CA1065068A

CA1065068A - Method of selectively precipitating metals from solutions

Info

Publication number: CA1065068A
Application number: CA256,912A
Authority: CA
Inventors: Thomas K. Mioen
Original assignee: Boliden AB
Current assignee: Boliden AB
Priority date: 1975-07-28
Filing date: 1976-07-14
Publication date: 1979-10-23
Also published as: SE7508535L; FI762151A; FR2319579B1; DE2631611A1; SE396364B; FR2319579A1

Abstract

A B S T R A C T O F T H E D I S C L O S U R E
A METHOD OF SELECTIVELY PRECIPITATING
METALS FROM SOLUTIONS
The invention is a method for selectively precipitating aluminum and iron from solutions initially having a pH of less than 4. The method involves reduction of any ingoing trivalent iron to divalent iron and then neutralizing the solution to a pH exceeding 4 so as to precipitate aluminum hydroxide. The iron in the remaining solution is then oxidized to trivalent state so that the iron content is caused to pre-cipitate out in a compact readily filtered form.

Description

- 10650~8 The present invention relates to a method of precipitating aluminium and iron from acid solutions.
Such solutions, for example, may comprise wash solutions ob-tained from gas cleaning plants or metal recovery plants. When working-up iron-oxides obtained by roasting pyrites, for example, in the manner described in Swedish Patent Specification 319 785 or the Canadian Patent Specification 865 647, by purification through chlorinating volatilization of the iron-oxides, the outgoing chlorine-containing gas is washed, for instance, in washing -towers 10 in which the metal chlorides are quantitatively absorbed and dis-solved in the acid washing liquid.
Acid solutions obtained by treating certain minerals or metal- ;
lurgical products and/or intermediate products hydro-metallurgically can also be treated according to the inventionO Such acid solutions may be obtained when, for example, ores, minerals or roasted pro-ducts are leached, electrolyzed or treated with acids in such manner that iron and aluminium is dissolved to a greater or lesser extent together with other valuable metals, or in certain cases ~ -in which substantially only iron and aluminium are dissolved out.
20 An example of the first type of acid solutions mentioned comprises ~~ -leaching solutions obtained when working-up metal-containing laterite ores or shales and residual solutions subsequent to re-covering metals electrolytically, for example, such as is described in the Swedish l~atent Specification 7507507-7 (Patent Application 1975-07-01~. An example of the other type of acid solution mention-ed comprises solutions obtained when digesting or dissolving so-called red sludge from the Bayer-process for the manufacture of aluminium and when leaching aluminium-iron-silicate, for example andalusite and kaoliniteO
30Thus, in respect of solutions of the aforementioned type containing salts of aluminium and iron there is a need either to ~
separate these elements, so that pure iron and/or aluminium can be ;

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1(~650~;8 produced as a produc-t, or to conver-t the elements to a form which will enable -them to be safely deposited.
As will readily be understood, it is necessary to be able to treat such acid solutions so that they have no harmful effect on the environment. Normally, -the elements contained in such solu-tions are precipitated out as a mixed hydroxide precipitate by neutralizing the solution to a pH lying between 4 and 7, using, for example, lime-stone, milk of lime (slaked lime), sodium hydroxide or carborate.
In order to ensure that all the iron is precipitated out, it is necessary to oxidize the iron -to its trivalent state.
Mixed precipitates of aluminium hydroxide and iron hydroxide are bulky and have a gelatinous and slimy consistency. Consequently such precipi-tates are difficult to filter and to free from any liquid enclosed therein or adhering thereto. Because of thisJ such mixed precipitates are both difficult to work-up and difficult to dispose of.
It is a well known fact that aluminium hydroxides begin to precipitate out at a pH of 4~3, using lime sodium hydroxide, etc., and that they have precipitated out quantitatively at a pH of 4.8.
By quantitative precipitation is meant here that all aluminium is precipitated out to the maximum extent permitted by the solubility product. The corresponding pH-values for the hydroxide precipitation of tri-valued iron are 2.8 and 3.5. On the other hand, iron (II) hydroxide will not precipitate ou-t until a pH of approximately 9 is reached. See, for example, in this respect MODERN KEMI (No. 4)(1973), page 30.
In many cases it is desirable to use the solution, subsequent to precipitating out Al and Fe, in a closed hydro-metallurgical process, for example, by returning residual solution to a leaching step arranged upstream of the precipitation step. In such instances a high pH is an obvious disadvantage, since the amount of acid used during the leaching process will then be unnecessarily high~

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106~68 It has now been surprisingly found that when aluminium and iron are slectively precipitated out from solutions having a pH
of less than 4, there are obtained precipitates which can be readily separated by,for example, filtration and worked-up or disposed of without, unless desired, it being necessary to completely neutralize the solution or to make the solution basic, i.e. raising -the pH
to abou-t 7 or thereabove.
In accordance with the invention, a reductant is admixed with the solution in a quantity sufficient to convert any ingoing tri-valent iron present in the solution to a divalent sta-te by reduction, whereafter a neutralizing agent is added until a pH which at least -exceeds 4 is reached so that aluminium hydroxide is precipitated out quantitatively, whereafter an oxidizing agent is added to the solu tion so that the ingoing iron is oxidized to a trivalent state and ... ~ . ... .
so that the iron content is caused to precipitate out in a compact, readily filtered form.
Thus, in accordance with the invention the iron content of the solution shall be in a divalent state when the aluminium is to be precipitated out. When the original solution contains ions of tri-~0 valent ~nn, these are therefore reduced by adding a reductant priorto the precipitation of the aluminiumO Sulphur dioxide or metallic iron, for example in the form of scrap iron, can be used to advantage as a reductant. In the case of a sulphate solution, it is also suit-able to effect the aforementioned reduction by adding a sulphidic material, such as iron~sulphide for example.
Subsequent to the quantitative precipitation of aluminium from the solution, the solution is oxidized by adding thereto a gas con-taining free oxygen, such as air or oxygen gas or some other suit-able oxidizing agent such as chlorine gas, nitric acid or an agent which gives off chlorine or oxygen. In certain cases oxidation can ;

be effected, to advantage, by ano~ic oxidation in an electrolysis -cell. The oxidation is conveniently carried out at ~evated pressure, :
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for example in an autoclave or similar apparatus. During the oxida-tion process, the iron is converted from a divalent to a trivalent state.
The iron is caused to precipitate out in a trivalent state in a compact, readily filtered form in conjunction with the oxidizing process or subsequent to the said processO The precipitate may have the form of a complex basic iron sulphate, basic iron oxides or hematite. The most important parameters for obtaining the desired precipitate are the pH value of the solution, the amount of iron contained therein, the precipitation temperature and time.
When alkaliions or ammonium ions are present in the leaching solution or are added thereto, the iron content of the solution can be caused to precipitate out at a pH which is higher than 1 and which is preferably lower than 3, as basic iron sulphate also in-cluding alkali ions and/or ammonium ions and having a structure which is principally A-Fe3-(S04)2 (OH)6, where A can be one or more of the ions Na j K or NH4 . Such a structure coincides with the mineral jarosite. When precipitating out jarosite, the temperature is suitably maintained at at least 90G. A lower ^temperature will extend the precipitation time.
In the absence of alkali ions or ammonium ions, it is also pos-sible within the same pH range to precipitate the iron out as a ~
precipitate called hydronium-jarosite (corresponding to CarphosidOEite) -having the same principle structure as that shown above for jarosite but where A is H30 -ionO
The iron can also be precipitated out in an autoclave at elevated pressure in the form of hematite. It is disclosed in the literature, TMS Paper A73-65 (1973), that when oxidizing iron (II) solutions, hematite can be precipitated out at an oxygen pressure of 20 k,p/cm2 and at a temperature of 200 C in solutionsh~ing a ~nal-acid content of 60 g/1 H2S04~ -The iron can also be precipitated out in the form of basic i~n .: . ,,. ~ ... . .. . .

oxide having a composition corresponding to goethi-te at a pH
below approximately 6, suitably within a pH range of 2-4. In this case no extra ~ditons to the solution are required, with the excep-tion of a neutralizing agent and an oxidizing agent. A suitable pre-cipitation temperature in this case is 85Co OH-ions from the solustion ore consumed during -the iron precipitation process, and hence the pH falls during said process.
During the precipita-tion process, the pH of the solution can be regulated by adding protolytes, such as a base. When the iron is to be precipitated ou-t as jarosite or hematite, pH values of the desired low magnitude can be obtained by adding acid to the solution prior to causing the iron to precipitate. Thus, the iron can be caused to precipitate out in the desired form during the entire precipition process by continuously measuring the pH of the solu-tion during said process and adjusting the system in accordance with the measurements , recorded. -The aluminium and iron precipltates obtained in accordance wltn the invention can be readily worked-up and can be safely disposed of.
When the original solution contains soluble salts of, for example, potassium, sodium and magnesium in addition to iron and aluminium, said salts can be readilyrecovered, for example, by evap~rating the residual solution subsequent to precipitating out the iron and aluminium therefrom. A residual solution having a low pH is favourable in respect to the resovery of said salts, it being assumed that the acidity is derived from hydrochloric acid in ch~oride solutions and sulphuric acid in sulphate solutions. When, subsequent to recovering the metal content of the solution, -the re-sidual solution comprises a sulphate solution which shall be destroyed in a roasting or combustion furnace and the S02-gas thus formed shall be utilized, it is advantageous for the solution to be as acid as -possible(having a high sulphate content or containing free sulphuric "

~ 1065068 acid). In such cases, also jarosite precipitated out of the solution and containing sulphate ions may be charged to the furnace at the same time, thereby to split-off and utilize the sulphur content.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of selectively precipitating aluminium and iron from solutions having a pH of less than 4, characterised in that the solution is first admixed with a reductant in a quantity sufficient to convert any ingoing trivalent iron in the solution to divalent iron by reduction, whereafter a neutralizing agent is added until the pH of the solution exceeds at least 4, so that the aluminium hydroxide is precipitated out quantitatively, whereafter an oxidizing agent is added to the solution so as to oxidize ingoing iron to a trivalent state and so that the iron content is caused to precipitate out in a compact, readily filtered form.

2. A method according to claim 1, characterised in that sulphur dioxide or metallic iron is used as the reductant.

3. A method according to claim 1, characterised in that a gas containing free oxygen is used as the oxidizing agent.

4. A method according to claim 1, characterised in that free chlorine gas or a compound which gives-off free chlorine gas is used as the oxidizing agent.

5. A method according to claim 1, characterised in that the iron is oxidised at an elevated pressure.

6. A method according to claim 1, characterised by regulating the pH of the solution during the precipitation of iron therefrom by adding protolyte to said solution.

7. A method according to claim 1, characterised in that the iron content of the solution is precipitated out as iron oxide hydroxide.

8. A method according to claim 1, characterised in that the iron is precipitated out from a sulphate-containing solution as a complex basic iron sulphate at a pH which exceeds 1 but which is less than 3.

9. A method according to claim 8, characterised in that alkali ions or ammonium ions are added to the solution.

10. A method according to claim 8, characterised in that the iron is precipitated out at a temperature greater than 90°C.

11. A method according to claim 7, characterised in that the iron is precipitated out from a solution having a pH
less than approximately 6.

12. A method according to claim 11, characterised in that the iron is precipitated out at a temperature of approxi-mately 85°C and at a pH of between 2 and 4.