urals.
Bauxite deposits in the Southern Urals were discovered in October 1935. They represent a sheet-like deposit extending among limestones. The most common bauxites found here are marginal, flagstone and block jasper-shaped bauxites.
In terms of their mineralogical composition, South Ural bauxites belong to the boehmite (“Ivanov Log”) and diaspore (“Kukshik”) types. Their chemical composition is more or less homogeneous and is characterized by the following figures:
53-57% A1 2 0 3; 18-23% Fe 2 0 3; 5-7% SiO 2 and 11-13% H 2 0.
In the upper layer of the formation, varieties of white bauxite are sometimes found with Al 2 03 content up to 78% and SiO 2 only 0.4%.
The South Ural bauxite deposits should be classified as a first-class raw material base for our aluminum industry.
Related posts: As noted earlier, bauxite contains up to 100 elements of the periodic table in various combinations. The number of minerals is also close to 100. From a technological point of view, all bauxite minerals can be divided into three groups. The first includes aluminum-containing minerals - gibbsite, boehmite, diaspore. The second includes minerals that complicate or disrupt the technology for producing alumina. These are silica-containing minerals, various silicates and aluminosilicates, carbonates, sulfides, and organic substances. The third group is ballast compounds, which do not undergo changes during technological processing and are removed from the technological cycle in the form of sludge. These include iron oxides and titanium-containing compounds. It should be noted that this division is arbitrary, since it does not take into account all the qualities of minerals, as well as the fact that under different production conditions the behavior of minerals can be exactly the opposite. For example, the mineral calcite (CaCO3), which is a harmful impurity in the Bayer process, is converted into a useful component in the sintering process, etc.Silicon-containing bauxite minerals and their leaching behavior. The silica content (SiO2) in bauxite varies widely (2-20%) and is characterized by the silicon module. Silica in bauxite is found in free and bound forms. Silicon-containing bauxite minerals include opal SiO2*H2O, chalcedony SiO2, α-quartz SiO2, as well as various aluminosilicates and silicates (kaolinite Al2O3*2SiO2*2H2O, chamosite (Mg, Al, Fe)12 [(Si, Al)8O20]( OH)16 and other minerals). According to the reactivity of dissolution in alkaline aluminate solutions, silica-containing minerals can be arranged as follows: silica hydrogel - opal mineral - kaolin minerals - quartz.
Kaolinite- the main silica-containing mineral of bauxite. Its group also includes dikkit and nacrite.
The heating curves of this mineral have 2 endothermic effects in the range from 400 to 600 °C and one exothermic effect at 900 °C. In minerals with a disordered structure, another endothermic effect appears at 100-200 °C.
When heated, the following transformations occur:
When heated, kaolinite transforms into metakaolinite, then into silicon spinel, and the final product is mullite with crystalline stone.
Kaolinite and minerals of its group interact with alkali-aluminate solutions to form sodium hydroaluminosilicate (see formula (4.9)). The intensity of its dissolution depends on the concentration of the alkali-aluminate solution and its temperature. Thus, when the Na2O content increases from 120 to 220 g/l at a process temperature of 105 °C, kaolinite completely dissolves. Reducing the temperature of the aluminate solution to 70 °C, compared to 105 °C, leads to a sharp decrease in the solubility of the mineral. Under the conditions of the Bayer hydrochemical method, kaolin minerals are completely decomposed first.
Quartz usually included in bauxite in the form of α-modification: α-SiO2. Its content in bauxite is variable and ranges from 3 to 11%. There is conflicting information about the behavior of quartz in alkaline solutions. In particular, the authors F.F. Wolf and O.I. Pudovkin believe that α-SiO2 does not dissolve in strong alkali-aluminate solutions with a Na2O concentration of 300 g/l and a caustic modulus of the solution of 4-7 units. According to other researchers, with sufficiently fine grinding, the solubility of quartz is not inferior to the solubility of silicic acid gel. Subsequently, using electron microscopy, the authors S.I. Kuznetsov and others showed that individual α-SiO2 crystals dissolve in alkaline solutions already at 100 °C. Thus, quartz under the conditions of the Bayer hydrochemical method is an active component. At elevated temperatures (220-230 °C) during autoclave leaching of bauxite, quartz dissolves completely.
Chamosite(Fe2+, Mg)23 * (Fe3+, Al)0.7 * (Si1.4*Al0.6)O5 * (OH)4 - this mineral belongs to the group of layered aluminosilicates. The term “chamosite” often refers to ferruginous chlorite. In alumina-containing raw materials, bauxite is also the main silica-containing mineral. It is most often found in the bauxite deposits of SUBR, Timan and YuUBR. The chemical composition of chamosites is very variable. There are chamosites with a predominance of di- and trivalent iron.
The content of the main components in them varies within the following limits: SiO2 = 18-33%, Al2O3 = 20-30%, Fe2O3 = 1-18%, FeO = 2-39%, MgO = 0.6-6.5%, H2O = 7-11%.
It was experimentally established that in aluminate solutions in the Bayer process, the solubility of chamosite depends on its chemical and mineralogical composition. In particular, deeply oxidized chamosite containing FeO ≤ 1% dissolves by 96% in 4 hours already at 95 °C. Low-oxidized chamosite with a FeO content of about 11.5% under the same conditions dissolves by 25-35%.
The interaction of chamosite with NaOH can be described by the following reaction:
This reaction may be one of the reasons for the increase in pressure in autoclaves and the appearance of divalent iron in solutions. It was found that during the processing of a new type of raw material - Timan bauxite at the Ural aluminum smelters, the number of blow-offs in autoclave batteries sharply increased, which also confirms the version of the decomposition of chamosites and chlorites during leaching.
It should be noted that the release of hydrogen during this reaction can be dangerous.
The process of converting bauxite silica into GASN occurs in 2 stages (Fig. 4.12):
1) dissolution of silica in an alkaline aluminate solution (see formula (4.6));
2) crystallization of GASN from solution (see formulas (4.7), (4.8)). The solubility of GASN decreases with increasing temperature; for this reason, aluminosilicate solutions are better and more deeply desiliconized when the process is carried out at temperatures of 150-170 °C.
Most researchers believe that the chemical composition of the released HASN is not constant, depends on the temperature, composition and concentration of the aluminate solution and corresponds to the conventional formula nNa2O*Al2O3*(1.4-2)SiO2*xH2O. This aluminosilicate, in its composition and form, belongs to a natural mineral called “sodalite”: 7(Na2O*Al2O3*SiO2)*2NaAlO2*nH2O.
The formation of insoluble compounds with silica causes the main losses of aluminum oxide and alkali with red mud in the form of HASN (see formula (3.4) - losses of Na2O and Al2O3 in the form nNa2O*Al2O3*(1.4-2)SiO2*xH2O).
Silica (SiO2) is one of the most harmful impurities when processing bauxite using the Bayer method. Hence the restriction on the use of bauxite with a low silicon module, less than 7-8 units.
In the presence of lime, part of the bauxite silica binds into a new compound called “aluminum hydrogarnet” (3CaO*Al2O3*0.55SiO2*5.5H2O), which leads to a decrease in alkali losses with red mud. In this case, the following chemical reaction occurs:
For example, when leaching North Ural bauxite without adding lime, red mud is formed with a Na2O content of 6 to 8%. When adding 3 wt.% CaO to this bauxite pulp, the alkali content in the red mud is reduced to 3-4%.
The rate and completeness of dissolution of free quartz depends on the particle size, the concentration of the aluminate solution and the temperature of the process (see Fig. 4.13, 4.14). Amorphous silica and its gel dissolve faster in caustic alkalis than quartz. Coarse-grained quartz dissolves more slowly than highly dispersed quartz.
The dissolution of silicon minerals and the release of insoluble GASN compounds from aluminate solutions during the leaching process leads to overgrowth of heat exchange equipment when heating bauxite with a circulating solution in heat exchangers, as well as to losses of useful components. Therefore, to weaken this harmful effect, it is recommended to keep bauxite pulp in wet mixers at a temperature of 100 ° C for 4-6 hours before heating. This leads to the creation of conditions for the transfer of the soluble part of bauxite silica into sodium hydroaluminosilicate even before leaching of the main aluminum-containing bauxite minerals.
Interesting curves were obtained by I.S. Lileev when he studied the behavior of dissolved silica in low-modulus aluminate solutions with αk = 1.7 at t = 70 °C. Three regions of the silica state were clearly identified (see Fig. 4.15). Region I is the region of the equilibrium state of the solution. Region II, bounded on the state diagram by the equilibrium line (OS), is the region of the equilibrium state of silica, and the limiting supersaturation line (OA), called the metastable region. A solution in the metastable region can remain in a state of unstable equilibrium for any length of time, retaining silica. Region III belongs to the labile region and is absolutely unstable. Being in this area leads to spontaneous (spontaneous) crystallization of GASN. Subsequently, the behavior of silica was studied under the same conditions, but only in the region of increased alumina concentrations in solution. Thanks to the averaging and approximation of the obtained experimental data, it was possible to derive equations for the limitation of these regions.
By extrapolating experimental data on the behavior of silica in concentrated aluminate solutions, mathematically processed results of the behavior of silica in solution were obtained and the region of the metastable state of silica was clearly identified.
The pattern of silica retention in the metastable region was also confirmed in solutions with high alumina concentrations. It was also shown that diluting these concentrated solutions to generally accepted concentrations allows silica to remain in the metastable region (RH curve), which allows for the subsequent separation of red mud from the aluminosilicate solution.
Region I: region of the equilibrium state of silica.
Region II: region of the metastable state of silica.
Region III: the region of the labile state of silica, in which silica is practically not retained in solution and is intensively released from it in the form of HASN.
Extremely high concentrations of silica in aluminate solutions have to be dealt with when leaching ore and cake. Desiliconization of aluminate solutions through GASN is possible due to the extremely low silica content (OS curve) in the equilibrium state region. The region above the OA curve is the region of the labile state of silica, where it practically cannot be retained by the solution and is released from it.
Iron-containing bauxite minerals and their leaching behavior. Constant companions of the main rock-forming minerals of bauxite - aluminum oxide and hydroxide and kaolinite - are iron compounds. Iron-containing bauxite minerals are represented by four classes of compounds: oxides, sulfides and sulfates, carbonates and silicates. From the first, most common class of minerals, hematite and hydrohematite, goethite and hydrogoethite, limonite and hematogel, as well as magnetite and maghemite should be distinguished. It has been established that diaspore bauxites are richer in sulfides compared to boehmite-gibbsite and gibbsite bauxites. Iron carbonates are present predominantly in gibbsite bauxites.
Goethite(α-FeOOH) is a constant companion of bauxites and is the main mineral of gibbsite bauxites in tropical countries and Mediterranean deposits. The crystal lattice of goethite is similar to diaspore, and γ-FeOOH in its structure corresponds to boehmite.
Under the conditions of the Bayer process, goethite in alkaline solutions, being dehydrated, turns into hematite α-Fe2O3. Without affecting the chemistry of the Bayer process, goethite can disrupt the thickening process of red mud. This is due to its ability to reversibly dehydrogenate and hydrogenate. If bauxite is fired until the mineral goethite is completely dehydrated, the thickening process occurs without complications.
Lepidocrocite(γ-FeOOH) is a rare mineral in bauxites; its structure corresponds to boehmite. This mineral is an unstable compound, and in alkaline aluminate solutions it recrystallizes into maghemite - γ-, α-Fe2O3, Fe2O3. This connection is magnetic.
Hematite(α-Fe2O3) is the main iron-containing mineral of SUBR bauxite. The amount of hematite from the total Fe2O3 content in bauxite is often 80-90%. Hematite is part of the beans and the cementing mass. Often finely dispersed and found in close association with other minerals. In bauxite, hematite is so finely dispersed that it is impossible to isolate it in its pure form. Artificial hematite can be obtained by dehydrating goethite by heating or treating it with an alkaline solution. Hematite is practically insoluble in alkaline aluminate solutions and is a ballast impurity in the Bayer process. Hematite is weakly magnetic, and this is explained by the presence in it of a small amount of magnetite Fe3O4 and maghemite γ-Fe2O3.
Maghemite(γ-Fe2O3) - highly magnetic. IN natural conditions found in sedimentary rocks rich in organic matter. It can also be obtained by dehydrating lepidocrocite or goethite. When heated, it irreversibly transforms into hematite.
Magnetite((FeIIFeIII2)O4) is an inert component of bauxite and does not interact with alkali-aluminate solutions.
Iron carbonates. The most common mineral is siderite FeСO3.
Found in monohydrate and gibbsite bauxites. Its amount in these bauxites is variable. The average content in Red October bauxites is 6%. In some batches - up to 30%. Siderite is rarely a pure mineral. It contains manganese and magnesium in noticeable quantities (from 5 to 30%). The replacement of iron with calcium occurs in more limited quantities (up to 10%). This mineral is a very harmful impurity, because it intensively and irreversibly interacts with alkaline solutions, which leads to their decausticization.
In particular: FeCO3+ 2NaOH + H2O = NaCO3+ Fe(OH)3 + 1/2 H2.
The formation of hydrogen can lead to increased pressure in autoclaves. Fe(OH)3 is a finely dispersed colloidal component of red mud; its presence in red mud increases the consumption of rye flour during thickening. In addition, alkaline solutions are contaminated with divalent iron, the content of which ranges from 0.008 to 0.725 g/l. During decomposition, iron is released together with aluminum hydroxide and reduces the quality of the resulting product.
Iron sulfide minerals. Almost all sulfur (92-95%) in bauxite is represented by iron sulfide minerals: pyrite, melnikovite-pyrite, pyrrhotite, marcasite, chalcopyrite.
According to the reactivity of dissolution in alkaline solutions, they are arranged in the following series: melnikovite-pyrite → pyrrhotite → marcasite → pyrite → chalcopyrite. The most common mineral is pyrite (FeS2), a typical representative of sulfide iron in bauxite. There is a colloidal variety: melnikovite. In alkaline aluminate solutions in the Bayer method, pyrite dissolves by 10-20%, and melnikovite by 100%. Isomorphic substitutions of iron with nickel and cobalt up to 14-20% are possible. Iron sulfide minerals have a negative effect on Bayer and sintering processes. Therefore, there are restrictions on the sulfur content in bauxite raw materials. It has been experimentally established that it is cost-effective to process bauxite using both the Bayer method and the sintering method with a sulfur content of no more than 1%. The presence of sulfides leads to irreversible losses of alkali in the form of sulfides, polysulfides and sodium sulfates. At the moment, methods have been developed for purifying alkali-aluminate solutions from sulfur and iron impurities by adding copper or zinc oxide to solutions.
The chemical reaction of pyrite decomposition in alkaline aluminate solutions is presented below:
The extraction of sulfur into solution depends on the mineralogical form and structure of the sulfide. Melnikovite has the most reactivity. The decomposition of sulfide minerals mainly occurs at temperatures above 180 °C, and increases with heating. An increase in the concentration of alkali in the solution has a similar effect. This problem arises acutely when bauxite with a sulfur content of more than 1% is received for processing. With such a sulfur content, the contamination of solutions with iron sharply increases, and the quality of the resulting alumina decreases. Iron goes into solution in the form of the compound Na2*2H2O - sodium hydroxothioferrate. In addition, it was noticed that equipment corrosion is increasing (the service life of heat exchange equipment using evaporation is reduced from 4.5 years to 9 months). Pipelines are also being rapidly destroyed.
V.V. Grachev established a direct dependence of the contamination of solutions with iron on the content of sulfide sulfur in the solution (see Table 4.2).
Thus, it was shown that the higher the content of sulfide sulfur in the solution, the more dissolved iron it contains. Subsequently, the presence of four forms of sulfur in alkaline aluminate solutions was established: S2- - sulfide, S2O3b2- - thiosulfate, SO3b2- - sulfite, SO4b2- - sulfate.
During oxidation during the leaching process, the following changes occur in the transition forms of sulfur:
S2- → S2О3в2- → SO3в2- → SO4в2-
The behavior of these forms of sulfur oxidation during the leaching of sulfide minerals is presented in Fig. 4.16.
The activation energy for the transition of sulfide sulfur to various forms has been calculated and has the following values: I. Ea = 2100 kJ/mol to S2O3b2-; II. Еа = 4396 kJ/mol to SO3в2-; III. Еа = 6007 kJ/mol to SO4в2-.
From the data presented it is clear that the first stage is the least energy-intensive; it occurs at temperatures below 100 °C. It has been experimentally proven that complete oxidation of sulfide sulfur to sulfate sulfur requires a certain time (see Table 4.3).
The rate of interaction depends on the contact surface and the solubility of oxygen in the aluminate solution, i.e., very highly dispersed oxygen must be supplied.
Iron is an integral companion of sulfur; it is also found in aluminate solutions in various forms and undergoes the following changes during the oxidation of sulfur:
2- - iron hydroxysulfate (red);
3- - gives the solution a green color at 25 ° C;
3-n - hydroxoaqua complex.
During the decomposition process, this hydroxoaqua complex of iron coprecipitates with aluminum hydroxide, introducing itself into its crystal lattice, and contaminates the resulting hydroxide with iron impurities, further reducing the quality of the resulting alumina.
Ways to combat sulfide minerals:
1) roasting above 600 °C allows you to destroy sulfide minerals and remove most of the sulfur in the form of gases, but complete removal of sulfur cannot be achieved;
2) flotation of pyrite of bauxite raw materials (water flotation of pyrite and its flotation in alkali-aluminate solutions were experimentally tested at the Department of Metallurgy of Light Metals UPI by F.F. Fedyaev, V.S. Shemyakin, V.V. Saltanov, etc.) . Subsequently, industrial tests of this technology were carried out at the processing plant in V. Pyshma and at the Bogoslovsky aluminum smelter. However, this technology has not received industrial implementation;
3) radiometric and photometric enrichment during ore preparation of bauxite raw materials are currently the most promising areas;
4) addition of ZnO to aluminate solutions. As a result, ZnS is formed, which removes sulfide sulfur with red mud. The content of ferrous iron in the solution is sharply reduced. For the first time this technology, developed at the Department of Metallurgy of Light Metals UPI V.V. Grachev, T.A. Uncoated and others, was successfully used at the Ural Aluminum Smelter in the mid-70s-80s. last century.
Titanium-containing bauxite minerals and their leaching behavior. Titanium oxide TiO2 is contained in all bauxites, both in free form and in the form of various chemical compounds. The total amount of TiO2 in bauxite is variable and ranges from 1 to 10%. In particular, in bauxites of the Altai deposit - 2-4% TiO2, Krasnooktyabrsky - 1.5-2.5% TiO2, Tatar - 2-10% TiO2, Gayansky - 1-2% TiO2.
The main titanium minerals: anatase, rutile, occasionally brookite, ilmenite; less commonly sphene, titanomagnetite, perovskite.
Rutile(TiO2) is a common mineral in bauxite. In some cases, up to 8-10% Fe(II) and Fe(III) are present. Rutile is the carrier of uranium and thorium in bauxite. In alkaline solutions, rutile can form a number of compounds such as sodium titanates and silicates. In the presence of lime, a perovskite compound is formed - CaO*TiO2. Chemically, rutile is less active than anatase.
Anataz(TiO2) is the most common titanium mineral in bauxite. Contains up to 1% iron and tin. The structure of anatase is similar to rutile, and the differences lie in the different arrangement of the [TiO6] octahedra. In technological processes of alumina production, it serves as a source of alkali losses due to the formation of sodium titanates. In the presence of calcium oxide, perovskite crystallizes. With increasing temperature, anatase activity increases sharply.
Ilmenite(FeO*TiO2) - is part of the cemented mass of bauxite. Ilmenite is inert in the Bayer process.
Sphene(CaO*TiO2*SiO2) - in bauxites, SUBR is present in the form of large isolated grains or accumulations of small grains with undeveloped edges. The color is yellow-green or brownish-gray. Sphene is also found in the cementing mass of bauxite, less commonly in beans. In the technological process, sphene is also inert.
Titanomagnetite(TiO2*Fe3O4) - more often found in diaspore-boehmite bauxites in the form of inclusions on large black crystals with a metallic luster. The mineral is inert in the technological process.
The behavior of titanium minerals during bauxite leaching was studied for the first time at VAMI. The data obtained showed that when artificially obtained rutile was treated with an alkaline or aluminate solution, the TiO2 content in the solution turned out to be insignificant - from 12 to 100 mg/l (see Fig. 4.17).
In the presence of lime additive, the TiO2 content in the solution is not detected.
It was later found that the addition of TiO2 during leaching of North Ural bauxite, as well as pure diaspore and boehmite, reduces the extraction of alumina into the solution (Fig. 4.17, 4.18). In the presence of lime, introduced based on the ratio CaO:TiO2≥1, the addition of TiO2 does not reduce the yield of alumina into the solution. The role of lime in this case is reduced to the formation of calcium titanate: 2CaO*TiO2*nH2O.
During the experiment, it was noticed that when the diaspore is dissolved in an alkaline aluminate solution in the presence of TiO2, the walls of the autoclaves become covered with a solid white coating, which is not washed off with water. Chemical and X-ray analysis of this plaque showed that it is insoluble sodium metatitanate - NaНТiO3.
TiO2 + NaOH = NaНТiO3
TiO2 + 2NaOH = Na2TiO3 + H2O
Na2ТiO3 + Н2О = NaНТiO3 + NaOH
Based on this, it was assumed that the same film could cover diaspore or boehmite crystals. Its thickness was established to be 18 angstroms. The sharp negative effect of titanium on diaspore dissolution is shown in Fig. 4.19.
Thus, the negative effect of titanium oxide on the dissolution of diaspore and boehmite is shown. This is explained by the fact that a protective film of sodium metatitanate has time to form on the crystal already during the heating of the pulp at a lower temperature than the leaching temperature of diaspore bauxite, i.e., before noticeable dissolution of the diaspore mineral and boehmite. With prolonged stirring, the particles that make up the film aggregate into larger flakes, the film is destroyed and the rate of dissolution of diaspore and boehmite increases. Two forms of sodium titanate have been established:
1) Na2O*3TiO2*2.5H2O - needle-shaped crystals obtained in solutions with Na20R concentrations up to 400 g/l;
2) 3Na2O*5TiO2*3H2O - small equiaxed crystals obtained in solutions with a Na2O concentration of more than 400 g/l.
Later, titanium compounds 5Fe2O3*TiO2*Al2O3 and 8Fe2O3*6Al2O3*TiO2*SiO2 were discovered in the red mud of Hungarian factories, which were called “Dorr sands”.
Below is a series of dissolution activities of the main titanium minerals in alkaline aluminate solutions:
TiO2 gel → anatase → rutile
At the moment, bauxite with the following content of titanium oxide in the raw material is supplied to the Ural aluminum smelters: SUBR - 1.5-2% TiO2, Middle Timan bauxite - 3-4% TiO2. Moreover, in the Subrovsky bauxite, the titanium mineral is presented in the form of anatase, and in the Middle Timan bauxite - in the form of rutile.
Carbonate-containing bauxite minerals and their leaching behavior. Among the minerals containing calcium carbonate, the following minerals are found: calcite CaCO3, dolomite MgCO3*CaCO3, hydromagnesite 4MgCO3*Mg(OH)2*4H2O and siderite FeCO3. All these minerals are easily decomposed under autoclave leaching conditions:
MeCO3 + 2NaOH = Na2CO3 + Me (OH)2
Carbonates are very harmful impurities in raw materials, because they convert the expensive caustic alkali NaOH into carbonate Na2CO3.
Calcite(CaCO3) is the most common carbonate in bauxite. The heating curve has one endoeffect in the region of 800-950 °C, which is explained by the dissociation reaction: CaCO3 → CaO + CO2. Calcite is actively decomposed by alkalis and the more strongly, the higher the temperature of the solution and the concentration of alkali in it. This mineral is one of the harmful impurities in bauxite due to the decausticization of active alkali in solution according to the reaction CaCO3 + 2NaOH = Na2CO3 + Ca(OH)2.
The highest calcite content was observed in North Ural bauxites - up to 7% CO2, so SUBR currently uses various mechanical methods for bauxite enrichment. In North Ural bauxites, calcite is disseminated into beans and cementing mass. It also fills cracks and voids, forming brushes and coarse-crystallized ores in them. When wet grinding and leaching bauxite, calcium carbonate reacts with alkali, turning it into soda. The equilibrium constant of this reaction at a temperature of 25 °C is calculated using the following formula:
where αCO3в2-, α(ОН)- - ion activity; LpCaCO3, LpCa(OH)2 - the product of the solubility of CaCO3 and Ca(OH)2.
With heating, the equilibrium constant of the reaction increases, since the solubility product of calcite increases and the solubility product of lime decreases; at 200 °C it is equal to unity. It was found that in a weakly heated aluminosilicate solution, namely during wet grinding of bauxite (t = 95 ° C), calcite decomposes to form soda and 3-calcium aluminate, which under these conditions is less soluble than lime. In particular:
3CaCO3 + 2NaAl(OH)4 + 4NaOH = 3CaO*Al2O3*6H2O + 3Na2CO3.
In Fig. Figure 4.20 shows the solubility isotherms of solid phases formed in the Na2O-CaO-Al2O3-CO2-H2O system at various temperatures, obtained by M.G. Leitezen and T.A. Potapova. This diagram shows the stability regions of 3CaO*Al2O3*6H2O.
All aluminate solutions with a composition above curve I are enriched in soda and do not interact with calcite. Solutions located below curve I decompose calcite with the formation of 3-calcium aluminate, and the region of its stability increases with increasing concentration of caustic alkali in the solution. It was later found that at elevated temperatures, 3-calcium hydroaluminate becomes unstable and decomposes with alkali according to the reaction
3CaO * Al2O3 * 6H2O + 2NaOH = 2NaAl(OH)4 + Ca(OH)2
Thus, the data presented show that during wet grinding of diaspore bauxites containing calcite impurities, this mineral completely decomposes with the formation of 3-calcium hydroaluminate and soda, and this hydroaluminate, when leached, further decomposes into lime and sodium aluminate. It has been established that calcium carbonates accelerate the leaching of diaspore bauxites, but they should be considered as harmful impurities, since during the decomposition of carbonates, decausticization of the alkali occurs and the accumulation of soda in aluminate solutions. Subsequently, during evaporation, sodium carbonate is released from the solution in the form of “red soda” and sent to the sintering stage for its causticization. In addition, great difficulties are created when evaporating alkaline aluminate solutions, since the heating tubes of evaporators quickly become overgrown with soda, causing the productivity of the devices to sharply decrease. For these reasons, diaspore bauxite containing more than 3-4% CO2 is not recommended for processing into alumina using the Bayer method. An increase in CO2 content above the recommended norm leads to the need to increase the power of the sintering stage.
Phosphorus and small traces of bauxite. The phosphorus content in bauxite in the form of P2O5 varies from traces to 8.0% and on average ranges from 0.4-0.6%.
Phosphorus concentrations are not determined by the mineral or genetic type of bauxite, nor by the age of the deposits.
The phosphorus content (P2O5) in bauxites from various deposits is as follows: in SUBR bauxites - 0.67%; in bauxites of the YuUBR - 0.20%; in STBR bauxite - 0.27%.
The most probable phosphorus minerals in bauxite are apatite 3 [Ca3PO4] * [Ca F, Cl)2]; vivianite Fe3(PO4)2 * 8H2O; francolite Ca10(PO4)6 * [A], AF2, (OH)2, CO3, O; evansite Al3(PO4)2 * 3Al(OH)3 * 12H2O.
The maximum P2O5 content in SUBR bauxite is 0.8%. Phosphorus is considered a very harmful impurity. When processing bauxite using the Bayer method, phosphorus is almost completely transferred into an alkaline aluminate solution, forming the compound Na3FO4. Subsequently, with a slight decrease in the temperature of the solution, sodium phosphate crystallizes, encrusting heat pipes, heating surfaces of heat exchangers and evaporators, reducing the duration of their operation. The presence of phosphorus affects the grain size of aluminum hydroxide (crushes it), this causes a decrease in the quality of the commercial product.
The pattern of distribution of small impurities in bauxites of various geological or lithologic-mineralogical types has been poorly studied. However, elements such as zirconium, vanadium, chromium, nickel and cobalt are present in all bauxite. Currently there are 43 identified in bauxite chemical element, 27 of which are small impurities (their content in bauxite is less than 0.1%). The mineralogical forms of minor impurities in bauxites have not been sufficiently studied. Most impurities, such as gallium and scandium, do not form independent minerals, but due to the proximity of the radii of their ions to the radii of aluminum ions, they enter the lattices of diaspora, boehmite and gibbsite minerals. When processing bauxite using the Bayer method, scandium and other rare earth elements are completely converted into red mud, in which their content increases by 1.5-2 times from the initial content in bauxite. Red mud currently belongs to man-made waste and is a raw material base for the production of these elements.
The content of small impurities in bauxite is presented in table. 6.5. Of greatest interest are those impurities that tend to accumulate in solutions during cyclic production - V, Ga, Cr.
Vanadium and its behavior during leaching. Vanadium may be associated with ferric oxide. A relationship has been noted between its content and the amount of iron oxide in bauxite.
The dependence is expressed by the following formula, %: V2O5 = 4.8 * Fe2O3 * 10v-3, where Fe2O3 - percentage in bauxite. In addition, a connection between vanadium and aluminum minerals was noticed due to the proximity of their ionic radii. There is an increase in the V2O5 content with an increase in the silicon module of bauxite, which may be a consequence of the inclusion of vanadium in aluminum hydroxide minerals. The highest vanadium content is observed in such alumina raw materials as high-iron blast furnace slag. In the hydrochemical processing of alumina production, vanadium is distributed approximately equally between the alkali-aluminate solution and the solid phase (red mud).
Accumulating in the aluminate solution during decomposition, it falls out of the solution along with aluminum hydroxide, reducing its quality. The V2O5 content in factory circulating solutions ranges from 1.1 to 1.5 g/l, so these solutions can serve as a source for obtaining vanadium from them. The main method for isolating vanadium from alkaline aluminate solutions is the crystallization method, based on reducing the solubility of vanadium compounds depending on the concentration of the solution and a decrease in temperature. Currently, the extraction of this product is carried out only at the Pavlodar aluminum smelter.
Gallium and its behavior during leaching. Gallium: tmelt = 30 °C, t = 2000 °C; has a high heat capacity. This element does not form independent minerals, but can isomorphically replace aluminum in its hydroxides. It has been noted that there is more of it in diaspore bauxites, since crystalline GaOOH is isomorphic to the AlOOH diaspore and can be incorporated into its crystal lattice. In the technological stages of alumina production, gallium oxide interacts with alkali and goes into solution in the form of dissolved sodium gallate:
Some gallium leaves technological process with red mud as a result of coprecipitation and chemical interaction of gallate anion with metal cations. The gallium content in the main products obtained in the Bayer process is given in table. 4.4.
A significant amount of commercial gallium entering the world market is produced by the aluminum industry as a by-product of bauxite processing. Research and industrial practice have shown that about 2/3 of gallium oxide from bauxite goes into solution, and 1/3 remains in red mud. By sintering red mud with limestone and soda, and then treating it with an alkaline aluminate solution, the remaining gallium can be extracted from bauxite. In the same way, gallium can be extracted from bauxite processed using the sintering method. The source of gallium in alumina production is aluminate solutions, previously purified from impurities. At foreign alumina refineries, gallium is extracted from Bayer process solutions by electrolysis on a mercury anode. We have developed methods for electrochemical deposition on a gallium cathode, as well as cementation of gallium from solutions with aluminum gallamide. A large amount of work was carried out at the Institute of Chemical Technology of the Ural Branch of the Russian Academy of Sciences under the leadership of S.P. Yatsenko on obtaining ultra-pure metal corresponding to the TU 48-4-350-84 grade. They also showed that the optimal scale of gallium production at an alumina plant with average productivity 0.5-1.0 million tons of alumina is a workshop for producing 5-10 tons of gallium per year. In this case, the gallium concentration established in circulating solutions depends little on the scale of gallium production.
Gallium has a number of valuable properties and is used in LEDs, lasers, and solar batteries. It has found wide application as a component of low-melting alloys, solders, diffusion-hardening compounds, as well as in dental materials.
Chromium and its behavior during leaching. Chromium compounds are usually found in bauxites in small quantities (0.02-0.04%), but some bauxites contain up to 3.0% Cr2O3. In addition to its supposed connection with iron hydroxide, chromium is associated in bauxite with boehmite; Trivalent chromium is soluble in alkaline solutions to form sodium hexahydrooxochromate. If there is an excess of alkali, these compounds can accumulate in aluminate solutions, turning them greenish. If chromium and bauxite enter the sintering stage, then after its oxidation with oxygen during the sintering process, sodium chromates are formed, which are highly soluble in water and alkaline solutions; in them, chromium is in the 6-valent form. In these reactions this compound is very toxic. The color of 6-valent chromium in alkaline solutions is red. To remove 6-valent chromium, you can use various reducing agents, in particular Ns2S, FeSO4*10H2O. Chromium passes into the 3-valent state and is released from alkaline solutions in the form of Cr(OH)3, and a certain amount of aluminum coprecipitates with it, i.e., the loss of aluminum with red mud increases slightly.
Organic substances in bauxite and their behavior in alkali-aluminate solutions. Bauxites from deposits of all types contain organic substances of various origins. These are mainly products of decomposition of plant residues that have migrated into deposits; mineralized plant residues are less commonly observed. The average content of organic substances in bauxite is as follows: in the form of bitumen - up to 0.052%, humins - up to 0.036%.
Humic compounds include high molecular weight compounds. According to the accepted classification, humic substances are divided into 3 groups:
1) water-soluble - fulvic acids;
2) soluble in alcohol - hematamilanic acids and their derivatives;
3) insoluble in either water or alcohol - humic acids.
The permissible organic content in alumina production solutions should be less than 3% oxygen. Organic matter is very harmful to the technological process, since its presence affects the speed and completeness of bauxite leaching. Humins slow down the decomposition of aluminate solutions, reduce the surface tension of solutions, which leads to foaming, as well as slowing down the thickening of red mud. Roasting, and in some cases washing, of bauxite can reduce the maximum concentration of organic substances in aluminate solutions. At the moment, the fight against organics in alkaline aluminate solutions comes down to the use of antifoaming agents in the form of various organic surfactants that allow them to extinguish foam, as well as the oxidation of organics with oxygen or ozone. The material balance of the distribution of organic substances is presented in Table. 4.5.
The numerator of the fraction is the percentage of the total amount of organic substances, and the denominator is the percentage of the amount of dissolved organic substances.
Thus, from this material balance it is clear that the bulk of organic matter - 83% - was released from the cycle with waste red mud. Organic substances are removed from the solution mainly with soda (co-precipitated with red soda) and Al(OH)3. Aluminum hydroxide obtained by decomposition in the Bayer branch is colored pink by organic substances, in contrast to the snow-white hydroxide obtained by sintering. The more organic substances there are, the more of them leave the cycle in these ways. It has been established that organic substances are capable of accumulating in aluminate solutions to a certain limit, when an equilibrium occurs between their intake and removal from the solution. At this equilibrium, the content of these substances must remain below the limit, otherwise additional measures are needed to purify the solutions.
Studies have shown that humic substances are almost completely leached from bauxite in the form of highly soluble alkaline humates. Bitumen is leached by no more than 10%, and when the autoclaved pulp is diluted and thickened, they completely precipitate. Humins are oxidized during leaching and partially at other stages to form sodium oxalate and resinous substances. These resinous substances, consisting mainly of carboxylic acids, color aluminate solutions brown, and at high contents their solutions turn black.
Studying the effect of organic substances on the leaching process, M.N. Smirnov showed that organic substances containing alcohol groups accelerate the leaching of diaspore bauxites. Moreover, it was found that they increase the activity of lime, increasing its solubility in aluminate solutions. Resinous substances (sodium oxalate and acetate) do not affect the extraction of alumina from diaspore bauxite. Organic substances representing bitumen reduce the rate of dissolution of diaspores in bauxite. According to M.N. Smirnov, such substances, when leached, envelop particles of aluminum minerals in bauxite and make it difficult for the aluminate solution to access them. Organic substances slow down the decomposition of aluminate solutions, the crystallization of recycled soda and the thickening of red mud, and also complicate the evaporation of the mother liquor. Resinous organic substances reduce the surface tension of aluminate solutions and thereby contribute to their foaming during transportation and mixing. Particularly strong foaming is observed in mixers after grinding bauxite, in red mud washers, as well as in decomposers.
From bauxite, from 3.8 to 11.9% of organic impurities, which are various shapes organics (see Fig. 4.21). During long-term circulation in the Bayer cycle, the content of organic matter in the circulating solution is almost 30 times higher than its supply with bauxite. The main carriers of this impurity are the circulating solution, the first industrial water and the seed hydroxide. Organic substances complicate the process of thickening red mud, decomposition of aluminate solutions, crystallization of vanadium and cementation of gallium. There are three main groups of organic substances in alkaline aluminate solutions: humins and primary products of their decomposition with a molecular weight greater than 500, intermediate (phenolic acids and benzene carbonates) and low molecular weight products. Alcohols, phenols, ketones, and aliphatic carboxylic acids have the ability to form foam (Table 4.6).
The combined Bayer-sintering scheme ensures the maintenance of the optimal amount of organic substances in circulating solutions by removing them with red mud, aluminum hydroxide and, especially, circulating soda. When processing bauxite only using the Bayer method, it is necessary to specially separate organic impurities from solutions in order to reduce their content in the recycled materials.
In the history of the development of the aluminum industry, there is a well-known example when a newly built alumina plant, operating according to the Bayer method, had to be closed after several months of operation due to severe contamination of the circulating solutions with organic substances.
