Minerals and ores of tungsten

Of the minerals of tungsten, the minerals of the wolframite group and scheelite are of practical importance.

Wolframite (xFeWO4 yMnWO4) is an isomorphic mixture of iron and manganese tungstates. If the mineral contains more than 80% iron, then the mineral is called ferberite. If the mineral contains more than 80% manganese, then the mineral is called hubernite.

Scheelite CaWO4 is practically pure calcium tungstate.

Tungsten ores contain a small amount of tungsten. The minimum content of WO3, at which their processing is expedient. is 0.14-0.15% for large deposits and 0.4-0.5% for small deposits. In ores, tungsten is accompanied by tin in the form of cassiterite, as well as minerals of molybdenum, bismuth, arsenic, and copper. Silica is the main waste rock.

Tungsten ores are enriched. Wolframite ores are enriched by the gravity method, and scheelite - by flotation.

Schemes of enrichment of tungsten ores are diverse and complex. They combine gravity separation with magnetic separation, flotation gravity and flotation. By combining various enrichment methods, concentrates containing up to 55-72% WO3 are obtained from ores. Extraction of tungsten from ore to concentrate is 82-90%.

The composition of tungsten concentrates varies within the following limits,%: WO3-40-72; MnO-0.008-18; SiO2-5-10; Mo-0.008-0.25; S-0.5-4; Sn-0.03-1.5; As-0.01-0.05; P-0.01-0.11; Cu-0.1-0.22.

Technological schemes for the processing of tungsten concentrates are divided into two groups: alkaline and acidic.

Methods for processing tungsten concentrates

Regardless of the method of processing wolframite and scheelite concentrates, the first stage of their processing is the opening, which is the transformation of tungsten minerals into easily soluble chemical compounds.

Wolframite concentrates are opened by sintering or fusion with soda at a temperature of 800-900 ° C, which is based on chemical reactions:

4FeWO4 + 4Na2CO3 + O2 = 4Na2WO4 + 2Fe2O3 + 4CO2 (1)

6MnWO4 + 6Na2CO3 + O2 = 6Na2WO4 + 2Mn3O4 + 6CO2 (2)

When sintering scheelite concentrates at a temperature of 800-900°C, the following reactions occur:

CaWO4 + Na2CO3 = Na2WO4 + CaCO3 (3)

CaWO4 + Na2CO3 = Na2WO4 + CaO + CO2 (4)

In order to reduce the consumption of soda and prevent the formation of free calcium oxide, silica is added to the mixture to bind calcium oxide to a sparingly soluble silicate:

2CaWO4 + 2Na2CO3 + SiO2 = 2Na2WO4+ Ca2SiO4 + CO2 (5)

Sintering of scheelite concentrate, soda and silica is carried out in drum furnaces at a temperature of 850-900°C.

The resulting cake (alloy) is leached with water. During leaching, sodium tungstate Na2WO4 and soluble impurities (Na2SiO3, Na2HPO4, Na2AsO4, Na2MoO4, Na2SO4) and excess soda pass into the solution. Leaching is carried out at a temperature of 80-90°C in steel reactors with mechanical stirring, operating in batch mode, or in continuous drum rotary kilns. Extraction of tungsten into solution is 98-99%. The solution after leaching contains 150-200 g/l WO3. The solution is subjected to filtration, and after separating the solid residue, it is sent for purification from silicon, arsenic, phosphorus and molybdenum.

Silicon removal is based on the hydrolytic decomposition of Na2SiO3 by boiling a solution neutralized at pH = 8-9. Neutralization of excess soda in the solution is carried out with hydrochloric acid. As a result of hydrolysis, slightly soluble silicic acid is formed:

Na2SiO3 + 2H2O = 2NaOH + H2SiO3 (6)

For purification from phosphorus and arsenic, the method of precipitation of phosphate and arsenate ions in the form of sparingly soluble ammonium-magnesium salts is used:

Na2HPO4 + MgCl2+ NH4OH = Mg(NH4)PO4 + 2NaCl + H2O (7)

Na2HAsO4 + MgCl2+ NH4OH = Mg(NH4)AsO4 + 2NaCl + H2O (8)

Purification of molybdenum is based on the decomposition of molybdenum sulfosalt, which is formed by adding sodium sulfide to a solution of sodium tungstate:

Na2MoO4 + 4NaHS = Na2MoS4 + 4NaOH (9)

With the subsequent acidification of the solution to pH = 2.5-3.0, the sulfosalt is destroyed with the release of poorly soluble molybdenum trisulfide:

Na2MoS4 + 2HCl = MoS3 + 2NaCl + H2S (10)

From a purified solution of sodium tungstate, calcium tungstate is first precipitated with CaCl2:

Na2WO4 + СaCl2 = CaWO4 + 2NaCl. (eleven)

The reaction is carried out in a boiling solution containing 0.3-0.5% alkali

while stirring with a mechanical stirrer. The washed precipitate of calcium tungstate in the form of a pulp or paste is decomposed by hydrochloric acid:

CaWO4 + 2HCl = H2WO4 + CaCl2 (12)

During decomposition, a high pulp acidity of the order of 90-120 g/l HCl is maintained, which ensures the separation of phosphorus, arsenic and partly molybdenum impurities from the precipitate of tungstic acid, which are soluble in hydrochloric acid.

Tungstic acid can also be obtained from a purified solution of sodium tungstate by direct precipitation with hydrochloric acid. When the solution is acidified with hydrochloric acid, H2WO4 precipitates due to the hydrolysis of sodium tungstate:

Na2WO4 + 2H2О = 2NaOH + H2WO4 (11)

The alkali formed as a result of the hydrolysis reaction reacts with hydrochloric acid:

2NaOH + 2HCl = 2NaCl + 2H2O (12)

The addition of reactions (8.11) and (8.12) gives the total reaction for the precipitation of tungstic acid with hydrochloric acid:

Na2WO4 + 2HCl = 2NaCl + H2WO4 (13)

However, in this case there are great difficulties in washing the precipitate from sodium ions. Therefore, at present, the latter method of tungstic acid precipitation is used very rarely.

The commercial tungstic acid obtained by precipitation contains impurities and therefore needs to be purified.

The most widely used ammonia method of purification of technical tungstic acid. It is based on the fact that tungstic acid dissolves well in ammonia solutions, while a significant part of the impurities it contains are insoluble in ammonia solutions:

H2WO4 + 2NH4OH = (NH4)2WO4 + 2H2O (14)

Ammonia solutions of tungstic acid may contain impurities of molybdenum and alkali metal salts.

Deeper purification is achieved by separating large crystals of ammonium paratungstate from the ammonia solution, which are obtained by evaporating the solution:

12(NH4)2WO4 = (NH4)10W12O41 5Н2О + 14NH3 + 2H2O (15)

tungsten acid anhydride precipitation

Deeper crystallization is impractical in order to avoid contamination of the crystals with impurities. From the mother liquor enriched with impurities, tungsten is precipitated in the form of CaWO4 or H2WO4 and returned to the previous stages.

Paratungstate crystals are squeezed out on filters, then in a centrifuge, washed with cold water and dried.

Tungsten oxide WO3 is obtained by calcining tungstic acid or paratungstate in a rotary tube furnace with a stainless steel tube and heated by electricity at a temperature of 500-850 ° C:

H2WO4 = WO3 + H2O (16)

(NH4)10W12O41 5Н2О = 12WO3 + 10NH3 + 10H2O (17)

In tungsten trioxide intended for the production of tungsten, the content of WO3 must be at least 99.95%, and for the production of hard alloys - at least 99.9%

Basic enrichment

For some beneficiation factories in the pre-beneficiation, the first Xinhai will use a moving screen jigging machine, and then enter into finishing operations.

Gravity enrichment

For wolframite gravity technology, Xinhai usually adopts a gravity process that includes multi-stage jigging, multi-stage table and middling regrinding. That is, after fine crushing, worthy ores, which, through the classification of the vibrating screen, carry out multi-stage jigging and produce coarse sand from jigging and from gravity. Then the ballast products of jigging of a large class will enter the mill for regrinding. multi-stage table, then coarse sand is produced from gravity and from the table, then the tails from the table will enter the tailings hopper, the middlings from the table will then return to the regrinding cycle, and the gravity coarse sand from the jigging and the table will enter into finishing operations.

recleaning

The wolframite finishing operation usually uses a combined flotation and gravity separation technology or a combined flotation - gravity and magnetic separation technology. At the same time, returns the companion element.

The finishing operation usually uses a combined method of flotation and enrichment table and washing of pyrites through flotation. at the same time, we can enter into the flotation separation of sulfur pyrites. after that, wolframite concentrates are produced, if wolframite concentrates contain scheelite and cassiterite, then wolframite concentrates, scheelite concentrates and cassiterite concentrates are produced through the combined flotation and gravity separation technology or the combined flotation technology - gravity and magnetic enrichment.

Fine sludge treatment

The fine sludge processing method in Xinhai is usually as follows: first, it conducts desulphurization, then, according to the properties of fine sludge and material, gravity, flotation, magnetic and electric enrichment technology is used, or the combined enrichment technology of several technologies is used to return tungsten ore, and at the same time time will spend the utilization of associated ore minerals.

Practical examples

The Xinhai wolframite object was taken as an example, the ore distribution size of this mine was inhomogeneous, ore sludge was very strong. The original process flow used by the processing plant, which includes crushing, pre-washing, gravity and refining, due to a number of technological defects, has resulted in huge loss of fine-grade tungsten ores, high cost of dressing, such as the poor state of comprehensive dressing performance. In order to improve the condition of wolframite sorting, this concentrator commissioned Xinhai to undertake a technical reconstruction task. After careful research on the properties of the ore and processing technology of this factory, Xinhai has optimized the processing technology of this factory's wolframite and added fine sludge processing technology. and ultimately achieve ideal enrichment rates. The enrichment factor of the factory before and after the transformation is as follows:

After the conversion, the extraction of tungsten ore has increased significantly. And softened the effect of fine sludge on the wolframite sorting process, achieved a good recovery rate, effectively improved the economic efficiency of the factory.

The main tungsten minerals are scheelite, hübnerite and wolframite. Depending on the type of minerals, ores can be divided into two types; scheelite and wolframite (huebnerite).
Scheelite ores in Russia, and also in some cases abroad, are enriched by flotation. In Russia, the process of flotation of scheelite ores on an industrial scale was carried out before the Second World War at the Tyrny-Auz factory. This factory processes very complex molybdenum-scheelite ores containing a number of calcium minerals (calcite, fluorite, apatite). Calcium minerals, like scheelite, are floated with oleic acid, the depression of calcite and fluorite is produced by mixing in a liquid glass solution without heating (long contact) or with heating, as at the Tyrny-Auz factory. Instead of oleic acid, tall oil fractions are used, as well as acids from vegetable oils (reagents 708, 710, etc.) alone or in a mixture with oleic acid.

A typical scheme of scheelite ore flotation is given in fig. 38. According to this scheme, it is possible to remove calcite and fluorite and obtain concentrates that are conditioned in terms of tungsten trioxide. Ho apatite still remains in such quantity that the phosphorus content in the concentrate is above the standards. Excess phosphorus is removed by dissolving apatite in weak hydrochloric acid. The consumption of acid depends on the content of calcium carbonate in the concentrate and is 0.5-5 g of acid per ton of WO3.
In acid leaching, part of the scheelite, as well as powellite, is dissolved and then precipitated from solution in the form of CaWO4 + CaMoO4 and other impurities. The resulting dirty sediment is then processed according to the method of I.N. Maslenitsky.
Due to the difficulty of obtaining a conditioned tungsten concentrate, many factories abroad produce two products: a rich concentrate and a poor one for hydrometallurgical processing into calcium tungstate according to the method developed in Mekhanobre I.N. Maslenitsky, - leaching with soda in an autoclave under pressure with transfer to a solution in the form of CaWO4, followed by purification of the solution and precipitation of CaWO4. In some cases, with coarsely disseminated scheelite, finishing of flotation concentrates is carried out on tables.
From ores containing a significant amount of CaF2, the extraction of scheelite abroad by flotation has not been mastered. Such ores, for example in Sweden, are enriched on tables. Scheelite entrained with fluorite in the flotation concentrate is then recovered from this concentrate on a table.
At factories in Russia, scheelite ores are enriched by flotation, obtaining conditioned concentrates.
At the Tyrny-Auz plant, ore with a content of 0.2% WO3 is used to produce concentrates with a content of 6о% WO3 with an extraction of 82%. At the Chorukh-Dairon plant, with the same ore in terms of VVO3 content, 72% WO3 is obtained in concentrates with an extraction of 78.4%; at the Koitash plant, with ore with 0.46% WO3 in concentrate, 72.6% WO3 is obtained with a WO3 recovery of 85.2%; at the Lyangar plant in ore 0.124%, in concentrates - 72% with an extraction of 81.3% WO3. Additional separation of poor products is possible by reducing losses in the tailings. In all cases, if sulfides are present in the ore, they are isolated before scheelite flotation.
The consumption of materials and energy is illustrated by the data below, kg/t:

Wolframite (Hübnerite) ores are enriched exclusively by gravity methods. Some ores with uneven and coarse-grained dissemination, such as the Bukuki ore (Transbaikalia), can be pre-enriched in heavy suspensions, separating about 60% of waste rock at a fineness of -26 + 3 MM with a content of no more than 0.03% WO3.
However, with a relatively low productivity of factories (not more than 1000 tons / day), the first stage of enrichment is carried out in jigging machines, usually starting from a particle size of about 10 mm with coarsely disseminated ores. In new modern schemes, in addition to jigging machines and tables, Humphrey screw separators are used, replacing some of the tables with them.
The progressive scheme of enrichment of tungsten ores is given in fig. 39.
Finishing of tungsten concentrates depends on their composition.

Sulfides from concentrates thinner than 2 mm are isolated by flotation gravity: concentrates after mixing with acid and flotation reagents (xanthate, oils) are sent to a concentration table; the resulting CO table concentrate is dried and subjected to magnetic separation. The coarse-grained concentrate is pre-crushed. Sulfides from fine concentrates from slurry tables are isolated by froth flotation.
If there are a lot of sulfides, it is advisable to separate them from the hydrocyclone drain (or classifier) ​​before enrichment on the tables. This will improve the conditions for separating wolframite on the tables and during concentrate finishing operations.
Typically, coarse concentrates prior to finishing contain about 30% WO3 with recovery up to 85%. For illustration in table. 86 shows some data on factories.

During gravitational enrichment of wolframite ores (hubnerite, ferberite) from slimes thinner than 50 microns, the extraction is very low and losses in the slime part are significant (10-15% of the content in the ore).
From sludges by flotation with fatty acids at pH=10, additional WO3 can be recovered into lean products containing 7-15% WO3. These products are suitable for hydrometallurgical processing.
Wolframite (Hübnerite) ores contain a certain amount of non-ferrous, rare and precious metals. Some of them pass during gravitational enrichment into gravitational concentrates and are transferred to finishing tailings. Molybdenum, bismuth-lead, lead-copper-silver, zinc (they contain cadmium, indium) and pyrite concentrates can be isolated by selective flotation from sulfide tailings, as well as from sludge, and the tungsten product can also be additionally isolated.

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Tungsten minerals, ores and concentrates

Tungsten is a rare element, its average content in the earth's crust is Yu-4% (by mass). About 15 minerals of tungsten are known, however, only minerals of the wolframite group and scheelite are of practical importance.

Wolframite (Fe, Mn)WO4 is an isomorphic mixture (solid solution) of iron and manganese tungstates. If there is more than 80% iron tungstate in the mineral, the mineral is called ferberite, in the case of the predominance of manganese tungstate (more than 80%), it is called hubnerite. Mixtures lying in composition between these limits are called wolframites. Minerals of the wolframite group are colored black or brown and have a high density (7D-7.9 g/cm3) and a hardness of 5-5.5 on the mineralogical scale. The mineral contains 76.3-76.8% W03. Wolframite is weakly magnetic.

Scheelite CaWOA is calcium tungstate. The color of the mineral is white, gray, yellow, brown. Density 5.9-6.1 g/cm3, hardness according to the mineralogical scale 4.5-5. Scheelite often contains an isomorphic admixture of powellite, CaMo04. When irradiated with ultraviolet rays, scheelite fluoresces blue - blue light. At a molybdenum content of more than 1%, fluorescence becomes yellow. Scheelite is non-magnetic.

Tungsten ores are usually poor in tungsten. The minimum content of W03 in ores, at which their exploitation is profitable, is currently 0.14-0.15% for large and 0.4-0.5% for small deposits.

Together with tungsten minerals, molybdenite, cassiterite, pyrite, arsenopyrite, chalcopyrite, tantalite or columbite, etc. are found in ores.

According to the mineralogical composition, two types of deposits are distinguished - wolframite and scheelite, and according to the shape of ore formations - vein and contact types.

In vein deposits, tungsten minerals mostly occur in quartz veins of small thickness (0.3-1 m). The contact type of deposits is associated with zones of contact between granite rocks and limestones. They are characterized by deposits of scheelite-bearing skarn (skarns are silicified limestones). The skarn-type ores include the Tyrny-Auzskoye deposit, the largest in the USSR, in the North Caucasus. During the weathering of vein deposits, wolframite and scheelite accumulate, forming placers. In the latter, wolframite is often combined with cassiterite.

Tungsten ores are enriched to obtain standard concentrates containing 55-65% W03. A high degree of enrichment of wolframite ores is achieved using various methods: gravity, flotation, magnetic and electrostatic separation.

When enriching scheelite ores, gravity-flotation or purely flotation schemes are used.

The extraction of tungsten into conditioned concentrates during the enrichment of tungsten ores ranges from 65-70% to 85-90%.

When enriching complex or difficult-to-enrich ores, it is sometimes economically advantageous to remove intermediate products with a content of 10-20% W03 from the enrichment cycle for chemical (hydrometallurgical) processing, as a result of which "artificial scheelite" or technical tungsten trioxide is obtained. Such combined schemes provide a high extraction of tungsten from ores.

The state standard (GOST 213-73) provides for the content of W03 in tungsten concentrates of the 1st grade not less than 65%, the 2nd grade - not less than 60%. They limit the content of impurities P, S, As, Sn, Cu, Pb, Sb, Bi in the range from hundredths of a percent to 1.0%, depending on the grade and purpose of the concentrate.

The explored reserves of tungsten as of 1981 are estimated at 2903 thousand tons, of which 1360 thousand tons are in the PRC. The USSR, Canada, Australia, the USA, South and North Korea, Bolivia, Brazil, and Portugal have significant reserves. Production of tungsten concentrates in capitalist and developing countries in the period 1971 - 1985 fluctuated within 20 - 25 thousand tons (in terms of metal content).

Methods for processing tungsten concentrates

The main product of the direct processing of tungsten concentrates (apart from ferrotungsten, smelted for the needs of ferrous metallurgy) is tungsten trioxide. It serves as the starting material for tungsten and tungsten carbide, the main constituent of hard alloys.

Production schemes for the processing of tungsten concentrates are divided into two groups depending on the accepted method of decomposition:

Tungsten concentrates are sintered with soda or treated with aqueous soda solutions in autoclaves. Tungsten concentrates are sometimes decomposed with aqueous solutions of sodium hydroxide.

Concentrates are decomposed by acids.

In cases where alkaline reagents are used for decomposition, solutions of sodium tungstate are obtained, from which, after purification from impurities, end products are produced - ammonium paratungstate (PVA) or tungstic acid. 24

When the concentrate is decomposed by acids, precipitation of technical tungstic acid is obtained, which is purified from impurities in subsequent operations.

Decomposition of tungsten concentrates. alkaline reagents Sintering with Na2C03

Sintering wolframite with Na2C03. The interaction of wolframite with soda in the presence of oxygen proceeds actively at 800-900 C and is described by the following reactions: 2FeW04 + 2Na2C03 + l/202 = 2Na2W04 + Fe203 + 2C02; (l) 3MnW04 + 3Na2C03 + l/202 = 3Na2W04 + Mn304 + 3C02. (2)

These reactions proceed with a large loss of Gibbs energy and are practically irreversible. With the ratio in wolframite FeO:MnO = i:i AG ° 1001C = -260 kJ / mol. With an excess of Na2C03 in the charge of 10-15% in excess of the stoichiometric amount, complete decomposition of the concentrate is achieved. To accelerate the oxidation of iron and manganese, sometimes 1-4% nitrate is added to the charge.

Sintering wolframite with Na2C03 at domestic enterprises is carried out in tubular rotary kilns lined with fireclay bricks. In order to avoid the melting of the charge and the formation of deposits (growths) in the zones of the furnace with a lower temperature, tailings from the leaching of cakes (containing iron and manganese oxides) are added to the charge, reducing the content of W03 in it to 20-22%.

The furnace, 20 m long and with an outer diameter of 2.2 m, at a rotation speed of 0.4 rpm and an inclination of 3, has a capacity of 25 t/day in terms of charge.

The components of the charge (crushed concentrate, Na2C03, saltpeter) are fed from the hoppers to the screw mixer using automatic scales. The mixture enters the furnace hopper, from which it is fed into the furnace. After exiting the kiln, the sinter pieces pass through the crushing rolls and the wet grinding mill, from which the pulp is sent to the upper polisher (Fig. 1).

Scheelite sintering with Na2C03. At temperatures of 800-900 C, the interaction of scheelite with Na2C03 can proceed according to two reactions:

CaW04 + Na2CQ3 Na2W04 + CaCO3; (1.3)

CaW04 + Na2C03 *=*■ Na2W04 + CaO + C02. (1.4)

Both reactions proceed with a relatively small change in the Gibbs energy.

Reaction (1.4) proceeds to an appreciable extent above 850 C, when decomposition of CaCO3 is observed. The presence of calcium oxide in the sinter leads, when the sinter is leached with water, to the formation of poorly soluble calcium tungstate, which reduces the extraction of tungsten into solution:

Na2W04 + Ca(OH)2 = CaW04 + 2NaOH. (1.5)

With a large excess of Na2CO3 in the charge, this reaction is largely suppressed by the interaction of Na2CO4 with Ca(OH)2 to form CaCO3.

To reduce the consumption of Na2C03 and prevent the formation of free calcium oxide, quartz sand is added to the mixture to bind calcium oxide into insoluble silicates:

2CaW04 + 2Na2C03 + Si02 = 2Na2W04 + Ca2Si04 + 2C02;(l.6) AG°100IC = -106.5 kJ.

Nevertheless, in this case, too, to ensure a high degree of tungsten recovery into the solution, a significant excess of Na2CO3 (50–100% of the stoichiometric amount) must be introduced into the charge.

The sintering of the scheelite concentrate charge with Na2C03 and quartz sand is carried out in drum furnaces, as described above for wolframite at 850–900°C. To prevent melting, leaching dumps (containing mainly calcium silicate) are added to the charge at the rate of reducing the content of W03 to 20-22%.

Leaching of soda specks. When cakes are leached with water, sodium tungstate and soluble salts of impurities (Na2Si03, Na2HP04, Na2HAs04, Na2Mo04, Na2S04), as well as an excess of Na2C03, pass into the solution. Leaching is carried out at 80-90 ° C in steel reactors with mechanical agitation, operating in hierio-

Concentrates with soda:

Elevator feeding the concentrate to the mill; 2 - ball mill operating in a closed cycle with an air separator; 3 - auger; 4 - air separator; 5 - bag filter; 6 - automatic weight dispensers; 7 - conveying auger; 8 - screw mixer; 9 - charge hopper; 10 - feeder;

Drum oven; 12 - roll crusher; 13 - rod mill-leacher; 14 - reactor with stirrer

Wild mode, or continuous drum rotary lixiviators. The latter are filled with crushing rods for crushing pieces of cake.

The extraction of tungsten from the sinter into the solution is 98-99%. Strong solutions contain 150-200 g/l W03.

Autoclave o-c One method of decomposition of tungsten concentrates

The autoclave-soda method was proposed and developed in the USSR1 in relation to the processing of scheelite concentrates and middlings. Currently, the method is used in a number of domestic factories and in foreign countries.

The decomposition of scheelite with Na2C03 solutions is based on the exchange reaction

CaW04CrB)+Na2C03(pacTB)^Na2W04(pacTB)+CaC03(TB). (1.7)

At 200-225 °C and the corresponding excess of Na2C03, depending on the composition of the concentrate, decomposition proceeds with sufficient speed and completeness. The concentration equilibrium constants of reaction (1.7) are small, increase with temperature, and depend on the soda equivalent (i.e., the number of moles of Na2C03 per 1 mole of CaW04).

With a soda equivalent of 1 and 2 at 225 C, the equilibrium constant (Kc = C / C cq) is 1.56 and

0.99 respectively. It follows from this that at 225 C the minimum required soda equivalent is 2 (i.e., the excess of Na2C03 is 100%). The actual excess of Na2C03 is higher, since the rate of the process slows down as equilibrium is approached. For scheelite concentrates with a content of 45-55% W03 at 225 C, a soda equivalent of 2.6-3 is required. For middlings containing 15-20% W03, 4-4.5 moles of Na2C03 per 1 mole of CaW04 are required.

CaCO3 films formed on scheelite particles are porous and up to a thickness of 0.1-0.13 mm their influence on the rate of scheelite decomposition by Na2CO3 solutions was not found. With intensive stirring, the rate of the process is determined by the rate of the chemical stage, which is confirmed by the high value of the apparent activation energy E = 75+84 kJ/mol. However, in case of insufficient stirring speed (which

Occurs in horizontal rotating autoclaves), an intermediate regime is realized: the rate of the process is determined both by the rate of supply of the reagent to the surface and the rate of chemical interaction.

0.2 0.3 0, it 0.5 0.5 0.7 0.8

As can be seen from Fig. 2, the specific reaction rate decreases approximately in inverse proportion to the increase in the ratio of molar concentrations of Na2W04:Na2C03 in solution. This is

Ryas. Fig. 2. Dependence of the specific rate of scheelite decomposition by a soda solution in an autoclave j on the molar ratio of Na2W04/Na2C03 concentrations in the solution at

Causes the need for a significant excess of Na2C03 against the minimum required, determined by the value of the equilibrium constant. To reduce the consumption of Na2C03, a two-stage countercurrent leaching is carried out. In this case, the tailings after the first leaching, in which there is little tungsten (15-20% of the original), are treated with a fresh solution containing a large excess of Na2C03. The resulting solution, which is circulating, enters the first stage of leaching.

Decomposition with Na2C03 solutions in autoclaves is also used for wolframite concentrates, however, the reaction in this case is more complicated, since it is accompanied by hydrolytic decomposition of iron carbonate (manganese carbonate is only partially hydrolyzed). The decomposition of wolframite at 200-225 °C can be represented by the following reactions:

MnW04(TB)+Na2C03(paCT)^MiiC03(TB)+Na2W04(paCTB); (1.8)

FeW04(TB)+NaC03(pacT)*=iFeC03(TB)+Na2W04(paCTB); (1.9)

FeC03 + HjO^FeO + H2CO3; (1.10)

Na2C03 + H2C03 = 2NaHC03. (l. ll)

The resulting iron oxide FeO at 200-225 ° C undergoes a transformation according to the reaction:

3FeO + H20 = Fe304 + H2.

The formation of sodium bicarbonate leads to a decrease in the concentration of Na2CO3 in the solution and requires a large excess of the reagent.

To achieve satisfactory decomposition of wolframite concentrates, it is necessary to grind them finely and increase the consumption of Na2C03 to 3.5-4.5 g-eq, depending on the composition of the concentrate. High-manganese wolframites are more difficult to decompose.

The addition of NaOH or CaO to the autoclaved slurry (which leads to causticization of Na2C03) improves the degree of decomposition.

The decomposition rate of wolframite can be increased by introducing oxygen (air) into the autoclave pulp, which oxidizes Fe (II) and Mil (II), which leads to the destruction of the crystal lattice of the mineral on the reacting surface.

secondary steam

Ryas. 3. Autoclave unit with a horizontally rotating autoclave: 1 - autoclave; 2 - loading pipe for the pulp (steam is introduced through it); 3 - pulp pump; 4 - pressure gauge; 5 - pulp reactor-heater; 6 - self-evaporator; 7 - drop separator; 8 - pulp input into the self-evaporator; 9 - chipper made of armored steel; 10 - pipe for pulp removal; 11 - pulp collector

Leaching is carried out in steel horizontal rotating autoclaves heated with live steam (Fig. 3) and vertical continuous autoclaves with stirring of the pulp with bubbling steam. Approximate process mode: temperature 225 pressure in the autoclave ~ 2.5 MPa, ratio T: W = 1: (3.5 * 4), duration at each stage 2-4 hours.

Figure 4 shows a diagram of an autoclave battery. The initial autoclave pulp, heated by steam to 80-100 °C, is pumped into autoclaves, where it is heated to

secondary steam

Ditch. Fig. 4. Scheme of a continuous autoclave plant: 1 - reactor for heating the initial pulp; 2 - piston pump; 3 - autoclave; 4 - throttle; 5 - self-evaporator; 6 - pulp collector

200-225 °C live steam. In continuous operation, the pressure in the autoclave is maintained by discharging the slurry through a throttle (calibrated carbide washer). The pulp enters the self-evaporator - a vessel under pressure of 0.15-0.2 MPa, where the pulp is rapidly cooled due to intensive evaporation. The advantages of autoclave-soda decomposition of scheelite concentrates before sintering are the exclusion of the furnace process and a somewhat lower content of impurities in tungsten solutions (especially phosphorus and arsenic).

The disadvantages of the method include a large consumption of Na2C03. A high concentration of excess Na2C03 (80-120 g/l) entails an increased consumption of acids for the neutralization of solutions and, accordingly, high costs for the disposal of waste solutions.

Decomposition of tungstate conc.

Sodium hydroxide solutions decompose wolframite according to the exchange reaction:

Me WC>4 + 2Na0Hi=tNa2W04 + Me(0 H)2, (1.13)

Where Me is iron, manganese.

The value of the concentration constant of this reaction Kc = 2 at temperatures of 90, 120 and 150 °C is equal to 0.68, respectively; 2.23 and 2.27.

Complete decomposition (98-99%) is achieved by treating the finely divided concentrate with 25-40% sodium hydroxide solution at 110-120°C. The required excess of alkali is 50% or more. The decomposition is carried out in steel sealed reactors equipped with stirrers. The passage of air into the solution accelerates the process due to the oxidation of iron (II) hydroxide Fe (OH) 2 into hydrated iron (III) oxide Fe203-«H20 and manganese (II) hydroxide Mn (OH) 2 into hydrated manganese (IV) oxide Mn02-lH20 .

The use of decomposition with alkali solutions is advisable only for high-grade wolframite concentrates (65-70% W02) with a small amount of silica and silicate impurities. When processing low-grade concentrates, highly contaminated solutions and hard-to-filter precipitates are obtained.

Processing of sodium tungstate solutions

Solutions of sodium tungstate containing 80-150 g/l W03, in order to obtain tungsten trioxide of the required purity, have so far been mainly processed according to the traditional scheme, which includes: purification from compounds of impurity elements (Si, P, As, F, Mo); precipitation

Calcium tungsten mag (artificial scheelite) with its subsequent decomposition with acids and obtaining technical tungstic acid; dissolution of tungstic acid in ammonia water, followed by evaporation of the solution and crystallization of ammonium paratungstate (PVA); calcination of PVA to obtain pure tungsten trioxide.

The main drawback of the scheme is its multi-stage nature, carrying out most of the operations in a periodic mode, and the duration of a number of redistributions. An extraction and ion-exchange technology for converting Na2W04 solutions into (NH4)2W04 solutions has been developed and is already being used at some enterprises. The main redistributions of the traditional scheme and new extraction and ion-exchange variants of the technology are briefly considered below.

Purification of impurities

Silicon cleaning. When the content of Si02 in solutions exceeds 0.1% of the content of W03, preliminary purification from silicon is necessary. Purification is based on the hydrolytic decomposition of Na2Si03 by boiling a solution neutralized to pH=8*9 with the release of silicic acid.

The solutions are neutralized with hydrochloric acid added in a thin stream with stirring (to avoid local peroxidation) to a heated solution of sodium tungstate.

Purification of phosphorus and arsenic. To remove phosphate and arsenate ions, the method of precipitation of ammonium-magnesium salts Mg (NH4) P04 6H20 and Mg (NH4) AsC) 4 6H20 is used. The solubility of these salts in water at 20 C is 0.058 and 0.038%, respectively. In the presence of an excess of Mg2+ and NH4 ions, the solubility is lower.

The precipitation of phosphorus and arsenic impurities is carried out in the cold:

Na2HP04 + MgCl2 + NH4OH = Mg(NH4)P04 + 2NaCl +

Na2HAsQ4 + MgCl2 + NH4OH = Mg(NH4)AsQ4 + 2NaCl +

After a long standing (48 hours), crystalline precipitates of ammonium-magnesium salts precipitate from the solution.

Purification from fluoride ions. With a high content of fluorite in the original concentrate, the content of fluoride ions reaches 5 g/l. Solutions are purified from fluoride - ions by precipitation with magnesium fluoride from a neutralized solution, to which MgCl2 is added. Purification of fluorine can be combined with hydrolytic isolation of silicic acid.

Molybdenum cleaning. Sodium tungstate solutions "must be cleaned of molybdenum if its content exceeds 0.1% of the W03 content (i.e. 0.1-0.2 t / l). At a molybdenum concentration of 5-10 g / l ( for example, in the processing of scheelite-powellite Tyrny-Auzsky concentrates), the isolation of molybdenum is of particular importance, since it is aimed at obtaining a molybdenum chemical concentrate.

A common method is to precipitate the sparingly soluble molybdenum trisulfide MoS3 from a solution.

It is known that when sodium sulfide is added to solutions of tungstate or sodium molybdate, sulfosalts Na23S4 or oxosulfosalts Na23Sx04_x (where E is Mo or W) are formed:

Na2304 + 4NaHS = Na23S4 + 4NaOH. (1.16)

The equilibrium constant of this reaction for Na2Mo04 is much larger than for Na2W04(^^0 » Kzr). Therefore, if an amount of Na2S is added to the solution, sufficient only for interaction with Na2Mo04 (with a slight excess), then molybdenum sulfosalt is predominantly formed. With the subsequent acidification of the solution to pH = 2.5 * 3.0, the sulfosalt is destroyed with the release of molybdenum trisulfide:

Na2MoS4 + 2HC1 = MoS3 j + 2NaCl + H2S. (1.17)

Oxosulfosalts decompose with the release of oxosulfides (for example, MoSjO, etc.). Together with molybdenum trisulfide, a certain amount of tungsten trisulfide co-precipitates. By dissolving the sulfide precipitate in a soda solution and re-precipitating molybdenum trisulfide, a molybdenum concentrate is obtained with a W03 content of not more than 2% with a loss of tungsten 0.3-0.5% of the initial amount.

After partial oxidative roasting of the precipitate of molybdenum trisulfide (at 450-500 ° C), a molybdenum chemical concentrate is obtained with a content of 50-52% molybdenum.

The disadvantage of the method of precipitation of molybdenum in the composition of trisulfide is the release of hydrogen sulfide according to reaction (1.17), which requires expenses for the neutralization of gases (they use the absorption of H2S in a scrubber irrigated with a sodium hydroxide solution). The selection of molybdenum trisulfide is carried out from a solution heated to 75-80 C. The operation is carried out in sealed steel reactors, gummed or coated with acid-resistant enamel. The trisulfide precipitates are separated from the solution by filtration on a filter press.

Obtaining tungstic acid from solutions of sodium tungstate

Tungstic acid can be directly isolated from a solution of sodium tungstate with hydrochloric or nitric acid. However, this method is rarely used due to the difficulty of washing precipitates from sodium ions, the content of which in tungsten trioxide is limited.

For the most part, calcium tungstate is initially precipitated from the solution, which is then decomposed with acids. Calcium tungstate is precipitated by adding a CaCl2 solution heated to 80-90 C to a solution of sodium tungstate with a residual alkalinity of the solution of 0.3-0.7%. In this case, a white finely crystalline, easily settled precipitate falls out, sodium ions remain in the mother liquor, which ensures their low content in tungstic acid. 99-99.5% W precipitates from the solution, mother solutions contain 0.05-0.07 g/l W03. The CaW04 precipitate washed with water in the form of a paste or pulp is decomposed by hydrochloric acid when heated to 90°:

CaW04 + 2HC1 = H2W04i + CaCl2. (1.18)

During decomposition, a high final acidity of the pulp is maintained (90–100 g/l HCI), which ensures the separation of tungstic acid from impurities of phosphorus, arsenic, and partly molybdenum compounds (molybdic acid dissolves in hydrochloric acid). Precipitates of tungstic acid require thorough washing from impurities (especially from calcium salts

and sodium). In recent years, continuous washing of tungstic acid in pulsating columns has been mastered, which greatly simplified the operation.

At one of the enterprises in the USSR, when processing sodium tungstate solutions, instead of hydrochloric acid, nitric acid is used to neutralize the solutions and decompose CaW04 precipitates, and the precipitation of the latter is carried out by introducing Ca(N03)2 into the solutions. In this case, the nitric acid mother liquors are disposed of, obtaining nitrate salts used as fertilizer.

Purification of technical tungstic acid and obtaining W03

Technical tungstic acid, obtained by the method described above, contains 0.2-0.3% impurities. As a result of acid calcination at 500-600 C, tungsten trioxide is obtained, suitable for the production of hard alloys based on tungsten carbide. However, the production of tungsten requires trioxide of a higher purity with a total impurity content of no more than 0.05%.

The ammonia method for purifying tungstic acid is generally accepted. It is easily soluble in ammonia water, while most of the impurities remain in the sediment: silica, iron and manganese hydroxides, and calcium (in the form of CaW04). However, ammonia solutions may contain an admixture of molybdenum, alkali metal salts.

From the ammonia solution, as a result of evaporation and subsequent cooling, a crystalline precipitate of PVA is isolated:

Evaporation

12(NH4)2W04 * (NH4)10H2W12O42 4Н20 + 14NH3 +

In industrial practice, the composition of PVA is often written in the oxide form: 5(NH4)20-12W03-5H20, which does not reflect its chemical nature as an isopoly acid salt.

Evaporation is carried out in batch or continuous devices made of stainless steel. Usually 75-80% of tungsten is isolated into crystals. Deeper crystallization is undesirable in order to avoid contamination of the crystals with impurities. It is significant that most of the molybdenum impurity (70-80%) remains in the mother liquor. From the mother liquor enriched with impurities, tungsten is precipitated in the form of CaW04 or H2W04, which is returned to the appropriate stages of the production scheme.

PVA crystals are squeezed out on a filter, then in a centrifuge, washed with cold water and dried.

Tungsten trioxide is obtained by thermal decomposition of tungstic acid or PVA:

H2W04 \u003d "W03 + H20;

(NH4) 10H2W12O42 4H20 = 12W03 + 10NH3 + 10H20. (1.20)

Calcination is carried out in rotary electric furnaces with a pipe made of heat-resistant steel 20X23H18. The calcination mode depends on the purpose of tungsten trioxide, the required size of its particles. So, to obtain tungsten wire grade VA (see below), PVA is calcined at 500-550 ° C, wire grades VCh and VT (tungsten without additives) - at 800-850 ° C.

Tungstic acid is calcined at 750-850 °C. Tungsten trioxide derived from PVA has larger particles than trioxide derived from tungstic acid. In tungsten trioxide, intended for the production of tungsten, the content of W03 must be at least 99.95% for the production of hard alloys - at least 99.9%.

Extraction and ion-exchange methods for processing solutions of sodium tungstate

The processing of sodium tungstate solutions is greatly simplified when tungsten is extracted from solutions by extraction with an organic extractant, followed by re-extraction from the organic phase with an ammonia solution with separation of PVA from an ammonia solution.

Since in a wide range of pH=7.5+2.0 tungsten is found in solutions in the form of polymeric anions, anion-exchange extractants are used for extraction: salts of amines or quaternary ammonium bases. In particular, the sulfate salt of trioctylamine (i?3NH)HS04 (where R is С8Н17) is used in industrial practice. The highest rates of tungsten extraction are observed at pH=2*4.

Extraction is described by the equation:

4 (i? 3NH) HS04 (opr) + H2 \ U120 * "(aq) + 2H + (aq) ї \u003d ї

Ї \u003d ї (D3GSh) 4H4 \ U12O40 (org) + 4H80; (aq.). (l.2l)

The amine is dissolved in kerosene, to which a technical mixture of polyhydric alcohols (C7 - C9) is added to prevent the precipitation of a solid phase (due to the low solubility of amine salts in kerosene). The approximate composition of the organic phase: amines 10%, alcohols 15%, kerosene - the rest.

Solutions purified from mrlibden, as well as impurities of phosphorus, arsenic, silicon and fluorine, are sent for extraction.

Tungsten is re-extracted from the organic phase with ammonia water (3-4% NH3), obtaining solutions of ammonium tungstate, from which PVA is isolated by evaporation and crystallization. The extraction is carried out in mixer-settler type apparatuses or in pulsating columns with packing.

The advantages of extraction processing of sodium tungstate solutions are obvious: the number of operations of the technological scheme is reduced, it is possible to carry out a continuous process for obtaining ammonium tungstate solutions from sodium tungstate solutions, and production areas are reduced.

Wastewater from the extraction process may contain an admixture of 80-100 mg/l of amines, as well as impurities of higher alcohols and kerosene. To remove these environmentally harmful impurities, froth flotation and adsorption on activated carbon are used.

Extraction technology is used at foreign enterprises and is also implemented at domestic plants.

The use of ion-exchange resins is a direction competing with extraction for the processing of sodium tungstate solutions. For this purpose, low-basic anion exchangers containing amine groups (often tertiary amines) or amphoteric resins (ampholytes) containing carboxyl and amine groups are used. At pH=2.5+3.5, tungsten polyanions are sorbed on resins, and for some resins the total capacity is 1700-1900 mg W03 per 1 g of resin. In the case of resin in the 8C>5~ form, sorption and elution are described by the equations, respectively:

2tf2S04 + H4W12044; 5^"4H4W12O40 + 2SOf; (1.22)

I?4H4WI2O40 + 24NH4OH = 12(NH4)2W04 + 4DON + 12H20. (l.23)

The ion-exchange method was developed and applied at one of the enterprises of the USSR. The required contact time of the resin with the solution is 8-12 hours. The process is carried out in a cascade of ion-exchange columns with a suspended resin bed in a continuous mode. A complicating circumstance is the partial separation of PVA crystals at the stage of elution, which requires their separation from the resin particles. As a result of elution, solutions containing 150–170 g/l of W03 are obtained, which are fed to the evaporation and crystallization of PVA.

The disadvantage of ion-exchange technology compared to extraction is the unfavorable kinetics (contact time 8-12 hours versus 5-10 minutes for extraction). At the same time, the advantages of ion exchangers include the absence of waste solutions containing organic impurities, as well as the fire safety and non-toxicity of resins.

Decomposition of scheelite concentrates with acids

In industrial practice, mainly in the processing of high-grade scheelite concentrates (70-75% W03), direct decomposition of scheelite with hydrochloric acid is used.

Decomposition reaction:

CaW04 + 2HC1 = W03H20 + CoCl2 (1.24)

Almost irreversible. However, the acid consumption is much higher than the stoichiometrically required one (250–300%) due to the inhibition of the process by tungstic acid films on scheelite particles.

The decomposition is carried out in sealed reactors with stirrers, lined with acid-resistant enamel and heated through a steam jacket. The process is carried out at 100-110 C. The duration of decomposition varies from 4-6 to 12 hours, which depends on the degree of grinding, as well as the origin of the concentrate (scheelites of various deposits differ in reactivity).

A single treatment does not always lead to a complete opening. In this case, after dissolving tungstic acid in ammonia water, the residue is re-treated with hydrochloric acid.

During the decomposition of scheelite-powellite concentrates with a content of 4-5% molybdenum, most of the molybdenum passes into the hydrochloric acid solution, which is explained by the high solubility of molybdic acid in hydrochloric acid. So, at 20 C in 270 g/l HC1, the solubilities of H2Mo04 and H2WO4 are 182 and 0.03 g/l, respectively. Despite this, complete separation of molybdenum is not achieved. Precipitates of tungstic acid contain 0.2-0.3% molybdenum, which cannot be extracted by re-treatment with hydrochloric acid.

The acid method differs from the alkaline methods of scheelite decomposition by a smaller number of operations of the technological scheme. However, when processing concentrates with a relatively low content of W03 (50-55%) with a significant content of impurities, in order to obtain conditioned ammonium paratungstate, two or three ammonia purifications of tungstic acid have to be carried out, which is uneconomical. Therefore, decomposition with hydrochloric acid is mostly used in the processing of rich and pure scheelite concentrates.

The disadvantages of the method of decomposition with hydrochloric acid are the high consumption of acid, the large volume of waste solutions of calcium chloride and the complexity of their disposal.

In the light of the tasks of creating waste-free technologies, the nitric acid method of decomposition of scheelite concentrates is of interest. In this case, the mother solutions are easy to dispose of, obtaining nitrate salts.

Vladivostok

annotation

In this paper, technologies for the enrichment of scheelite and wolframite are considered.

The technology of enrichment of tungsten ores includes: preliminary concentration, enrichment of crushed products of preliminary concentration to obtain collective (rough) concentrates and their refinement.

Keywords

Scheelite ore, wolframite ore, heavy medium separation, jigging, gravity method, electromagnetic separation, flotation.

1. Introduction 4

2. Preconcentration 5

3. Technology of beneficiation of wolframite ores 6

4. Technology of enrichment of Scheelite ores 9

5. Conclusion 12

References 13

Introduction

Tungsten is a silver-white metal with high hardness and a boiling point of about 5500°C.

The Russian Federation has large explored reserves. Its tungsten ore potential is estimated at 2.6 million tons of tungsten trioxide, in which the proven reserves are 1.7 million tons, or 35% of those in the world.

Fields under development in Primorsky Krai: Vostok-2, OJSC Primorsky GOK (1.503%); Lermontovskoye, AOOT Lermontovskaya GRK (2.462%).

The main tungsten minerals are scheelite, hübnerite and wolframite. Depending on the type of minerals, ores can be divided into two types; scheelite and wolframite (huebnerite).

When processing tungsten-containing ores, gravity, flotation, magnetic, as well as electrostatic, hydrometallurgical and other methods are used.

preliminary concentration.

The cheapest and at the same time highly productive methods of preconcentration are gravitational ones, such as heavy media separation and jigging.

Heavy media separation makes it possible to stabilize the quality of the food entering the main processing cycles, to separate not only the waste product, but also to separate the ore into rich coarsely disseminated and poor finely disseminated ore, often requiring fundamentally different processing schemes, since they differ markedly in material composition. The process is characterized by the highest density separation accuracy compared to other gravity methods, which makes it possible to obtain a high recovery of a valuable component with a minimum concentrate yield. When enriching ore in heavy suspensions, a difference in the densities of the separated pieces of 0.1 g/m3 is sufficient. This method can be successfully applied to coarsely disseminated wolframite and scheelite-quartz ores. The results of studies on the enrichment of tungsten ores from the Pun-les-Vignes (France) and Borralha (Portugal) deposits under industrial conditions showed that the results obtained using enrichment in heavy suspensions are much better than when enriched only on jigging machines - into a heavy fraction recovery was more than 93% of the ore.

Jigging in comparison with heavy-medium enrichment, it requires less capital expenditures, allows enriching the material in a wide range of density and fineness. Large-sized jigging is widely used in the enrichment of large- and medium-disseminated ores that do not require fine grinding. The use of jigging is preferable when enriching carbonate and silicate ores of skarn, vein deposits, while the value of the contrast ratio of ores in terms of gravitational composition should exceed one.

Technology of beneficiation of wolframite ores

The high specific gravity of tungsten minerals and the coarse-grained structure of wolframite ores make it possible to widely use gravity processes in their enrichment. To obtain high technological indicators, it is necessary to combine apparatuses with different separating characteristics in the gravitational scheme, in which each previous operation in relation to the next one is, as it were, preparatory, improving the enrichment of the material. A schematic diagram of the enrichment of wolframite ores is shown in fig. one.

Jigging is used starting from the size at which tailings can be identified. This operation is also used for separating coarsely disseminated tungsten concentrates with subsequent regrinding and enrichment of jigging tailings. The basis for choosing the scheme of jigging and the size of the enriched material are the data obtained by separating the density of the material with a size of 25 mm. If the ores are finely disseminated and preliminary studies show that large-sized enrichment and jigging are unacceptable for them, then the ore is enriched in suspension-carrying flows of small thickness, which include enrichment on screw separators, jet chutes, cone separators, locks, concentration tables. With staged grinding and staged enrichment of ore, the extraction of wolframite into rough concentrates is more complete. Rough wolframite gravity concentrates are brought to standard according to developed schemes using wet and dry enrichment methods.

Rich wolframite concentrates are enriched by electromagnetic separation, while the electromagnetic fraction can be contaminated with iron zinc blende, bismuth minerals and partially arsenic (arsenopyrite, scorodite). To remove them, magnetizing roasting is used, which increases the magnetic susceptibility of iron sulfides, and at the same time, sulfur and arsenic, which are harmful to tungsten concentrates, are removed in the form of gaseous oxides. Wolframite (hubnerite) is additionally extracted from sludge by flotation using fatty acid collectors and the addition of neutral oils. Rough gravitational concentrates are relatively easy to bring to standard using electrical methods of enrichment. Flotation and flotation gravity are carried out with the supply of xanthate and blowing agent in a slightly alkaline or slightly acidic medium. If the concentrates are contaminated with quartz and light minerals, then after flotation they are subjected to recleaning on concentration tables.

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