RU2521577C2 - Production of aluminium-bearing cake - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention can be used in nonferrous metallurgy for production of alumina. Aluminium-bearing cake is produced by sintering of charge from nepheline ore, limestone and return products at 1250-1300°C. Said cake is cooled to 1000°C at the rate of not over 14°C/min sufficient for segregation of impurities in the grain of dicalcium silicate that facilitates the formation of maximum amount of its β-modification.

EFFECT: higher yield.

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Description

The invention relates to the field of non-ferrous metallurgy and relates to a technology for the production of alumina.

A known method of producing alumina, including the preparation of a mixture of nepheline ore with intensifying additives of fluorides CaF ₂ , NaF, 3NaF · AlF ₃ , Na ₂ SiF ₆ in an amount of from 0.1 to 0.3% and coal 1.5-2% coal from mass of dry charge, its sintering at 1220-1280 ° С and processing of cake (Alumina production / Liner A.I., Eremin N.I., Liner Yu.A., Pevzner I.Z. - M .: Metallurgy, 1978. - S.231-232). The disadvantages of this method are the necessity of conducting the sintering process in the presence of mineralizers, fluorides of alkali or alkaline earth metals, low extraction of alumina from the cake.

A known method of processing alkaline aluminosilicate raw materials, including the preparation of a mixture of nepheline ore, limestone and circulating products, its sintering at a temperature of 1250-1300 ° C and the processing of cake (Production of alumina / Liner A.I., Eremin N.I., Liner Yu. A., Pevzner I.Z. - M.: Metallurgy, 1978. - p. 185-189). The disadvantage of this method is the low extraction of alumina from the cake, caused by non-compliance with the formation of the optimal phase structure of cake.

The closest in essence to the claimed invention is a method for producing aluminum-containing cake (patent RU No. 2364572) by mixing and sintering at a temperature of 1250-1300 ° C of a mixture prepared from nepheline ore, limestone and circulating products, maintaining the obtained cake at a temperature of 1160 ° C the course of time necessary for the formation of the maximum amount of α ' _L- modification, and at a temperature of 680-620 ° C for the time required for the formation of the maximum amount of β-modification of dicalcium silicate. The disadvantage of this method is the need to create a complex system of 2-stage cooling. For the implementation of the method on an industrial scale, large capital costs will be required to create conditions that ensure the presence of cake at high temperatures for a long time. From an economic point of view, these costs will be inefficient. Another disadvantage is the prolonged exposure of the cake at a temperature of 680-620 ° C to form the maximum amount of β-modification.

The purpose of the invention is to increase the degree of extraction of alumina from the cake due to a more complete transition of the polymorphic modification of dicalcium silicate α'- into the β-modification, simplifying the method of obtaining cake.

This goal is achieved by the fact that in the method of producing aluminum-containing cake, including the preparation of a mixture of nepheline ore, limestone and circulating products, its sintering at a temperature of 1250-1300 ° C, the cake is cooled to a temperature of 1000 ° C at a speed sufficient to pass the process of segregation of impurities in the grain of dicalcium silicate, contributing to the formation of the maximum amount of β-modification of dicalcium silicate.

It is known that aluminum-containing sinter obtained by sintering from nepheline ore, limestone and recycled products consists of 75% of dicalcium silicate. Currently, there are five polymorphic modifications of dicalcium silicate: α, α '(α' _H , α ' _L ), β, γ. The dicalcium silicate present in the cake is represented by α'- and β-polymorphic modifications.

The technical and economic efficiency of methods for producing alumina, based on the binding of silica to dicalcium silicate, is largely determined by the degree of interaction of the latter with aluminate-alkaline solutions and the products of this interaction. As a result of this interaction, the silica content in the aluminate solution increases, and alumina and alkali are lost in the form of Na ₂ O · Al ₂ O ₃ · 1.7 · 1.9SiO ₂ · H ₂ O and hydrogranates formed during the leaching of sodium hydroxide containing a variable amount of SiO ₂ and Н ₂ О) 3СаО · Al ₂ O ₃ · SiO ₂ (6-2n) · Н ₂ О, which have little solubility and pass into dump slurry, which leads to a decrease in the extraction of alumina and alkali from raw materials. Due to the lower solubility in the alkaline-aluminate solutions of the β-modification of dicalcium silicate compared with the α'-modification for alumina production technology, the preparation of cake with the maximum content of β-modification of dicalcium silicate is of the greatest importance.

Preservation at room temperature of the α'-form of Ca ₂ SiO _{4 is} achieved due to crystal chemical stabilization. Crystallochemical stabilization is based on the formation of Ca ₂ SiO ₄ solid solutions with some additives, one of which is alkaline aluminates. This fact is described in the book by V.Ya. Abramov, G.D. Stelmakova, I.V. Nikolayev, Physicochemical fundamentals of complex processing of aluminum raw materials (alkaline methods), Moscow, Metallurgy, 1985, p. 139, as well as in the book Liner A.I., Alumina Production, Moscow, Metallurgizdat, 1961, p. 397. Foreign atoms block the rearrangement of the lattice and thereby prevent the transformation of the high-temperature modification into the low-temperature one even at the nucleation stage.

In our case, during sintering of a multicomponent mixture, the process of nucleation and, consequently, the formation of polymorphic C ₂ S modifications is most affected by the presence of alkali metal impurities (Na, K) in the material. Moreover, to stabilize α'-C ₂ S requires more impurities than to stabilize β-C ₂ S.

A well-known widespread method of purification of metals from dissolved impurities, based on the effect of segregation of impurities during crystallization. According to the accepted theory (D.Mack Liin, Grain boundaries in metals, Moscow, 1960), segregation occurs if the action of external factors (mechanical, temperature, etc.) leads to variations in the chemical potential in the bulk of the material. As a result of this, there is a diffusion redistribution of elements, their accumulation in grain boundaries, leading to equalization of the chemical potential. Thus, by varying the temperature regime of processing the material, it is possible to displace part of the impurities to the grain boundary, thereby cleaning the central region of the grain. Such a redistribution of impurities in the grain volume significantly facilitates the nucleation process. In the purified central region of the grain during a polymorphic transition, a restructuring of the structure will occur with the formation of β-C ₂ S.

From the theory of segregation, it is known that the diffusion rate of impurity atoms, and, accordingly, the rate of temperature change (cooling mode) will have a decisive influence on the distribution of impurity atoms over the grain volume, and hence on the formation of phases in the cake. The greatest effect on the distribution of impurities will be observed at high temperatures under the condition of a mandatory decrease in the temperature of the material (Questions of atomic science and technology No. 3, 2003, Kinetics of grain-boundary segregation of impurities in polycrystals, VV Slezov, LN Davydov, O. Osmaev, R.V. Shapovalov, pp. 25-34). At a constant temperature, the segregation process will continue to an equilibrium state and further aging will not lead to the development of the process. Therefore, the purification of α′-C ₂ S grain from stabilizing impurities will not be completed, and the degree of conversion α ′ → β will not be complete.

Laboratory studies carried out on the sinter showed that when the sample is slowly cooled from an average sintering temperature of 1270 ° C to 1000 ° C with a speed of not more than 14 ° C / min for ~ 19 minutes, an almost complete transition α ′ → β-C is achieved ₂ S.

Sinter cooling from sintering temperature was carried out in the furnace in three modes, at a rate of 8 ° C / min, 14 ° C / min and 24 ° C / min. In the process of cooling, the cake was extracted from the furnace in the temperature range of 1160-600 ° C in increments of 40 ° C, followed by rapid cooling. Tables 1, 2, 3 show data on the content of phases in the cake, the amount of extraction of Al ₂ O ₃ from cake at various cooling conditions.

Table 1 The change in the phase composition of the cake when it is cooled at a rate of 8 ° C / min The temperature of the extraction of cake from the furnace, ° C The content in the spec,% The recovery of Al ₂ O ₃ from cake,% Cool time spec to temp., min β-C ₂ S α′-C ₂ S 1160 24 44 71.6 13.8 1120 45 23 80.1 18.8 1080 52 16 82.9 23.8 1040 60 8 86.2 28.8 1000 67 one 89.1 33.8 960 67 four 89.0 38.8 920 67 one 89.2 43.8 880 67 one 89.1 48.8 840 67 one 89.6 53.8 800 67 one 89.0 58.8 760 67 one 88.7 63.8 720 67 one 89.0 68.8 680 67 one 88.9 73.8 640 67 one 89.5 78.8 600 67 one 89.0 83.8 average (in the temperature range 1000-600 ° C) 89.1

table 2 The change in the phase composition of the cake when it is cooled at a rate of 14 ° C / min The temperature of the extraction of cake from the furnace, ° C The content in the spec,% The recovery of Al ₂ O ₃ from cake,% Cool time spec to temp., min β-C ₂ S α′-C ₂ S 1160 27 41 72.8 7.9 1120 47 21 80.9 10.7 1080 57 eleven 84.9 13.6 1040 63 5 87.4 16,4 1000 67 one 89.0 19.3 960 67 one 89.2 22.1 920 67 one 89.5 25.0 880 67 one 89.7 27.9 840 67 one 88.9 30.7 800 67 one 89.2 33.6 760 67 one 88.5 36,4 720 67 one 89.9 39.3 680 67 one 89.0 42.1 640 67 one 89.7 45.0 600 67 one 89.6 47.9 average (in the temperature range 1000-600 ° C) 89.3

Table 3 The change in the phase composition of the cake when it is cooled at a rate of 24 ° C / min The temperature of the extraction of cake from the furnace, ° C The content in the spec,% The recovery of Al ₂ O ₃ from cake,% Cool time spec to temp., min β-C ₂ S α′-C ₂ S 1160 eleven 57 66,4 4.6 1120 27 41 72.8 6.3 1080 47 21 80.9 7.9 1040 57 eleven 84.9 9.6 1000 62 6 86.5 11.3 960 62 6 86.7 12.9 920 62 6 86.5 14.6 880 62 6 87.0 16.3 840 62 6 86.4 17.9 800 62 6 87.0 19.6 760 62 6 86.8 21.3 720 62 6 87.0 22.9 680 62 6 86.8 24.6 640 62 6 87.0 26.3 600 62 6 86.8 27.9 average (in the temperature range 1000-600 ° C) 86.8

From the data obtained it follows that it is sufficient to cool the spec only to 1000 ° C at a rate of no more than 14 ° C / min so that the impurities blocking the α ′ → β-C ₂ S rearrangement pass to the grain boundary. Due to the absence of obstacles for phase adjustment in the purified grain region, the maximum amount of β-C ₂ S is formed. Therefore, the maximum level of impurity concentration in the grain, which ensures the maximum formation of β-C ₂ S, is achieved when the cake is cooled to 1000 ° C at a speed of no more 14 ° C / min.

Thus, the organization of special cooling conditions to increase the content of β-C ₂ S in the cake below a temperature of 1000 ° C is not required. From a technical point of view, this method can be implemented in sintering furnaces, for example, by extending the fuel burner in the furnace, thereby increasing the length of the cooling zone and contributing to the slow cooling of the cake to 1000 ° C at a speed of not more than 14 ° C / min.

When implementing the necessary conditions for cooling the cake according to the prototype (patent RU No. 2364572), α′-C ₂ S remains in the cake in the amount of 7%, table 4. With an increase in the cake holding time in the proposed temperature ranges from 60 minutes to 120 minutes, there is no noticeable decrease in the amount α′-C ₂ S in spec. The results obtained indicate the inhibition of the process of grain cleaning α′-C ₂ S from stabilizing impurities. Withstanding at a temperature of 1160 ° C, the segregation process continues until the equilibrium state. A further increase in exposure time will not lead to the development of the process. Cooling in the range of 680-620 ° C does not contribute to the development of the segregation process due to the low diffusion rate of impurity atoms, thereby their concentration level will not reach the minimum value for the formation of the maximum amount of β-C ₂ S.

Table 4 The change in the phase composition of the cake when it is cooled according to the conditions of patent RU No. 2364572 Modes Temperature ° C Holding time, min The content in the spec,% The recovery of Al ₂ O ₃ cake,% β-C ₂ S α′-C ₂ S one 1160 5 680-620 5 52 16 82.9 2 1160 thirty 680-620 thirty 58 10 85,4 3 1160 60 680-620 60 61 7 86.6 four 1160 120 680-620 120 61 7 86.6

As can be seen from the data in Table 2 and Table 4, the implementation of the proposed conditions for the cooling of the cake will increase the extraction of Al ₂ O ₃ from the cake from 86.6% to 89.3% and simplify the cooling mode.

A feature of the invention is that the cake is cooled to a temperature of 1000 ° C at a rate sufficient to undergo a process of segregation of impurities in the grain of dicalcium silicate, contributing to the formation of the maximum amount of β-modification of dicalcium silicate. Moreover, in comparison with the prototype, the extraction of aluminum oxide from the cake is 2-3% higher during its subsequent leaching.

Claims (1)

A method of producing aluminum-containing cake by mixing and sintering at a temperature of 1250-1300 ° C of a mixture prepared from nepheline ore, limestone and recycled products, characterized in that the cake is cooled to a temperature of 1000 ° C at a rate sufficient to undergo a process of segregation of impurities in the grain of dicalcium silicate, contributing to the formation of the maximum amount of β-modification of dicalcium silicate.