м.т.н. Аубакиров А.М.

Инновационный Евразийский университет, Казахстан

IMPROVEMENT OF FILTERING TECHNOLOGIES AND THE CRYSTALLIZATION OF SODIUM HYDROALUMINATE IN ALUMINIUM INDUSTRY

In recent times, due to the usage of bauxites from new deposits in the aluminium industry the impurity composition of aluminate solutions has changed. This reflects negatively on the several processing stages of production, particularly on the division producing rough gallium. Another reason prejudicial to the production of gallium is reduction of its content in the bauxites, as well as the loss of valuable rare metal in the derivation with ferruginous sands. This implies a decrease in the concentration of gallium in the aluminate solutions.

Emerging issues require further study and the development and implementation of new technological schemes.

Figure 1- Sodium hydroaluminate production unit:

КР1-K8 - Crystallizer of sodium hydroaluminate; ГЦ1,2 - hydrocyclone; ПГ –thickening repulpator; СГ - thickener; МП – mixer of pulp of filter power, ф1-3 – vacuum filter, Р1,2-receiver, HB1,2 - vacuum pump, H1-H12 –Centrifugal pump, Мф1,2 – filtrate mixer, МА1,2 – aluminate solution mixer.

Direct extraction from solutions by cooling became impractical; therefore a new technological scheme of processing of working solutions with obtaining artificial solutions rich in gallium has been developed. Weak working solutions and drainage of sodium sedimentation tank are cleared at the beginning of chlorides, sulfates and carbonates by evaporation of these solutions with concentration of 220 g/l to concentration of 320 g/l with all sodium salts precipitate. In order to achieve a concentration of 150-300 g/l in the thickened pulp, the evaporation and thickening should be carried out so as much as possible chlorides precipitate. Draining of the thickener is directed to the allocation of CGC by re-evaporation of the solution to the concentration of 420 g/l: in this case the solutions become sodium aluminate saturated, which precipitate as sodium hydroaluminate to Na2O · Al2O3 · 2,5H2O and subsequently release:

- due to cooling of the solution after the second stage of evaporation from 140 to 85-90 C;

- soaking for 10-14 hours;

- afterfiltration on filters BOU.

At the same time sodium hydroaluminate precipitates with about 25% moisture, which dissolves in water and has a caustic ratio of about 1,6 units and the concentration of about 160 g/l, which is pumped to the sintering workshop as aluminate solution at desiliconization stage, followed by extraction of alumina by means of decomposition, and cleaned filtrate of sodium hydroaluminate is directed to the extraction stage of CGC, which is carried out by salting out with sodium chloride, thickened pulp from the first evaporation stage is used as a chloride primer which is supplied with ratio of chlorine of thickened pulp to Al2O3 of filtrate of about 0,4 – 0,7.

Figure 2 – crystallizer:

1 – vat; 2 - mixer; 3 - hydraulic lock; 4 - step-bearing; 5,6,7 –coil; 8 –clamp; 9 – bracket; 10 – reducer; 11 - electric motor

The evaporated solution from the mixer with concentration of Na2Oky - 440 g/l, sodium percentage - 2.8 ÷ 3.0 and αky - 2,9 ÷ 3,0 in the amount of 15.4 m3/h is directed to the crystallizers of sodium hydroaluminate with PB pumps. Crystallizers operate in batch mode, each is receiving and consumable by turns. Solution in the crystallizers is stirred with mixer driven by electric motor through reducer SPC-45. The temperature of the solution supplied to the crystallization is of 105-115 0C. Crystallization is carried out until the caustic ratio of liquid phase of the pulp reaches 8.0 - 9.0 for 10-14 hours, with the pulp cooled to 80-85 0C which is achieved by a pair of registers which are fed with industrial water. In order to achieve the intended caustic ratio of liquid phase of the pulp of sodium hydroaluminate, the concentration of feed solution should not be less than 435 - 445 g/l Na2O_ky.

Received pulp of sodium hydroaluminate in the amount of 14,5 m3/h is pumped into the mixer and supplied to the filters BOU with 20 pumps. The pulp from the crystallizer is pumped is not in full, 1 m of the pulp is left for priming next crystallization of sodium hydroaluminate. This method of supplying the primer to the process is less appropriate because redistribution of sodium hydroaluminate according to granulometric composition may occur during unloading the crystallizer, in this case the largest crystals which are the products can be found at the bottom of the crystallizer and are used as the primer while the smallest ones are carried in the first place and sent to the next processing stage. The condition of the primer surface is very important in the crystallization process. The best primer is the primer with active surface. The smallest particles of sodium hydroaluminate have more active surface. Besides, the process of crystallization is heterophase since it consists of at least two phases. During heterophase processes where the main stage runs at the interface, the process speed depends on the total surface area that is on the specific surface area of supplied primer. When using smaller primer the total surface area where crystallization takes place increases significantly which will have an immediate effect on the overall speed of the process.

Complete unloading of the crystallizer with further classification of the obtained precipitate of sodium hydroaluminate, after which the finer fraction is returned back as the primer in the crystallizer and the coarser one is sent further to the next processing stage can improve the characteristics of crystallization and filtration of the obtained precipitate.

List of references:

1) Shalavina Ye.D., Romanov G.A., Yevseyev Yu.N. Obtaining gallium from aluminate solutions - Alma-Ata: Nauka, 1990. – p. 204

2) Yeremin N.I. Gallium - M., 1964.

3) Kassatkin A.G. Basic processes and devices of chemical technology. Publishing house "Chemistry", Moscow, 1971.

4) A.I. Ivanov, G.N. Kozhevnikov. Complex processing of bauxites. Yekaterinburg, 2003.

5) Utkin N.I. Production of non-ferrous metals. - 2nd ed. - M.: Intermet Engineering, 2004. - p. 442.

6) Ibragimov A.T., Budon S.V. Development of the technology of alumina production from bauxites in Kazakhstan. Pavlodar. 2010.