CA3032938A1 - Method for producing smelter-grade alumina from low-grade high-silicon aluminum-containing raw materials - Google Patents

Abstract

The group of inventions relates to metallurgy and can be used in processing low-grade high-silicon aluminum-containing raw materials. Aluminum-containing raw materials are milled and then broken up with hydrochloric acid, which is an acidic recycled mother solution. The chloride pulp that forms is separated into waste siliceous residue and clarified chloride solution. Aluminum chloride hexahydrate is crystallized from the clarified chloride solution. The aluminum chloride hexahydrate is thermally decomposed into aluminum oxide and then calcinated, with raw alumina forming as an intermediate product. The raw alumina is leached out with a recycled alkaline solution, and the resulting aluminate solution is decomposed. The 15% acidic mother liquor is subjected to pyrohydrolysis. The chloride ion concentration in the raw alumina is kept at the level of 0.2-5.0%, and the chloride ion concentration in the recycled alkaline solution is kept at the level of 40-90 g/L. After decomposition, the recycled alkaline solution in an amount of 10-40 mass % of the entire stream is evaporated until crystals of chlorine-containing compounds form, which are removed from the process. The quality of the alumina is improved and the energy expended to produce it is decreased.

Description

THE SMELTER-GRADE ALUMINA
PRODUCTION METHOD (EMBODIMENTS) The invention relates to the metallurgy sphere, in particular to acidic methods for producing alumina, and can be used in processing low-grade high-silicon aluminium-containing raw materials, including waste, such as ash from coal combustion. Smelter-grade alumina and its semi-finished product -aluminium hydroxide - have a wide range of industrial applications, primarily for the production of aluminium metal.
Alumina refineries worldwide produce high-quality smelter-grade alumina mainly using the Bayer technology from low-silicon (Bayer) bauxite, in which the Al2O3/SiO2 concentration ratio (alumina to silica ratio) is not lower than 3.
When the values of the alumina to silica ratio are in the range between 3 and 7, it is necessary to use combined Bayer-sintering processes, which are more energy-intensive ones. For high-silica aluminium-containing raw materials, for example, nepheline and kaolin, the industry uses only the sintering method, the energy costs of which are approximately 5 times higher than those of the Bayer process.
At the same time, acidic methods for the production of alumina from high-silicon aluminium-containing raw materials are being developed. Among them, the hydrochloric acid method is currently considered to be the most rational one.
It is known that alumina can be produced from high silica bauxites by the hydrochloric acid process, which includes the calcination of aluminium-containing raw materials at a temperature of up to 700 C, their treatment with hydrochloric acid, the salting out of aluminium hexahydrate chloride (A1C13.6H20) by saturating the clarified chloride liquor with gaseous hydrogen

2 chloride, the calcination of aluminium chloride to produce aluminium oxide (alumina), the spent liquor pyrohydrolysis, and the rectification of absorbed hydrochloric acid, including the return of hydrogen chloride at the stage of acid treatment and salting out in the form of an aqueous solution and gas, respectively (Elsner D., Jenkins D.H., and Sinha H.N. Alumina via hydrochloric acid leaching of high silica bauxites-process development. Light metals, 1984, p. 411-426).
According to the known method, only rectified hydrochloric acid is fed to the raw materials processing area, which eliminates the circulation and accumulation of impurities (for example, iron, sodium, potassium, calcium, etc.) in the acid cycle and as much as possible reduces their content in the aluminium chloride hexahydrate. The impurities are removed as oxides by complete pyrohydrolysis of the spent liquor following the crystallisation of A1C13.6H20.
Despite this, the phosphorus content in the end product is 1.5 times higher than the permissible limits for smelter-grade alumina.
The disadvantages of this method should also include very complex equipment and process flow and a lot of expensive equipment to provide the complete regeneration of hydrochloric acid, which entails high capital expenditure for the construction of an alumina refinery using this technology.
The area, in which complete pyrohydrolysis of the spent liquor after the crystallisation of AlC13.6H20 is implemented, is a very energy-intensive one and fuel costs significantly increase the production cost.
In addition, alumina produced by the calcination of aluminium chloride hexahydrate is fundamentally different from conventional smelter-grade alumina by low particle strength, tendency to dusting, 1.5 to 3 times less bulk density, and completely different rheological properties (very poor fluidity), which cause problems during its transportation and in the course of electrolytic production of aluminium. When such alumina is calcined, it is almost impossible to simultaneously achieve a low content of residual chlorine and a-phase, which is

3 one of the main requirements for smelter-grade alumina. In the event that the feed stock contains any phosphorus compounds, almost its entire volume will enter the finished product, as indicated by the authors of the known method.
There is a known method for aluminium and iron extraction from aluminium ore (patent CA2684696 published on November 27, 2008): the method includes the preparation of aluminium-containing raw materials (kaolinitic argillite), the leaching thereof with 6-molar hydrochloric acid at 110 C, the separation of the resulting suspension into solid and liquid phases, the distillation of the liquid phase and the washdown water of the solid phase to the extent of 90% including hydrogen chloride regeneration by rectification and its return to the leaching stage. The remaining 10% of the liquid phase are neutralised with caustic alkali up to pH > 10 to produce aluminium chloride liquor and separate the iron oxide precipitate. The aluminium chloride liquor is neutralised with hydrochloric acid up to pH = 3 4 and the aluminium is separated by liquid extraction and then converted to aluminium hydroxide and oxide (alumina).
This method also requires a very large amount of thermal energy to provide the boiling-down of the entire flow of the liquor and washdown water after the digestion of raw materials to an extent of 90% and a significant consumption of hydrochloric acid and caustic alkali to selectively recover iron and aluminium from the liquors.
The closest to the claimed method is a combined acid and alkali method to produce alumina by hydrochloric acid treatment of raw materials, including the separation of silica sediment, the crystallisation of aluminium chloride hexahydrate from the clarified chloride liquor, followed by its calcination to produce an intermediate alumina product, which the authors called 'raw' or 'crude' alumina due to the significant content of iron and other impurities (with the exception of silicon). This is followed by the leaching of the intermediate alumina

4 product with alkaline spent liquor, the precipitation of the resulting pregnant liquor, the water washing, and subsequent calcination of the separated aluminium hydroxide, the boiling-down of the spent liquor, which left the precipitation area and the water, which was used to wash down aluminium hydroxide, and the formation of alkaline spent liquor returned to the area for leaching the intermediate alumina product (Non-Ferrous Metal Production Reference.
Alumina Refining. Moscow: Metallurgiya, 1970, p. 236-237).
Essentially, processing of the intermediate alumina product is its alkaline recrystallisation according to a simplified Bayer process flow, which is used to remove iron, phosphorus, and other impurities and produce smelter-grade alumina both in terms of chemical composition and physical properties.
A serious drawback of the described flow is that iron, sodium, potassium, calcium, magnesium, and other impurities delivered along with the raw materials are accumulated during the acid cycle, and under this method the problem is solved by deep evaporation of the chloride liquor and chlorides crystallisation to the maximum possible extent under the circumstances. Following their calcination with crude alumina, a significant amount of alkali metal chlorides is fed to the Bayer alkaline cycle; such chlorides will inevitably accumulate in the circulating flow and their removal is not provided for.
The disadvantages of the known method for producing alumina also include the overall high energy costs and additional loss of alkali to an amount of 36-37 kg/tonne of alumina. For these reasons, the method has found no industrial application.
The invention is based on the problem of developing a method to produce smelter-grade alumina from low-grade (high-silicon) raw materials, which would allow processing poor high-silicon ores and waste.

The technical result is to improve the quality of alumina and reduce energy consumption when producing smelter-grade alumina from low-grade raw materials, in other words, when processing poor high-silicon ores and wastes.
The problem can be solved, and the above technical result can be achieved

5 by the proposed method for producing smelter-grade alumina, which includes the following stages:
extraction of aluminium-containing raw materials with hydrochloric acid, separation of the resulting chloride slurry into waste silica precipitate and clarified chloride liquor, crystallisation of aluminium chloride hexahydrate from the clarified chloride liquor, aluminium chloride hexahydrate thermal decomposition into aluminium oxide, followed by its calcination to produce crude alumina as an intermediate product, crude alumina leaching with the alkaline spent liquor and precipitation of the resulting aluminate liquor, water washing and subsequent calcination of the aluminium hydroxide separated, and boiling-down of the spent liquor, which left the precipitation area, and the water, which was used to wash down aluminium hydroxide to form alkaline spent liquor, which is returned to the intermediate alumina product leaching stage.
In addition, in order to optimise the process, the chloride ion concentration in the intermediate alumina product should be maintained at a level of 0.2-5.0 wt.
%, the chloride ion concentration in the alkaline spent liquor should be maintained at a level of 40-90 g/L, and the spent liquor, which left the precipitation area (10-40 wt. A of the total flow), should be boiled-down until the chlorine-containing compound crystals to be removed from the process are separated.

-

6 -According to an embodiment, the smelter-grade alumina production method includes the following stages:
milling of aluminium-containing raw materials, followed by their extraction with hydrochloric acid, which is acidic spent liquor, separation of the resulting chloride slurry into waste silica precipitate and clarified chloride liquor, crystallisation of aluminium chloride hexahydrate from the clarified chloride liquor, aluminium chloride hexahydrate thermal decomposition into aluminium to oxide, followed by its calcination to produce crude alumina as an intermediate product, crude alumina leaching with alkaline spent liquor and precipitation of the resulting aluminate liquor and subsequent calcination of the separated aluminium hydroxide, while about 15% of the acidic spent liquor is subjected to pyrohydrolysis, the chloride ion concentration in the crude alumina is maintained at 0.2-5.0%, the chloride ion concentration in the alkaline spent liquor is maintained at a level of 40-90 g/L, the alkaline spent liquor after precipitation in the amount of 10-40 wt.% of the total flow is boiled down until the chlorine-containing compound crystals to be removed from the process are separated.
According to the second embodiment, the smelter-grade alumina production method includes the following stages:
milling of aluminium-containing raw materials, followed by their leaching with hydrochloric acid spent liquor, which is acidic spent liquor, separation of the resulting chloride slurry into waste silica precipitate, which is dumped following its washing down with water, and clarified aluminium chloride liquor, while the water, which was used for washdown purposes, is supplied to the area to adiabatically absorb hydrogen chloride from the fumes resulted from the calcination of aluminium chloride hexahydrate and those

7 resulted from the pyrohydrolysis process and the amount of washdown water is determined by the amount of water for adiabatic absorption, aluminium chloride hexahydrate crystallisation from the clarified aluminium chloride liquor; after the crystals are separated, the resulting spent liquor is supplied to the rectification area, where the hydrogen chloride concentration in the spent liquor is reduced to form hydrogen chloride gas, which is dried and then supplied to the salting-out area; the spent liquor discharged from the rectification area is divided into two unequal parts: a larger one is supplied directly to prepare spent liquor, the other part is supplied to remove impurities by pyrohydrolysis, aluminium chloride hexahydrate thermal decomposition to form aluminium oxide, which is subsequently calcined to produce crude alumina as an intermediate product, while the calcination fumes are absorbed by water for washing down the waste silica precipitate, crude alumina leaching with the alkaline spent liquor according to the Bayer process and precipitation of the resulting aluminate liquor, water washing and subsequent calcination of the aluminium hydroxide separated, boiling-down of the spent liquor, which left the precipitation area, and the water, which was used to wash down aluminium hydroxide to form the alkaline spent liquor to be returned to the stage of leaching an intermediate alumina product, and the spent liquor is largely used for leaching aluminium-containing raw materials and only a part of it is fed to remove impurities by pyrohydrolysis.
Both embodiments of the method ensure the achievement of the general technical result that is increase in the quality of alumina and reduction in energy costs in the production of smelter-grade alumina from low-grade raw materials.

As additional measures, it is preferably to implement the following:

8 The spent liquor discharged from the precipitation area and the water, which was used for washing down aluminium hydroxide, are boiled down in two stages, with alkali metal carbonates being crystallised at the first stage and alkali metal chlorides being crystallised at the second stage.
Alkali metal chlorides, mainly sodium and potassium ones, are purified and, in the form of an aqueous solution, subjected to membrane or diaphragm electrolysis.
The chlorine and hydrogen formed in the course of membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides are used to synthesise hydrochloric acid, which is fed to extract the original aluminium-containing raw materials, and a part of the aqueous solution of alkali metal hydroxides formed during membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides is mixed with the alkaline spent liquor, which is returned to the intermediate alumina product leaching stage.
A part of the alkali metal hydroxide solution resulted from membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides is fed to neutralise the silica precipitate.
Drawing list Fig. 1 - Schematic alumina production process flow.
The invention is illustrated by the schematic alumina production process flow shown in Fig. 1, which clearly demonstrates the rational optimal combination of the acid and alkaline cycles of the technology both in terms of flows and modes, which, as a whole, provides the achievement of the technical result.
The crushed aluminium-containing raw materials, such as kaolin clay or kaolin argillite, are subjected to acidic extraction (leaching) with hydrochloric acid spent liquor, for example, under autoclave conditions. After leaching, the

9 slurry is separated into a precipitate containing about 90% silica (Si-stoff) and aluminium chloride liquor. The Si-stoff washed down with water is dumped.
The water used to wash down the Si-stoff is supplied to the area to adiabatically absorb hydrogen chloride (HCl) from the fumes resulted from the calcination of the aluminium chloride hexahydrate (ACH, AlC13.6H20) and those resulted from the pyrohydrolysis process. And the HC1 concentration in the aluminium chloride liquor is brought to 17-19%. In the course of absorption, due to the large amount of heat released during the HC1 absorption, the aluminium chloride liquor self-evaporates and all the water supplied for washing down the Si-stoff is removed from the cycle. Notably, the amount of Si-stoff washdown water is determined by the amount of water that can be evaporated at the adiabatic absorption stage.
Separated from the Si-stoff, the alumina chloride liquor is fed to the crystallisation (salting-out) area, where the hydrogen chloride gas produced by rectification is bubbled through the liquor and the HC1 concentration in the liquor is brought to 32%, with most (¨ 95%) of aluminium precipitating as aluminium chloride hexahydrate crystals. After the crystals (crystallised particles) are separated, the resulting spent liquor is fed to the rectification area, where the HC1 concentration in the spent liquor is reduced to almost 22-27% HCl) and a gaseous HCl is formed, which is dried to a content of ¨ 5% H20 and supplied to the salting-out area. Drying is carried out by cooling the gas with cooling water to a temperature of ¨ 35 C. The spent liquor discharged from the rectification area (contains 22-27% HCl) is divided into two unequal parts: a larger one is supplied directly to prepare acidic spent liquor; the other part is supplied to remove .. impurities by pyrohydrolysis.
A proportion of the spent liquor fed for pyrohydrolysis is determined by the permissible content of impurities in the spent liquor for leaching. The proportion of spent liquor for pyrohydrolysis will approximately be 15%.

Notably, the content of impurities in the aluminium chloride liquor will increase by about 6 times compared to that in leaching the ore with pure hydrochloric acid.
During pyrohydrolysis, all the free acid contained in the spent liquor and HC1 formed by hydrolysis of chlorides of the metals, including Al, Fe, Ca, Mg, passes 5 into the gas phase. Pyrohydrolysis products will include fumes and pyrohydrolysis cake consisting of iron oxide (Fe2O3) and partially Al, Ca, Mg oxides, and other minor impurities. The fumes resulted from the pyrohydrolysis contain regenerated HCl and are fed to the area to absorb HCl with the Si-stoff washdown water.

10 The produced ACH is supplied to the calcination area to produce crude alumina and fumes containing HC1). The fumes resulted from the calcination process are delivered to the absorption stage, where absorption is carried out using the Si-stoff washdown water. In order to restore the losses, fresh acid is added to the spent liquor, which is fed to the leaching stage; also, it can be added by washing down of the ACH product supplied for rectification.
Fresh water is added for sanitary purification of fumes resulted from the calcination and pyrohydrolysis processes (then it is used to wash down the Si-stoff).
The advantage of this process flow is that a significant part of the spent liquor is used for leaching the ore and only a part of it is fed to remove impurities by pyrohydrolysis. There are no large and complex boiling-down and salt rectification areas, the pyrohydrolysis area is minimised and not associated with the production of crude alumina but instead intended for partial removal of impurities, which significantly reduces energy costs.
It should be noted that the crude alumina as per the prior art method contains the minimum amount of impurities, including chlorides. In order to achieve this in the prior art method, it is necessary to maintain that the aluminium chloride liquor contains the minimum permissible level of impurities, for

11 example, iron, as well as potassium, sodium, calcium, magnesium, etc. that are fed with raw materials; for this, the acidic spent liquor should be purified from these impurities. The prior art method indicates that it is difficult to carry out such purification, for instance, from iron. The usual technique to do this is pyrohydrolysis, notably, complete evaporation of the acidic spent liquor at a temperature of up to 850 C as indicated in the similar method (Elsner D., Jenkins D.H., and Sinha H.N. Alumina via hydrochloric acid leaching of high silica bauxites - process development. Light metals, 1984, p. 423), so energy costs are very high in this case.
According to the claimed method, crude alumina is then delivered for alkaline recrystallisation, which is based on the known Bayer process. The product of the Bayer process is alumina, which is converted into smelter-grade alumina by calcination.
To prepare the spent liquor, fresh caustic alkali is also fed, which consumption is dependent on mechanical losses with waste mud and alumina product, as well as alkali loss during its decaustification with metal chlorides (A1C13, FeCl3, MgCl2, CaCl2) that the crude alumina contains. In this case, NaC1 and KC1 contained in the crude alumina just pass into solution and do not cause any loss of alkali.
A special feature of alkaline processing of crude alumina by the Bayer method (as opposed to the natural bauxite processing method) is the fact that due to an extremely small amount of the mud formed, it requires little water to wash down the mud. Due to this, the water balance in the alumina production process can be offset without boiling-down the entire flow of the spent liquor as the amount of water added to wash down the hydrate approximately corresponds to the amount of water removed along with alumina product. Moreover, it becomes possible to recover heat by autoclave leaching using slurry-slurry heat exchangers

12 without self-evaporation of the blow-off slurry since a small amount of mud does not require such a large quantity of condensate to wash down the mud.
The combined operations of deep boiling-down of a part of the spent liquor and crystallisation are needed to remove chlorine from the Bayer cycle. The solubility of NaCl in caustic solutions is significantly reduced only in the area with high Na2O concentrations; therefore, we are talking about deep boiling-down of a part of the spent liquor to a caustic alkali content of 25-33%
(Na2O).
The amount of spent liquor supplied to the boiling-down area is determined by the permissible level of chloride accumulation in the Bayer process. The higher the acceptable level of chlorides in the liquors is, the less proportion of the spent liquor to be fed to the boiling-down area and, accordingly, the amount of evaporated water (and heat energy consumption) will be (with the same content of chlorides in the crude alumina).
According to the experience of the authors, the permissible level of chlorides in the spent liquor used in the Bayer process is 90 g/L (for chloride ion C1-) in industrial conditions.
The crystallised sodium chloride and partially potassium chloride separated after the boiling-down process are fed for the well-known diaphragm or membrane electrolysis to release caustic alkali and hydrogen and chlorine gases, from which hydrogen chloride gas is synthesised. The caustic alkali and hydrogen chloride are returned to the acid and alkaline parts of the process, respectively, to compensate for the inevitable loss of these agents.
Thus, the claimed method is a closed-loop process flow that makes it possible to process low-grade (high-silicon) aluminium-containing raw materials to produce smelter-grade alumina.
Since crude alumina is an intermediate product but not a marketable one, it is not necessary for the content of impurities of iron, potassium, sodium, calcium, magnesium, etc. in it that are fed along with raw materials to be the

13 minimum allowable one. Therefore, the concentration of these impurities in the acid cycle can be increased, which will reduce the chloride liquor evaporation costs. To this end, at the crystallisation stage, it is advisable to most fully and quickly extract aluminium chloride hexahydrate into the solid phase using simple equipment and easily implemented process methods without worrying about the purity of the crystallised ACH, which is fed to the calcination area to produce crude alumina. Moreover, there is no need for calcining the product deeply during the calcination in order to completely decompose the chlorides. On the one hand, this reduces calcination-related heat costs; on the other hand, this does not create conditions for the formation of a hardly soluble a-phase in the crude alumina.
Residual chlorine represented mainly by potassium, sodium, calcium, and magnesium chlorides is extracted together with crude alumina to the acid phase of the process flow, where it will inevitably accumulate. However, the research carried out by the authors has shown that the accumulation of chloride ion in the alkaline spent liquor to a level of 40-90 g/L causes no significant decrease in the performance of the Bayer process. In order to avoid further accumulation of chlorine in the alkaline cycle of the process, a part of the spent liquor, after precipitation and in the amount of 10-40% of the total flow rate, is boiled-down until chlorine-containing compound crystals to be removed from the process are separated. Laboratory experiments and cyclic process calculations have shown that this method is sufficient to maintain the chloride ion concentration in the alkaline spent liquor at the required level and ensure the water balance of the Bayer process.
The selection method does not make it possible to determine an optimal combination of the operating parameters of such a multi-link cyclic technology, which the claimed method is. The authors solved this problem by using specially developed mathematical models of the mass heat balances of the process. At the same time, the authors unexpectedly found that the consumption of energy in the

14 form of fuel, heat, and electricity can be reduced, if we purposefully allow the accumulation of impurities in the acid and alkaline cycles of the process and crude alumina, which is an intermediate product passing from the acid cycle to the alkaline one.
The numerical experiments performed according to the results of optimisation iterative calculations based on the aforementioned mathematical models found the following: if about 15% is fed to the pyrohydrolysis area, the content of impurities (iron, sodium, potassium, magnesium calcium, etc.) is set at an equilibrium level that does not reduce the recovery of aluminium from raw materials to crude alumina, but leads to an increase in the concentration of the mentioned impurities in the aluminium chloride hexahydrate and further in the crude alumina. However, when leaching the crude alumina in the alkaline cycle of the process, the iron, calcium, and magnesium compounds immediately go into an insoluble precipitate and are removed. In this case, it should be considered that the smaller the proportion of acidic spent liquor fed to the pyrohydrolysis area is, the lower the energy costs associated with fuel combustion in that area are.
The pyrohydrolysis-related costs can be reduced, if the crude alumina is subjected to deep high-temperature calcination and washing down with water to remove soluble chlorides prior to leaching in the Bayer cycle, as provided for in the prior art method. In this case, the chloride ion content in crude alumina decreases to hundredths and tenths of a per cent, but the content of hardly soluble alpha alumina increases. Its alkaline processing is implemented through high-temperature autoclave leaching, which, as a consequence, leads to increased thermal energy consumption.
On the other hand, it is obvious that if the process temperature or the intensity of heat and mass exchange in the calcination area are reduced, the energy consumption will significantly decrease there, but the chlorine content in the crude alumina will increase and such chlorine in the form of chloride ion will continue accumulating in the alkaline cycle of the process. Chlorine transfer from the acid cycle to the alkaline one will inevitably lead to losses of both hydrochloric acid and caustic alkali. In the claimed method, these losses are compensated by removing some potassium and sodium chlorides from the 5 alkaline cycle and their electrolytic processing to produce NaOH and chlorine and hydrogen, gases from which HC1 is synthesised. But such regeneration requires thermal energy when evaporating potassium and sodium chlorides from alkaline liquors and also electricity for electrolysing an aqueous solution of these chlorides.
10 However, the numerical experiments have shown that despite the complexity of optimising the heat-and-mass balances of the process, the mathematical models developed by the authors allow finding some non-obvious alternative mutually linked combinations of process parameters in the acid and alkaline cycles, in order to minimise energy consumption while maintaining the

15 required quality of the smelter-grade alumina product. This can be achieved, when the chloride ion concentration in the crude alumina is maintained at a level of 0.2-5.0 wt.%, the chloride ion concentration in the alkaline spent liquor is maintained at a level of 40-90 g/L, and the spent liquor, which left the precipitation area (10-40 wt.% of the total flow), is boiled-down, until the chlorine-containing compound crystals to be removed from the process are separated. The claimed method was implemented experimentally with the presence of the above-mentioned optimal combination of process parameters.
Example 540 g of aluminium-containing raw materials (kaolin argillite) containing, wt.%: A1203 27.1; SiO2 56.8; Fe2O3 2.0; Na2O 0.31; K20 <0.15; TiO2 0.48; CaO
0.45; MgO 0.27; P205; 0.05; 11.8, crushed to a particle size of <100 1.1m, were mixed with 1,650 ml of 20% hydrochloric acid, placed in an autoclave and kept under stirring for 3 h at 160 C. The resulting chloride slurry was separated by

16 filtration, the solid precipitate (waste Si-stoff) was washed down with water.
The clarified aluminium chloride liquor was bubbled with dry hydrogen chloride gas at 70 C, until the separation of ACH crystals stopped. The crystallised ACH
was separated from the spent liquor using a filter and calcined at 600 C to produce crude alumina. The spent liquor was diluted with the Si-stoff washdown water to free 20% HCl to produce the acidic spent liquor, which was fed to repeatedly extract kaolin argillite using the acid, and all the above (cyclic) operations were performed repeatedly.
After a total of 6 above cycles were performed, the content of the acidic spent liquor components was stabilised as follows, %: AlC13 20.5-21.5; FeC13 3.9-4.2; TiC12 0.001; CaC12 0.4-0.48; NaCl 0.1-0.12; KCl 0.1-0.11. No reduction in the rate of aluminium recovery from the aluminium-containing raw materials was observed and it was 95.5-97.5 %. After each experiment, 15% volume of the spent liquor was replaced with pure hydrochloric acid (20% concentration) to simulate the removal of impurities from the cycle by pyrohydrolysis or by treatment with concentrated sulphuric acid to form low-soluble sulphates of the corresponding metals.
The average composition of the resulting waste Si-stoff, wt.%, was as follows: A1203 2.0; SiO2 90.5; Fe2O3 0.16; Na2O 0.2; K20 <0.15; TiO2 0.7; CaO
0.12; MgO <0.025; P205; <0.02; 4.2.
Following the stabilisation of the acidic spent liquor composition, another 10 cyclic experiments were carried out; as a result of them, the authors produced crude alumina with the following composition, wt.%: A120386.0; SiO2 0.08;
Fe2032.9; Na2O 0.61; K20 <0.15; <TiO2 0.05; CaO 0.3; MgO <0.025; P205; 0.06;
Cl- 3.5 ; 7Ø
In order to produce smelter-grade alumina from crude alumina by alkaline processing in the Bayer cycle, 500 g of crude alumina were dissolved for 2 hours

17 in an alkali green liquor with the following composition, g/L: A1203102.0;
Na20 174.0; NaCl 63.3 in an autoclave at 150 C.
The content of the resulting filtered green liquor was as follows, g/ L: A1203 167.3; Na20 149.2; NaCl 57.7. The precipitation of the liquor carried out in accordance with the Bayer technology resulted in the separation of alumina, from which the authors, after its washing down with hot water, %: and calcination at 1,100 C, produced alumina with the following chemical composition, % A1203 98.7; Si02 0.004; Fe203 0.008; Na20 0.15; K20 0.01; TiO2 0.001; Ca0 0.004;
Mg0 0.0025; P205; 0.0007; V205 0.0002; Cr203 0.0003 0.02; C1-0.013.
In determining the physical and mechanical properties of this alumina using standard methods, the authors found the following:
= - A1203 content 3 %
particle size distribution:
- size - 20 m, 2.4 %
- size - 45 um, 8.8 %
- size + 125 um, 3.9 %
specific surface area (BET) 74.2 m2/g loss on ignition at 1,100 C (LOI) 0.8%
humidity 0.8%, hardness index 9%
bulk density 0.97 g/cm3 gradient 28.6 time, for which a 100 g weight can outflow from the funnel with a 2.2 min 2.4 mm hole and a bell angle and vertical line of 9 54' (Alcoa test) Hydrogen fluoride (HF) adsorption capacity 23 mg/g, _

18 The produced alumina fully complies with Russian (GOST 30558-98 'Smelter-Grade Alumina') and international requirements for 'sandy' smelter-grade alumina despite the high content of chlorides in the alkaline cycle.
Due to the lack of published data on energy consumption in similar methods, except for the similar method (Elsner D., Jenkins D.H., and Sinha H.N.
Alumina via hydrochloric acid leaching of high silica bauxites-process development. Light metals, 1984, p. 411-426), the authors performed calculations on the consumption of heat and electricity to produce 1 kg of alumina and compare the results with a view to comparing the energy saving in all the technologies mentioned in this description of the invention. The results are shown below.
Energy Quality of produced consumpt Technology alumina ion kJ/kg Elsner D., Jenkins D.H., and Sinha H.N.
Alumina via hydrochloric acid leaching of Not smelter-grade 37.1 high silica bauxites-process development.
Light metals, 1984, p. 411-426 Family member - patent CA2684696 Not smelter-grade 56.3 Prior art - Non-Ferrous Metal Production Reference. Alumina Refining. Moscow: Smelter-grade 46.2 Metallurgiya, 1970, p. 236-237).
Industrial method for sintering with Smelter-grade 54.3 limestone and soda Proposed method Smelter-grade 38.4 It is obvious that in terms of energy saving in processing high-silica raw materials the proposed method is second only to family patent 1, which, however,

19 cannot provide the production of smelter-grade alumina. Other family patents require much higher energy consumption.
Energy saving specified for the claimed method can be optimally achieved when about 15% of the acidic spent liquor is subjected to pyrohydrolysis, the chloride ion concentration in the intermediate alumina product is maintained at a level of 0.2-5.0 wt.%, the chloride ion concentration in the alkaline spent liquor is maintained at a level of 40-90 g/L, and the spent liquor, which left the precipitation area (10-40 wt.% of the total flow), is boiled-down, until the chlorine-containing compound crystals to be removed from the process are separated. The specified concentration and flow intervals were calculated on the basis of a mathematical model of the aggregate mass balance of the acid and alkaline parts of the process. The total calculated energy consumption did not exceed 41.2 kJ/kg at any combination of the operating parameters within the declared intervals.
Although the description has some references to certain embodiments, numerous modifications should be obvious to specialists in this art and are not limited strictly to the example, description, and process flow.

Claims (16)

1. The smelter-grade alumina production method comprising the following stages:
milling of aluminium-containing raw materials, followed by their extraction with hydrochloric acid, which is acidic spent liquor, separation of the resulting chloride slurry into waste silica precipitate and clarified chloride liquor, crystallisation of aluminium chloride hexahydrate from the clarified chloride liquor, aluminium chloride hexahydrate thermal decomposition into aluminium oxide, followed by its calcination to produce crude alumina as an intermediate product, crude alumina leaching with alkaline spent liquor and precipitation of the resulting aluminate liquor and subsequent calcination of the separated aluminium hydroxide, while about 15% of the acidic spent liquor is subjected to pyrohydrolysis, the chloride ion concentration in the crude alumina is maintained at 0.2-5.0%, the chloride ion concentration in the alkaline spent liquor is maintained at a level of 40-90 g/L, the alkaline spent liquor after precipitation in the amount of 10-40 wt.% of the total flow is boiled down until the chlorine-containing compound crystals to be removed from the process are separated.

2. The method according to claim 1, wherein alkaline spent liquor is boiled down in two stages, with alkali metal carbonates being crystallised at the first stage and alkali metal chlorides being crystallised at the second stage.

3. The method according to claim 2, wherein alkali metal chlorides, mainly sodium and potassium ones, are purified and, in the form of an aqueous solution, subjected to membrane or diaphragm electrolysis.

4. The method according to claim 3, wherein the chlorine and hydrogen formed in the course of membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides are used to synthesise hydrochloric acid, which is fed to extract the original aluminium-containing raw materials, and a part of the aqueous solution of alkali metal hydroxides formed during membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides is mixed with the alkaline spent liquor, which is returned to the intermediate alumina product leaching stage.

5. The method according to claim 4, wherein a part of the alkali metal hydroxide solution resulted from membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides is fed to neutralise the silica precipitate.

6. The smelter-grade alumina production method comprising the following stages:
milling of aluminium-containing raw materials, followed by their leaching with hydrochloric acid spent liquor, which is acidic spent liquor, separation of the resulting chloride slurry into waste silica precipitate, which is dumped following its washing down with water, and clarified aluminium chloride liquor, while the water, which was used for washdown purposes, is supplied to the area to adiabatically absorb hydrogen chloride from the fumes resulted from the calcination of aluminium chloride hexahydrate and those resulted from the pyrohydrolysis process and the amount of washdown water is determined by the amount of water for adiabatic absorption, aluminium chloride hexahydrate crystallisation from the clarified aluminium chloride liquor; after the crystals are separated, the resulting spent liquor is supplied to the rectification area, where the hydrogen chloride concentration in the spent liquor is reduced to form hydrogen chloride gas, which is dried and then supplied to the salting-out area; the spent liquor discharged from the rectification area is divided into two unequal parts: a larger one is supplied directly to prepare spent liquor, the other part is supplied to remove impurities by pyrohydrolysis, aluminium chloride hexahydrate thermal decomposition to form aluminium oxide, which is subsequently calcined to produce crude alumina as an intermediate product, while the calcination fumes are absorbed by water for washing down the waste silica precipitate, crude alumina leaching with the alkaline spent liquor according to the Bayer process and precipitation of the resulting aluminate liquor, water washing and subsequent calcination of the aluminium hydroxide separated, and boiling-down of the spent liquor, which left the precipitation area, and the water, which was used to wash down aluminium hydroxide to produce the alkaline spent liquor to be returned to the intermediate alumina product leaching stage, and the spent liquor is largely used for leaching aluminium-containing raw materials and only a part of it is fed to remove impurities by pyrohydrolysis.

7. The method according to claim 6, wherein the chloride ion concentration in the intermediate alumina product is maintained at a level of 0.2-5.0 wt.%, the chloride ion concentration in the alkaline spent liquor is maintained at a level of 40-90 g/L and the spent liquor, which left the precipitation area (10-40 wt.%
of the total flow), is boiled-down until the chlorine-containing compound crystals to be removed from the process are separated.

8. The method according to claim 6, wherein the spent liquor discharged from the precipitation area and the water, which was used for washdown purposes, are boiled down in two stages, with alkali metal carbonates being crystallised at the first stage and alkali metal chlorides being crystallised at the second stage.

9. The method according to claim 8, wherein alkali metal chlorides, mainly sodium and potassium ones, are purified and, in the form of an aqueous solution, subjected to membrane or diaphragm electrolysis.

10. The method according to claim 9, wherein the chlorine and hydrogen formed in the course of membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides are used to synthesise hydrochloric acid, which is fed to leach the original aluminium-containing raw materials, and a part of the aqueous solution of alkali metal hydroxides formed during membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides is mixed with the alkaline spent liquor, which is returned to the intermediate alumina product leaching stage.

11. The method according to claim 10, wherein a part of the alkali metal hydroxide solution resulted from membrane or diaphragm electrolysis of the aqueous solution of alkali metal chlorides is fed to neutralise the silica precipitate.

12. The method according to claim 6, wherein the hydrogen chloride concentration in the aluminium chloride liquor is brought to about of 17-19%
and in this case, in the course of absorption, due to the large amount of heat released during the HC1 absorption, the aluminium chloride liquor self-evaporates and all the water supplied for washing down the waste silica precipitate is removed from the cycle.

13. The method according to claim 6, wherein the alumina chloride liquor is fed to the crystallisation (salting-out) area, where the hydrogen chloride gas produced by rectification is bubbled through the liquor, and the concentration in the liquor is brought to about 32%, with most (¨ 95%) of aluminium precipitating as aluminium chloride hexahydrate crystals.

14. The method according to claim 6, wherein following the separation of crystals (crystallised particles), the resulting spent liquor is fed to the rectification area, where the hydrogen chloride concentration in the spent liquor is reduced to form hydrogen chloride gas, which is dried to a water content of about 5% and next fed to the salting-out area, while the drying is carried out by cooling the gas with cooling water to a temperature of about 35 °C.

15. The method according to claim 6, wherein a proportion (approximately 15%) of the spent liquor to be fed for pyrohydrolysis is determined by the permissible content of impurities in the spent liquor to be fed for leaching and, during pyrohydrolysis, all the free acid contained in the spent liquor and hydrogen chloride formed by hydrolysis of chlorides of the metals, including Al, Fe, Ca, Mg, passes into the gas phase; the fumes resulted from the pyrohydrolysis contain regenerated hydrogen chloride and are fed to the area to absorb hydrogen chloride with water to washdown the waste silica precipitate.

16. The method according to claim 6, wherein at the stage of calcination producing crude alumina and fumes containing hydrogen chloride, the fumes resulted from the calcination process are delivered to the absorption area, where absorption is carried out with water for washing down the waste silica precipitate, while the spent liquor supplied to the leaching area is added with fresh acid to compensate for losses, and fresh water is added for sanitary purification of the fumes resulted from the calcination and pyrohydrolysis processes.