A METHOD OF PREPARING WATER PURIFYING AGENTS

Abstract: In the manufacture of basic aluminium chloride and/or basic aluminium sulphate chloride from aluminium raw material, aluminium chloride and optionally a water-soluble sulphate together with a quantity of water at least corresponding to the number of moles of chloride to be removed from the aluminium chloride may be heated to a temperature of 100-700`C. The duration of the heating treatment depends on the temperature selected, so as to obtain a crystalline basic aluminium chloride or a cry basic aluminium sulphate chloride of the desired basicity. The method may also be applied in the manufacture of substantially iron-free basic aluminium chloride and/or basic aluminium sulphate chloride from iron-containing bauxite, shale or other iron containing aluminium minerals, wherein is involved a precipitation of an aluminium sulphate chloride of low iron content with hydro-chloric acid gas. Hydrochloric acid gas released during the heating process is returned to the precipitation stage.

Description: A METHOD OF PREPARING WATER PURIFYING AGENTS The present invention relates to a method of producing basic aluminium chloride and/or basic aluminium sulphate chloride from aluminium raw material and particularly producing an iron-free basic aluminium chloride and/or aluminium sulphate chloride by treating silicon- containing and iron-containing bauxite, shale and/or other iron- contaminated aluminium minerals with sulphuric acid, while recovering at the same time iron-containing aluminium salts. The world's need for drinking water increases together with the in¬ crease in world population, while the world ' s available supply of drinking water has been diminished by the increase in contamination of the environment. When viewed against this background, it will be obvious that the need for water-purifying agents will greatly in¬ crease. Water which is suitable to drink must first be disinfected and clear, i.e. freed from colloids and suspended particles, which are unable to settle. It has been found that aluminium compounds, primarily basic aluminium chloride with a certain sulphate content or basic aluminium sulphate chloride are particularly effective substances for clarifying waters. The presence of sulphate results in a higher degree of separation of contaminats, and thereby in clearer water, than when solely aluminium chloride is used. In the production of aluminium-containing chemical products, the most common starting material is aluminium hydroxide. Aluminium hydroxide is mainly prepared by a method according to the Bayer- process. This process, however, requires a starting material in the form of a high-grade bauxite with but a low silica content, which otherwise gives rise to losses of alkali and aluminium in the form of sodium aluminium silicate in the so-called red mud. Bauxites containing more than 5% silica are not used. Neither is it possible ^ to utilize the iron content of bauxite in this context, because said iron content is lost in the red mud. Particularly when.manufact uring aluminium-containing chemical products for water purifying purposes, such as basic aluminium chloride containing a certain amount of sulphate, it is necessary to use as a starting material pure aluminium hydroxide, in order to fulfil the specific requ.n-.e- ents placed on the composition of the final product. Thus, methods are known, such as the method described in SE-A- 7308968-2, in which the aluminium hydroxide is neutralized with sulphuric acid, and the aluminium-sulphate content is converted to a solid basic aluminium chloride of low sulphate content, by reactio with a mixture of calcium hydroxide and calcium chloride. At the same time, however, considerable quantities of calcium sulphate (gypsum) are obtained, which creates serious dumping problems. In other known methods, there is added to the aluminium hydroxide a mixture of hydrochloric acid and sulphuric acid, whereafter the resultant aluminium sulphate chloride is caused to react with calciu hydroxide, to form a solid, basic aluminium chloride of low sulphate content. In this case the amount of calcium sulphate produced is only half that produced by the other methods. The amount produced, however, is still very large and consequently causes handl ng and dumping problems. Further, a number of methods are known (FR-A 1.558,347, FR-A5-71.422 NO-C-761136, FR-A5-77.06530, and FR-A5-77.06531 ) for treating silica containing and iron-containing aluminium raw materials with sulphuri acid, for the purpose of producing aluminium oxide for use in the manufacture of aluminium. In general, the continued treatment proces for producing aluminium oxide comprises treating the resultant slurr with a sulphuric acid-hydrochloric acid solution, followed by precipi tion with hydrogen chloride, while cooling the solution. The result ant crystals are then decomposed at a temperature of 300-400 C, to drive off the water, and the whole of the chloride content and the decomposition product are calcined at high temperatures, to form a pure aluminium oxide. These methods, however, pertain to the manufacture of a completely different product than the present invention, and hence also lack those steps required when producing the aforesaid water-purifying agents based on basic aluminium chloride and characteristic of the method according to the present invention. The object of the present invention is to eliminate the disadvantages inherent of the aforesaid methods for producing water-purifying agents based on basic aluminium chloride. In accordance with the invention, basic aluminium chloride and/or basic aluminium sulphate chloride is produced from an aluminium raw material, which may be aluminium chloride or oxidic, hydroxidic and other aluminium raw materials which are able to be converted to aluminium chloride. According to a preferred embodiment of the invention, an aluminium-chloride based water-purify ng agent substantially free from iron is produced in a relatively simple and economically advantageous manner from heavily contaminated raw materials, and in particular from raw materials heavily contaminated with iron and silicon, while being able to control the composition of said agent. The characteristic features of the invention are set forth in the fol l owing claims. The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a block schematic illustrating a preferred embodiment of the invention, in which the starting material comprises highly contaminated aluminium raw materials. The aluminium raw material, which is heavily contaminated - primarily with iron and silicon - and which may, if necessary, be crushed and ground and optionally calcined, is digested ,(1) with a mixture of con¬ centrated sulphuric acid and water, and optionally with re-circulated c: π acid mother liquor containing chlorides and sulphates of iron and aluminium. Subsequent to a completed leaching at elevated temperatu a useable product can be obtained from part of the digestion soluti by causing the solution to cool and therewith to crystallize (6) in the form of aluminium iron sulphate, which can be used to advantage as a water-purifying agent but which, because of its iron content, is utilized only as a flocculating agent in the purification of sewage water and other contaminated water deemed unsuitable for purification as drinking water. When necessary, the digestion solut is evaporated prior to said crystallization. This product, which contains mainly iron sulphate and aluminium sulphate, is well suite inter alia - as a precipitating agent for sewage purification proce That part of the digestion solution which is not caused to crystall is then diluted (2), normally with a liquid, which may be water or chloride-containing liquid or sulphate-containing liquid obtained f the process, or mixtures thereof. Subsequent to diluting said part said solution, insoluble material is filtered off (3), this insolub material comprising mainly undissolved silicon compounds originatin from the aluminium raw material, said insoluble material being eith returned to the digestion stage or dumped. Subsequent to heating th filtrate (4), hydrochloric acid gas is introduced thereinto, to cau aluminium sulphate chloride to crystallize out (5). The relationshi between the sulphate content and the chloride content is controlled with the temperature of crystal ization, in a manner such as to obt a relationship which lies at least in the proximity of that desired the end product. As indicated by the arrow extending in Figure 1 fr stage (5), and associated text, if desired part of the product can taken directly from said stage in the form of iron-containing alumi sulphate chloride in solution form. The aluminium sulphate chloride obtained is very easy to filter. By washing (7) the primarily obtained aluminium sulphate chloride with concentrated hydrochloric acid, there is surprisingly obtained an aluminium chloride which is totally free from iron. The sulphate co is also very low in the aluminium chloride washed with hydrochloric acid. Alternatively, if desired part of the product can be taken ou in the form of a solid, iron-free aluminium chloride, as shown by the arrow at 7. By heating (8) the aluminium sulphate chloride, washed with hydro- chloric acid to the extent desired in accordance with the above, to a given temperature of at least 100°C and at most about 700°C, over a given period of time, a quantity of hydrocloric acid gas, of such magnitude is driven off and returned to the crystallization stage (5) that there is obtained a solid, basic aluminium sulphate chloride having particularly good properties with regard to water purification. By dissolving (9) the resultant solid, basic aluminium sulphate chloride in water, there can be prepared a liquid product exhibiting the same good properties, whereat the water-purifying agent is able to mix more readily with the contaminated water at the place of use. The composition of the end product according to the present invention is adjusted by first roughly adjusting the ratio of sulphate to chloride, by controlling the crystallization temperature, as before- mentioned. A low crystallization temperature of about 10-40°C can be maintained, by cooling the filtrate, whereat an aluminium sulphate salt of very low chloride content is obtained. By maintaining a crystallization temperature of about 40-70°C, it is possible to obtain an aluminium sulphate salt of high chloride content. After the subsequent purifying step (7), the aluminium sulphate chloride crystals are heated, in accordance with the aforegoing. Figure 2 illustrates a heating process for finely adjusting the composition of the basic aluminium sulphate chloride to a preferred composition. In this respect, temperature and time are held sub¬ stantially within the area between the two curves of Figure 2 during the heating process. As will be seen from the Figure, this area is defined upwardly, i.e. towards longer times, by the relationship t (mi-,) and downwardly, i.e. towards shorter times, by the relationship t em) - 77 • e - 5 - 0 • 10"3τ -° C - The available choice of a suitable calcination temperature is governed by the particular apparatus used and the technics applied. At very high calcination temperatures, about 700°C and thereabove, the preferred composition is reached so quickly that it is difficult to monitor the method, and hence the precision when adjusting the composition may be lost. In the following examples and in the claims, by the term basicity is meant generally the ratio 0H/A1 in the general formula for basic aluminium chloride Al Cl (OH) and basic aluminium sulphate chloride x y Al Cl (OH) (S0^,) 2 .Thus all compositions in which x and x + 2z are less than 3 are basic. The invention wil l now be il l ustrated by means of a number of working examples. Example 1 Aluminium chloride hexahydrate, A1C1, •_ SH^O, was heated at 400 - 600 for different lengths of time. The contents of aluminium and chlorine were determined and the basicity calculated. The values obtained are given in table I below. In addition, the pH was determined in a solution of 5% of the product in water. Basic aluminium chlorides whose basicity is greater than 2.5 are not soluble in water. In those cases when the product did not dissolve, this is indicated in the table with the word "insoluble". i r CD Exampl e 2 Ground bauxite containing 49.5% A1 2 0 3 , 18.3% Fe 2 0 3 , 18.3% Fe 2 0 3 , 2.8% Ti0 2 and 2.8% Si0 2 , was digested with concentrated sulphuric 5 acid and water for 2 hrs at 110°C. 90% of the aluminium and the iron entered the solution. The digestion solution was diluted with water to form a solution containing 3.3% Al, 1.5% Fe, and 21.4% S0 4 - The insoluble material, mainly silicon compounds and titanium com¬ pounds, were filtered off. Hydrochloric acid gas was introduced 0 into " the filtrate, wherewith crystallization took place. The temperature of the solution was 45-50°C during the whole of the crystallizing process. Crystals were taken out on four different "occasions during the total crystallizing process, and treated per se. The crystals of the four different batches had the following 5 compositions: Crystal batch Al Fe SO, Cl number % % % % 1 11 .0 0.25 4.2 40.3 0 2 10.7 0.20 7.9 36.7 3 11 .0 0.18 3.2 41 .1 4 11 .1 0.10 1 .5 43.0 25 Subsequent to the crystallization process, 11% of the input alumin¬ ium and 91% of the input iron were found present in the mother liquor. The crystals obtained had a very low content of iron and sulphate. 30 For the purpose of obtaining a basic aluminium sulphate chloride, the crystal-batch 4 was calcined for 20 minutes at 200°C, whereat a product having the following composition was obtained: Al Fe S0 4 Cl ~, ~ 0/ 0/ Cl 01 - J - J ro /o A 19.0 0.17 2.5 32.3 The product is soluble in water and has a basicity, i.e. a mole ratio of OH/Al, of 1.4. Example 3 Part of the digestion solution obtained in accordance with Example 2, was crystallized directly by lowering the temperature of the solution, to form a product suitable for water-purification purposes, there being crystallized an aluminium-iron-sulphate. A product containing 13.4% water-soluble A1 2 0 3 , 4.6% water-soluble Fe 2 0 3 and 2.4% water- insoluble substance was obtained. That part of the solution which did not crystallize was diluted with water to form a solution containing 3.3% Al , 1.5% Fe and 21.4% SO,. The insoluble product, mainly compounds of silicon and titanium, was filtered off. The filtrate was cooled to 15-25°C, whereafter hydro¬ chloric acid gas was introduced into the filtrate, to effect said crystallization. A temperature of from 15 to 25°C was maintained during the whole of the crystallization process. Subsequent to being dried at 40°C the crystallized product contained 9.5% AT, 0.4% Fe, 30.8% S0 4 and 14.3% Cl . The crystals contained 91% of the input aluminium and less than 10% of the input iron. It will be apparent from this that aluminium crystallizes much more selectively than iron. The crystals were washed with concentrated hydrochloric acid, whereat after drying at 40°C a product having the following composition was obtained: Al Fe so 4 Cl % % % % 10.8 0.015 7.4 36.6 By washing the crystals, the iron content could be lowered from 0.4% to 0.015% and the sulphate content from 30.8% to 7.4%. The method also produced an aluminium sulphate chloride which was very poor in iron. By calcinating the product for 20 minutes at 200°C, there was obtained a basic aluminium sul phate chloride of the following compos¬ ition: Al Fe SO, Cl % % % 18.5 0.025 12.4 -29.8 This product is solubl e in water and has a basicity of 1 .6. Exampl e 4 An aluminium -sulphate chloride was produced from diluted, filtered bauxite digestion solution, crystallized by adding hydrochloric acid gas thereto. The aluminium sulphate chloride was washed very thorough ly with concentrated hydrochloric acid, there being obtained an aluminium chloride of the following composition: Al Fe S0 4 Cl % % % % 10.6 0.001 0.34 40.9 This aluminium chloride is of sufficiently good quality to provide an electrolysis-pure aluminium oxide when calcined. The product was calcined at 500°C for 5 minutes, whereat there was obtained a water-soluble aluminium chloride, with a basicity of 1.3. Exemple 5 A solution which is made acid with sulphuric acid and obtained when leaching alun-shale can be treated by neutralizing with limestone. If the neutralizing process is interrupted when the pH is 4.2, there is obtained a precipitate comprising mainly gypsum and aluminium hydroxi One such precipitate containing 8.2 Al, 1.3% Fe, 4.0% P0 4 , 13.6% Ca, and 34.0% S0 4 , was leached with a stoichio etric quantity of sulphuri acid for 1 hr at 95°C, whereat the major part of the aluminium, iron and phosphate was dissolved by passing hydrochloric acid gas down into the filtrate at 55 C, there were obtained crystals of the following composition: 11.1% Al, 0.024% Fe, 0.18% P0 4 , 1.8% S0 4 and 42.7% Cl . The iron content was lowered to 0.004%, by washing the crystals with concentrated hydrochloric acid. The crystals were heated to 450°C for 10 minutes, whereafter the crystals, which were soluble in water, contained 22.0 % Al and 31.3% Cl, which corresponds to a basicity of 1.9. -- u

Claims: CLAIMS 1. A method of producing basic aluminium chloride and/or basic aluminium sulphate chloride, characterized in that aluminium chloride, any water-soluble sulphate needed and a quantity of water corresponding at least to the number of moles of chloride to be removed from the aluminium chloride are heated to a temperature of 100-700 C for a pre-determined length of time in dependence on the selected temperature, thereby to obtain a crystalline basic alumi- nium chloride and/or sulphate chloride having the desired basicity. 2. A method according to claim 1, wherein basic aluminium chloride is produced, characterized in that the aluminium chloride is alumi¬ nium chloride hexahydrate. 3. A method according to claim 1, wherein basic aluminium sulphate chloride is produced, characterized in that the aluminium chloride is aluminium sulphate chloride hydrate. 4. A method according to claim 1, wherein a substantially iron-free basic aluminium chloride and/or basic aluminium sulphate chloride is produced from iron-containing bauxite, shale or other iron-containin aluminium-containing minerals by digesting the starting material in an acid environment in a manner known per se; separating the major part of the iron content as an aluminium iron sulphate; precipi¬ tating an aluminium sulphate chloride of low iron content by means of hydrochloric acid gas; and washing said precipitate with a hydrochloric-acid containing washing liquid in such quantity as to decrease the sulphate content of the precipitate to a pre- determined extent; characterized in that subsequent to being washed, the precipitate is separated from at least the major part of the washing so as to constitute and correspond to the mixture of alumi¬ nium chloride, water soluble sulphate and water being heated up and thereby to obtain a crystalline basic aluminium chloride of the desired basicity containing the intended amount of sulphate, and in S UBSTITUTE SHEET that hydrochloric acid gas released during said heating process is returned to the precipitation stage. 5. A method according to any one of claims 1-4, characterized in that the precipitate is heated for such a period of time and at such a temperature as to form a basic aluminium sulphate chloride having the composition - AlCl χ (0H) y (S0 4 ) z , where 0.5≤x -≤2.5 0.5-Sy -S2.5 0 2 z *l 6. A method according to claim 4, characterized in that the precip¬ itate is washed with so much concentrated hydrochloric acid that solid •aluminium chloride is formed directly; and in that said solid aluminium chloride is then heated to form an aluminium chloride which is sub¬ stantially free of sulphate. 7. A method according to claim 4, characterized in that the pre¬ cipitate is washed with dilute hydrochloric acid, so as to dissolve said precipitate, whereafter aluminium chloride having a lower sulphate content than the precipitate is precipitated from the solu¬ tion by re-crystallization, by introducing hydrochloric acid gas into said solution.

Inventors: NILSSON R, SCHOEN G
Applicants: BOLIDEN AB, NILSSON R, SCHOEN G
Application year: 1980
Main classification: C01F7
IPC codes: C01F7