DE1179919B - Process for the production of aluminum oxide - Google Patents

Description

Process for producing alumina The invention relates to a improved process for the production of pure alumina from aluminum-containing starting materials.

The usual procedure for making purified alumina from ores, such as B. bauxite, consists generally in the dissolution of the ores in Caustic soda for the elimination of iron impurities and subsequent precipitation of aluminum hydrate from the alkaline solution.

The precipitate is then roasted to produce alumina.

Under certain circumstances, however, an acid process can have economic advantages bring against the alkaline process.

The digestion of aluminum-containing ores with sulfuric acid has up to now had little success, particularly because of the high operating and capital costs and because of the difficulty in producing alumina of a suitable purity for subsequent reduction into metallic aluminum. The difficulties are the slow and costly crystallization of aluminum sulfate from the digestion solution and avoid unwanted iron contamination of the product.

It has already become known, starting from alumite in the digestion solution by adding A1203 to produce a ratio SO1: A1203 of 0.9 in order to subsequently to obtain a basic Al2 (S04) a by heating the solution in the absence of air, which was free from contamination by Fe and alkalis.

The purpose of this invention is to overcome the difficulties mentioned above to overcome and create a process for the production of aluminum oxide that can be run economically and at the same time an aluminum oxide of one supplies such purity that it is used for the production of metallic aluminum the known methods can be used.

According to the invention, a process for the production of aluminum oxide by digesting A1203-containing minerals with sulfuric acid using pressure and elevated temperature, separating the insoluble residue, de-ironing the solution by adding a reducing agent, hydrolysis of the aluminum sulfate solution and heating the precipitated basic A12 (SO4 ) 31 proposed, whereby in the filtered digestion solution a weight ratio of SO3; A120, set between 1.2 and 2.3 and basic aluminum sulfate is precipitated from the resulting solution at 180 to 300 ° C under pressure and nitrogen atmosphere and converted into A1203 in a known manner by heating.

Usually the precipitated mixture consists essentially of basic aluminum sulphate Al2 (S04), with a proportion of aluminum oxide hydrate. Corresponding hereinafter reference is made to "basic aluminum sulfate", which includes a portion of aluminum hydrate, which is produced during digestion. Preferably will the ratio of SO,: A120, set to a value between 1.4 and 1.6, and the precipitate is kept in a pressure vessel within the temperature limits between 210 and 230 ° C.

The designation »harmful impurities« is used as an indication of those impurities whose presence in the end product exceeds one certain limit the product unsuitable for reduction into metallic aluminum power. It is possible, before the hydrolysis, to determine the ratio of SO,: A120, in the digestion solution by adding a salt of an alkali metal (e.g. sodium sulfate) or an alkali metal hydroxide (e.g. sodium hydroxide), but results this measure a product that is up to an undesirable amount is contaminated with alkali metal, e.g. B. contains when using sodium hydroxide the aluminum oxide end product a few percent Na20. It is also possible that Reduce the ratio of S03 to A120 $ by adding lime. That reduces that mentioned ratio by separation of SO3 as calcium sulphate from the solution; however, the soluble calcium that remains in the solution forms a significant one harmful pollution in the sense mentioned above, and the loss of sulphate in the calcium sulfate residue would be a serious economic disadvantage in the overall process represent.

The invention offers various possibilities for setting the Weight ratio SO3: A1203 of the digestion solution, without a significant amount of harmful impurities to introduce. The measures can either be taken individually or used in combination to achieve the desired weight ratio of S03: A1203 to obtain: dissolution of metallic aluminum in the digestion solution, whereby the concentration of alumina in the solution is increased and the ratio from S03: A1203 is decreased at the same time.

Electrolysis in a cell with a semi-permeable membrane. By doing When current is passed through this cell, the sulfate ions move through the semi-permeable membrane and enter the anolyte. Due to the selective Property of the membrane, the aluminum ions are retained and remain in the catholyte. In this way, if the current continues to pass, both the Sulphate content and the ratio S03: A1803 of the catholyte are reduced.

Heat the digestion solution for about 30 minutes at a temperature (e.g. from 100 to 140 ° C) with an aluminum-containing substance such. B. Bauxite. Included an additional amount of aluminum oxide is dissolved. That way becomes a average reduction in the ratio of S03 to A1203 in the solution achieved. This type of setting can also be used at temperatures above 140 ° C but at the expense of sulfate loss. At temperatures below 100 ° C the reaction proceeds relatively slowly.

Addition of basic aluminum sulfate to the digestion solution, no matter whether or not it has already been made in some other way. Each Basic sulfate of aluminum can be used for this purpose as all of these Mixtures have a ratio of SO3: A1203 less than 2.35. Is preferred for this purpose the basic sulphate is used, which is produced in the pressure digestion stage and that has a ratio of SO3: A1203 approximately equal to 1. Under these In some circumstances it is of course necessary to have a high yield of basic To get sulfate in the digestion stage. As stated above, there is a disadvantage of the earlier acid processes in a contamination of the alumina products by iron. In the present invention, it was found before some of the iron impurities is reduced in the digestion solution in a known manner, this is already reduced could be if the digestion solution for about 30 minutes at 100 to 140 ° C with the addition of z. B. bauxite was heated.

Insoluble iron oxides precipitate and are filtered out before hydrolysis can be.

If the setting of the S03: A1203 ratio of the digestion solution by adding metallic A1 or by electrolysis, the Iron in this case already reduced to the metallic state, so that a separate Reduction stage is not required. Such a reduction process can, however provided if deemed necessary.

Crushed bauxite is digested with sulfuric acid. That happens under pressure with stirring and aeration at temperatures above 110 ° C.

After the solid residues have been separated out and filtered, the filtrate becomes by one or a combination of the enumerated possibilities given above got employed. For example, an excess of fine grained bauxite is used the digestion solution heated, but with a preferred maintenance of the temperature below 140 ° C, but not below 100 ° C. After another filtration, the remaining Liquid with a small amount of aluminum metal or some other reducing agent treated in order to convert all dissolved iron compounds into the metallic state convict.

The set liquid, which has a ratio of S03: A1203 of approximately 1.9, is placed in a pressure vessel together with basic aluminum sulfate brought that circulates from a subsequent stage to the ratio of S03: A180 $ to decrease even further. The contents of the pressure vessel are then pressurized in a non-oxidizing atmosphere at a temperature preferably above 190 ° C heated for approximately 30 minutes. This becomes the basic Precipitated aluminum sulfate, and after cooling and filtration, the filtrate is in returned to the work process.

The precipitated basic aluminum sulfate is divided into two parts, one of them is returned to the precipitation stage, the rest of the basic aluminum sulfate is then roasted at a temperature of 1000 to 1200 ° C to add aluminum oxide form. The sulphurous gases that are excreted during this roasting will be returned to the digestion.

The yields are good for economical operation of the process The hydrolysis is necessary for two reasons: To have sufficient basic aluminum sulfate for circulation and for roasting.

If the hydrolysis yield is low, it can be very low Amount of aluminum sulphate to be transferred to the roasting stage, as the main part is required for the setting level. In this case most of the resolved works Aluminum sulfate continuously around, taking for maintaining the process additional costs must be incurred.

It should therefore at least approximately 400 /, or more of the basic Aluminum sulphate must be available for the roasting stage. This in turn requires a Yield of hydrolysis on the order of 65 ° / o. The yield the hydrolysis generally increases with the temperature used in the precipitation stage is used, and with the degree of adjustment. Therefore it is possible to have three groups to be distinguished from operating conditions 1. At a temperature in the range of 300 ° C an adjustment of the ratio S03: A1203 of z. B. 2.2 to 2.34; 2. at a temperature in the range of 180 ° C, an adjustment of the ratio S03 : A1203 from approximately 1.2 to 1.3; 3. at a temperature in the range of 200 to 250 ° C with an intermediate degree of setting the ratio of S03 : A1203 on the order of 1.3 to 2. The application of extreme temperatures as in 1 is uneconomical, since this requires expensive facilities and high operating costs required are. The application of temperatures in the range of 180 ° C according to 2 is just as uneconomical, as the degree of adjustment has a ratio of S03: A1203 Requires a very high hydrolysis yield of, for example, 1.2 and only one low proportion of these products of hydrolysis for the. Roast level available stands. The working process according to Fig. 3, however, gives a suitable yield hydrolysis, which can be obtained at an economical cost.

At a temperature of approximately 220 ° C. , hydrolysis is possible with a ratio of SO3: A1203 between the values 2.34 and 1.00; however, at either end of this ratio, the yield is low or the process is uneconomical.

The results that were obtained with a production plant with an excess of 102 g of A1202 to 1 liter of the solution set are summarized in the tables below. Weight of Ratio percentages generated Hydrolysis alumina S03: A1303 yield basic in grams Weight- in a @p sulfates per liter proportions roasting of the set solution Effect of adjustment on hydrolysis with 220'C 1.88 42 100 43 1.76 49 74 42 1.65 59 55 45 1.53 70 43 51 1.41 82 35 63 1.29 '92 28 78 Effect of adjustment on hydrolysis with 1801> C 1.76 26 50 15 1.65 34 21 10 1.53 45 10 8 1.41 55 3 4 1.29 67 1 2 At 180 ° C, the weight of the aluminum oxide produced in the roasting stage per liter of the solution used is low in all ratios, and it drops as the ratio of SO3: A1803 decreases. On the other hand, at 220 ° C, the amount of alumina increases as this ratio decreases.

There may be disadvantages in handling very large ones Quantities of solid basic aluminum sulphate are produced in the hydrolysis stage, and it is preferred, for this reason, at an intermediate ratio from 1.4 to 1.6 to work.

Let a number of examples illustrate the process. Example 1 A digestion solution was prepared by digesting crushed. Bauxite made with sulfuric acid with stirring and ventilation at a temperature of 110 ° C. The residue was filtered off and a filtrate containing 45 g of A1203 per liter was obtained, and the solution thus adjusted was used as a catalyst in an electrolytic cell with an anion-selective semipermeable membrane, in which part of the sulfate ions was removed from the liquid. The ratio SO3: A180 $ in the liquid was 2.39. An electric current with a current density of 400 amps per square meter was passed until the ratio of SO3: A180 $ was reduced to a value of 1.18 . The digestion solution set in this way was then treated with sulfur dioxide at 100 ° C. in order to reduce further iron to the metallic state if necessary, and then heated to about 200 ° C. in an autoclave and kept under a nitrogen atmosphere for a period of 30 minutes. The resulting product was a basic aluminum sulfate containing more than 90 "/ the amount of alumina originally introduced into the autoclave. It was then roasted at 1100 ° C. Analysis of the roasted product (essentially A1803) revealed less than 0.020 /, Silicon and less than 0.020 / 0 iron, other impurities in undesirable quantities could not be determined by a spectroscopic analysis. that the filtered solution contained 34 g A120 $ per liter and a ratio of SO: A1802 had 2.22 the adjusted solution was then treated at 100 ° C with sulfur dioxide, to reduce residual iron in the metallic state 200 ml.. the adjusted solution were placed in an autoclave together with 16 g of basic aluminum sulfate, the ratio of SO3: A1803 being changed to a value of 1.5 rde. The contents of the autoclave were then heated to 220 ° C. under a nitrogen atmosphere and this temperature was maintained for a period of 30 minutes. The solid product obtained was a basic sulfate of aluminum, had the same composition as the solid material originally added, and weighed 49 g. The product contained 730% of the aluminum oxide originally placed in the autoclave.

Example 3 400 ml of a digestion solution were produced in which 51 g of A120 $ per liter were included. These Solution was with 18 g bauxite heated to 130 ° C in an autoclave for 30 minutes. Which resulting liquid, after filtration, had a ratio of 1.90 (S03: A1203) on. This level of adjustment was mainly achieved through the resolution of Bauxite and, to a lesser extent, by precipitation of 1.66 g of SO3 from the original Solution. Part of the liquid thus adjusted became 27.8 g of basic aluminum sulfate added so that the mixture had an overall ratio SO3: AI20, of 1.58. The adjusted liquid was then treated with sulfur dioxide at 100 ° C. The mixture was then heated for 30 minutes at a temperature of 220 ° C heated, and the hydrolysis product, basic sulfate of aluminum, weighed in of 45.9 g, representing a gain of 18.1 g of basic aluminum sulfate. The product contained 60 ° / o of the total amount of alumina in the Autoclave was introduced. In summary, the following can be said: a) The yield of solid hydrolysis products rises with the temperature of the hydrolysis and falls with it the ratio of S03: A120 ,.

b) The yield of the hydrolysis does not depend on the process or the procedures of the setting used.

c) The additional intended setting by adding basic Aluminum sulfate adds to the previously partially discontinued liquids certain that a high yield will be obtained in the hydrolysis step.

d) Additional recruitment can also be made by means other than can be achieved by adding basic aluminum sulfate. The expense of such others In most cases, however, funds are not affordable.

e) Under suitable conditions a product can be obtained, that has a smaller ratio of silicon and iron and other impurities contains than it normally does for aluminum oxide, which is used to produce metallic Aluminum is used is mandatory.

Claims (3)

Claims: 1. Process for the production of aluminum oxide by Digestion of minerals containing A120, with sulfuric acid under application of pressure and elevated temperature, separating the insoluble residue, de-ironing the solution by adding a reducing agent, hydrolysing the aluminum sulfate solution and heating of the precipitated basic Al $ (S04) ,, d a d u r c h g e -k e n n n z e i c h n e t, that in the filtered digestion solution by adding A1,0, -containing substances or by electrolysis of the digestion solution in one with a semi-permeable membrane provided cell a weight ratio SO3: A1203 between 1.2 and 2.3 is set and basic from the resulting solution at 180 to 300 ° C under pressure and an N2 atmosphere Aluminum sulfate precipitated and converted into A1203 in a known manner by heating will. 2. The method according to claim 1, characterized in that the weight ratio brought to a value between 1.4 and 1.6 and the hydrolysis at a temperature is carried out between 210 and 230 ° C. 3. The method according to any one of claims 1 and 2, characterized in that the setting of the weight ratio of SO,: A1203 with an A1203-containing material by adding preferably bauxite is carried out at a temperature between 100 and 140 ° C. Considered Publications: F u 1 d a - G i n s b e r g, »Alumina and Aluminum«, 1st part (1951), Pp. 180 to 202; Gmelins Handbuch, B. Edition, No. 35, p. 57.