IE46818B1 - Process of hydrometallurgical treatment for eliminating impurities from a solution containing dissolved metals - Google Patents

Abstract

A process of hydrometallurgical treatment for removing metal impurities from zinc sulfate solution containing Fe+++ ions includes adding to said solution a silicate capable of forming dissolved silicic acid in situ, and a neutralizing base to provide a pH between 1.5 and 4.5. The metal impurities are precipitated as a solid residue on a base of iron silicate.

Description

This invention relates to a process of hydrometallurgical treatment for eliminating metal impurities from a solution containing dissolved metals to be extracted, more particularly for eliminating iron impurities from a zinc sulphate solution.

In hydrometallurgy, there are three main industrial processes for selectively eliminating iron as an easily dacantable and filterable residue from zinc sulphate solutions issuing from attack of oxidised zinc ores, more particularly roasted zinc ores.

This residue is essentially made of jarosite, goethite or hematite according to the process involved.

These processes have, however, the drawback that such residues have generally no commercial or industrial interest and even raise important storage problems in respect to environmental pollution.

Moreover, these processes involve expensive and cumbersome plants, when making allowance for flow rates and retention times involved.

One of the main purposes of this invention consists of providing a process of very simple conception, allowing a residue containing eliminated impurities to be produced and able to be directly used, without any further important treatment, in some industrial applications or as raw material in the manufacture of industrial products, for example building materials. 46618 - 3 This invention also relates to the residue so obtained, when used for example as embankment or as aggregate in cement or concrete.

Other details and feacures of this invention will become 5 apparent from the description such as hereinafter given, by way of non-limitative examples, of several particular embodiments of the invention, with reference to accompanying drawings.

Figure 1 is a block diagram of a first embodiment of 10 a process according to the invention.

Figure 2 is a block diagram of a variant of the embodiment of the process according to the invention illustrated by Figure 1.

In the Figures, the same reference numbers relate to 15 identical or similar elements.

This invention relates in general to a process of hydrometallurgical treatment for eliminating metal impurities from a solution containing dissolved metal values to be extracted as metals.

More particularly, this invention relates to a process for eliminating metal impurities, more particularly iron, from a solution of zinc sulfate to be submitted to an electrolysis for extraction of zinc as metal. This solution is usually obtained from an attack of a residue of an oxidised zinc ore by means of sulfuric acid, more particularly roasted zinc ore coming from lixiviation of this ore.

This process is characterised in that to said solution, a silicate, such as an alkaline-earth metal or zinc silicate is added, so as to form in situ dissolved silicic acid and to precipitate metal impurities at a pH between 1.5 and 4.5, - 4 4-6818 forming a well filterable and easily washable silicate residue. Preferably the solid silicate residue is caused to precipitate at a temperature between 40 and 85°C.

When a zinc silicate is used, the solution is first maintained at a pH not above 1.5 so as to allow this zinc silicate to be quickly dissolved, and then an alkaline-earth metal silicate, preferably calcium silicate, and/or lime is added to bring up and maintain the pH of the solution between 1·5 and 4.5 and to- induce precipitation of impurities, forming a solid silicate residue mainly made of iron silicate.

In some cases, oxidised zinc ore can also be used to bring the pH of the value not above 1.5, the formation of silicic acid being then obtained by addition of an alkalineearth metal silicate which then precipitates said impurities, forming said solid silicate residue, when the pH is maintained between 1.5 and 4.5; this may be carried out by adding a basic enough silicate and/or lime.

In other cases, only an alkaline-earth metal silicate may be used, more particularly calcium silicate, since such silicates are still enough dissolved at a pH between 1.5 and 4.5· and thus allow precipitation of ions which are to be eliminated, after dissolution of these silicates.

The real amount of silica dissolved in the solution remains always fairly small when the pH is maintained between 1.5 and 4.5 since as silicate is dissolved, it reacts in a substantially instantaneous manner with iron and possibly other metal ions to be eliminated, forming a solid silicate residue.

The preferential limits so as to cause precipitation in the solution are in the range of 2.5 to 3. - 5 In order to prevent a too large excess of dissolved silica after said precipitation, it is desirable not to exceed the stoichiometrical amount of added silicate with respect to impurities to be eliminated.

Figure 1 more clearly illustrates the various steps of a particular embodiment of the process according to the invention, such as applied to extraction of zinc from oxidised zinc ore.

This process consists of subjecting in I ore 1 to an attack by means of sulfuric acid 2, called return acid, mainly coming from electrolysis of zinc sulfate for precipitating zinc metal at the cathode, this electrolysis being not illustrated by the figures.

Said attack is known and may be made in one or two steps. In the case of a one-step attack, such as illustrated by Figure 1, an ore excess is used so as to obtain an attack ending in a neutral medium.

In II, there are separated by decantation, on the one hand a solution 3 containing zinc sulfate which, after a purification step (not shown) is sent to the above-mentioned electrolysis, and on the other hand, a residue 4, generally called neutral residue, having a concentration of 200 to 400 gr/1 of solids for example, which is subjected to a hot acid attack at a temperature generally between 85 and 95°C by means of more concentrated acid in order to mainly dissolve zinc ferrites. This acid attack of the neutral residue may take place in one or more counter-current steps.

Figure 1 shows a two-step attack. The latter comprises attack of the neutral residue 4 in HI by means of more concentrated acid 10 and a return acid admission 2, separa46818 - 6 i tion by decantation in IV of a dissolved iron-loaded solution 5 and submitting thickened portion 6 to a new attack in V, being generally more acid than in the preceding step, bymeans of return acid 2 and a make-up of fresh concentrated acid 7, for example about 98% acid. The reaction products are then subjected to a filtration in VI. The filtration cake is treated with wash water 8 and a solid residue 9, rich in lead and noble metals possibly present in the ore 1 is obtained.

A portion 10 of the filtrate, forming said more concentrated acid, is recycled to attack in III, while the other portion 11, diluted with wash water 8, at the same time as solution 5, at a resulting acidity of 5 to 250 gr/litre of H^SO^, advantageously of 30 to 60 gr/1 of H^SO^, preferably of 30 to 45 gr/1 of H^O^, according to the invention, are subjected to neutralisation in VII with a new amount 12 of oxidised zinc ore until a pH which is not above 1.5, preferably comprised between 0.5 and 1.5, is obtained.

By decantation, a residue 13 is separated in VIII, this residue being optionally sent back to the neutral residue attack in III for solubilizing zinc ferrites.

Dissolved silica, in the silicic acid state, is formed in IX, according to the invention, in the acid solution made of the overflow 14 coming from above-mentioned decantation, by addition of a basic enough silicate 15, such as calcium silicate, for example as a slag, while maintaining the pH value between 1.5 and 4.5, preferably between 2.5 and 3, thus precipitating a silicate residue containing impurities, in particular iron.

When carrying out this precipitation in oxidising - 7 medium, for example by admitting air or oxygen 16 and/or optionally an oxidising agent 17, such as Mn0_, Fe++ is +++ also precipitated, after oxidation to Fe ; which would be present in the solution.

The attack time in IX is generally about 1 to 4 hours and essentially depends on the presence of Fe++ ions.

The reaction product formed in IX is subjected to a decantation in X. The thickened portion 18 is then filtered in XI, followed by a treatment with wash water 19. The overflow 20 coming from decantation in X, as well as the filtrate 21 diluted with wash water 19 join the solution 3 which after a final purification known per se is submitted to electrolysis for zinc extraction.

The decantation in X could possibly be omitted in many cases since generally the concentration in solids is relatively high. The reaction product formed in X would be then immediately subjected to the filtration in XI.

The filtration cake 22 formed in XI is a solid silicate residue containing substantially all the impurities to be eliminated from the zinc sulfate solution to be submitted to electrolysis, in particular iron, antimony, arsenic, aluminium, tin and germanium which are generally present in a zinc ore.

Figure 2 relates to a variant of the embodiment illustrated by Figure 1,which is different from the latter due to the fact that the hereinabove described attack IX takes place without prior separation of a residue coming from a neutralisation VII.

In this variant, there is advantageously used in the above steps VII, IX, a basic enough silicate of an alkalineearth metal, in particular of calcium, as a slag, optionally together with zinc oxide and/or lime, so as to precipitate impurities, forming a solid silicate residue which is then separated in the same way as in the embodiment illustrated by Figure 1.

Hereinafter some laboratory tests are described, which were carried out to determine the chemical working conditions of the process according to the invention.

Example 1 300 ml of a lixiviation solution containing 10 gr/1 of Fe, 150 gr/1 of Zn and 5.4 gr/1 of Mg were added together with 29 gr of slag into a reactor containing 100 ml of the same lixiviation solution but without iron. The addition of reactants was regulated so as to obtain a pH > 1.5 during the reaction. The reaction mixture at 70—80°C was'stirred with a propeller.

The warm mixture was filtered under vacuum. The filtrate composition was as follows: 149.8 gr/1 of Zn? gr/1 of Mg; 0.05 gr/1 of Al; 0.7 gr/1 of Fe.

Example 2 300 ml of a lixiviation solution containing 10 gr of Fe, 150 gr of Zn, 8.4 gr of Mg and a trace of Cu were added together with 26 gr of slag into a reactor containing 100 ml of the same lixiviation solution but deprived of iron.

The addition of reactants was regulated so as to maintain a pH >1.5 during the reaction. The mixture was maintained in an oxidising condition by air injection. The whole was stirred at a temperature of 70—80°C.

The warm mixture was filtered under vacuum. The filtrate composition was as follows: 149.8 gr/1 of Zn; 8.3 gr/1 of Mg and 0.1 gr/1 of Al. >46818 Example 3 300 ml of a lixiviation solution containing 10 gr of Fe, 150 gr of Zn and 11.4 gr of Mg were added together with gr of slag into a reactor containing 100 ml of the same lixiviation solution but deprived of iron. The addition of reactants was regulated so as to maintain a pH 1.5 during the reaction. A small amount of organic or inorganic . o oxidising agent was added and the reaction medium at 70—80 C was stirred with a propeller. The warm mixture was filtered under vacuum. The filtrate composition was as follows: 149.8 gr/1 of Zn; 10.4 gr/1 of Mg and 0.2 gr/1 of Al.

These three examples allow the following conclusions to be drawn: -substantially the whole iron, as well divalent as trivalent, may be eliminated. —the Mg concentration is maintained below a determined limit.

A practical example of the process according to the invention on an industrial scale, and in continuous, based on the block diagram of Figure 2 in combination with that of Figure 1, was developed as follows. 19.5 metric tons/hour of roasted zinc ore having the following analysis: Zn Fe Pb A^2°3 60% 9% 1.77% 0.249% were contacted in I with 79 cubic metres/hour of return acid 2 having a concentration of 176 gr/1 of H^SO^.

The thickened residue in II containing zinc ferrites, at a concentration of 400 gr/1 of solids, is submitted in 818 - 10 III to a hot acid attack, at atemperature in the range of 90 to 95°C where it is contacted with recycled acid lo forming the main portion of the filtrate coming from the filtration in VI, and with a return acid admission 2, so as to obtain in III a mean acidity of about 30 gr/1. The thickened residue 6 deprived of a portion of ferrites is retreated in V with a more acid solution at a rate of 100 to 500 gr/1, formed by return acid 2 and an admission 7 of about 98% concentrated fresh sulfuric acid.

The residue 9 resulting from this attack is substantially deprived of the whole zinc and, on the contrary, contains the whole lead and the noble metals which may be present in the treated ores.

The other portion 11 of the filtrate, diluted.with wash water 8, is added to the overflow 5 so as to obtain an acid solution containing about 30 to 40 gr/1 of I^SO^.

This solution which contains, in addition to zinc, a high concentration of dissolved iron and of other noxious metals for electrolysis, is then submitted to the proper treatment according to the invention for precipitation of these iron and noxious metals, maintaining substantially the whole zinc in solution. According to the second variant illustrated by Figure 2, to this solution is added a calcium silicate 12, 15, taking care to bring up and maintain the pH between 1.5 and 4.5, If necessary by addition of other basic substances, such as lime, and optionally an oxidising agent 16, and air 17 is injected, which moreover improves the evaporation of the solution, thus reducing the presence of dissolved CaSO^.

The precipitation reaction of Fe+++ is very fast and -Ills substantially complete after 1 hour. It is, however, of interest to extend the reaction term in IX up to about 4 hours in order to oxidise Fe++ susceptible to issue for example from the presence of sulfur in the ore and the slag.

++ +++ As a matter of fact, oxidation of Fe to Fe is a relatively slow phenomenon. Care has to be taken at the end of the iron precipitation that the pH of the obtained solution is near that of the solution coming from the neutral lixiviation in I, so that the solution is directly usable in the purification to which it has to be subjected before electrolysis.

The relatively low acidity, of a pH of about 3 to 4, of the solution to be filtered in XI allows the use of less expensive materials than in known purification processes.

The efficiency of zinc extraction is about 99.5%.

It has been found that the silicate residue so obtained is quite well filterable and does not require intensive counter-current washing.

This residue is a very stable and non toxic product and furthermore presents properties such as, according to the invention, this product can be used as embankment, as aggregate in cements and as starting material for manufacturing building blocks.

This residue finds a particularly interesting application in the cement field due to the fact that it contains iron as a form allowing an easy handling on an industrial scale.

Moreover, the presence in such a product of gypsum coming for example from the neutralisation with slag of the sulfuric acid present in the acid solution, of silica mainly 6 8ΐθ - 12 issuing from slag but also susceptible to partially issue from the treated solution, and of alumina, makes the use of this product particularly useful in the cement field as additive to starting materials before introduction of the latter into the furnace or after passing through the latter.

Thus the invention also includes the product formed by the dried or undried filtration cake, resulting from the described process, and its valuable application, in particular in the cement field and obviously in any industry susceptible IO to consume a product containing the above-mentioned constituents .

It has also been found, such as resulting from hereinabove given laboratory tests, that the magnesium concentration in the solution to be subjected to zinc electrolysis may be 15 decreased by means of a coprecipitation with silicate derivatives and in particular with iron silicate.

As already mentioned, the silicic acid concentration in the medium is always very low and is substantially constant during the whole precipitation step.

Due to the high reactivity of silicates of alkaline, < earth metals, in particular of the slag, it is not necessary to operate the reaction by varying the pH conditions and thus to work in a batchwise manner. The silicic acid such as produced-is immediately precipitated by iron, carrying along other noxious metals to the later electrolysis, such as magnesium, as also already mentioned hereinabove.

The first neutralisation step to a pH not above 1.5 may advantageously be carried out with an oxidised or silicated zinc ore, which thus still increases the interest of the - 13 process. If zinc oxide is used, the quality of the final silicate residue is moreover considerably reduced.

Besides, the introduction of zinc silicate in this first step is a very practical means for extracting zinc from ores which are treated with difficulty by means of other processes which are specially provided for silicate ores.

As a matter of fact, it has been found that as long as the pH does not exceed 1.5, the reaction of zinc silicate dissolution is fast and complete, and the residue so formed is quite well filterable. This is not generally the case in solutions of a higher pH. As an example, at a pH of about 3, if the solution contains aluminium, the latter will tend to precipitate as a silico-alumina form at the surface of zinc silicate grains and will inhibit the dissolution reaction of the latter. It has been thus found that it is not possible to use, on an industrial scale, a silicated zinc ore instead of a silicate of alkaline-earth metals to cause the precipitation of metal impurities of solutions at a pH higher than 1.5 without substantially reducing the reaction rate and the zinc yield.

This problem does not exist with slag having a high calcium silicate content due to the high reactivity of this silicate at pHs between 1.5 and 4.5.

It is also to be noted that the invention is not limited to the purification of zinc sulfate solutions issuing from lixiviation of zinc ores or to the hereinabove described variants. Thus many other hydrometallurgy applications could be contemplated for precipitation of noxious impurities as a silicate residue, optionally according to variants of the process of the invention other than those explicitly described - 14 ' hereinbefore.

The acidity conditions in the precipitation of the silicate residue will have for example to be adapted to the kind of the impurities to be eliminated and to the kind of metal to be extracted by electrolysis or of the acid issued, or optionally to another extraction operation so as to obtain an as selective as possible separation.

Some steps of the variants of the process, such as illustrated by the Figures, could be divided in two or more stages. This could be for example the case for attack in IX where it could be first possible to essentially provide the formation of silicic acid by addition of basic substances, such as basic enough silicates and the separation of the basic substance which is not completely attacked at this stage, and then, in a second stage, the addition of for example an excess of slag or lime, in oxidising medium, in order to ensure a maximum iron precipitation.

Amongst some advantages of the process according to the invention, with respect to some conventional steps preceding those of the invention, for example in the treatment of residues issuing from lixiviation of zinc Ore, it is still to be noted a relative large purge of SO^ ions, improving the sulfuric acid control in the whole process allows concentrated solutions to be obtained with the benefit of heat produced by concentrated acid in dilution and during the attack.

Having regard to the preceding it can be said that the process according to the invention is more particularly useful for purification of solutions containing an amount of iron to be eliminated. - 15 The invention is in fact based on the finding that iron plays an important role in the chemical and precipitation phenomena occurring in the solution. Thus the precipitation mechanism is quite different in the elimination of silica from a silicated zinc ore, when dissolving the latter in a solution at a pH lower than 1.5 and then precipitating the dissolved silica by bringing the pH of this solution up to a value higher than 1.5 and by maintaining a relatively high temperature. In this case, silica solidifies by polymerisation, while in the process according to the invention, formation of iron silicate is more particularly contemplated, which does not require a high temperature.

It is overall important to note that unexpectedly it has been found that when the magnesium content in the solution to be purified exceeds a certain limit, a portion of this magnesium is coprecipitated with iron silicate formed. In such a manner, the presence of magnesium in the lime used to bring up the pH above 1.5 does not present any specific drawback.

Advantageously, according to the invention, in some cases it could even be useful to add an amount of iron ions to the solution after the pH of the latter has been brought above 1.5, if for example the silicate or magnesium content of the ore is relatively high, in order to form a sufficient amount of iron silicate which is necessary to obtain a quite well filterable and washable residue and a sufficient coprecipitation of other impurities existing in the solution. The iron amount added must preferably be substantially stoichiometrical with respect to silica which is dissolved or able to be dissolved in the solution. Λ6818

Claims (18)

1. CLAIMS:1. A process for removing one or more metal impurities from a solution containing one or more metals to be subsequently extracted from the solution,the process comprising adding a silicate to said solution so as to form in situ dissolved silicic acid and precipitating a solid silicate residue at a pH between 1.5 and 4.5, the solid silicate residue comprising said one or more metal impurities.

2. A process as claimed in Claim 1, wherein the silicate added is an alkaline-earth metal silicate.

3. A process according to Claim 2, wherein the silicate is calcium silicate.

4. A process as claimed in either of Claims 2 or 3, wherein the silicate is in the form of a slag.

5. A process as claimed in any of Claims 2 to 4, wherein to the solution, brought to or maintained at a pH between 1.5 and 4.5, is added the alkaline-earth metal silicate so as to form in situ dissolved silicic acid, the silicic acid being capable of causing, as fast as it is formed, precipitation of the solid silicate residue.

6. A process as claimed in any of Claims 1 to 5, wherein a neutralisation agent is used to bring the pH of the solution to or maintain the pH of the solution at between 1.5 and 4.5 and thus to cause precipitation of the solid silicate residue.

7. A process as claimed in any of Claims 1 to 6, wherein the pH of said solution is brought to or maintained at between 2.5 and 3 to cause precipitation of the solid silicate residue.

8. A process as claimed in any of Claims 1 to 7, wherein the solid silicate residue is precipitated under oxidising 4 6 818 - 17 conditions.

9. A process as claimed in any of Claims 1 to 8, wherein the solution is a zinc sulphate solution containing metal impurities and is derived from an acid attack on ferriferous residues.

10. A process as claimed in Claim 9, wherein the solution is neutralised by an oxidised and/or silicated zinc ore to bring the pH of the solution to a value not exceeding 1.5 before adding the alkaline-earth metal silicate and/or lime thereto.

11. A process according to Claim 10, wherein the solution is neutralised by the oxidised and/or silicated zinc ore to bring the pH of the solution to a value in the range of 0.5 to 1.5.

12. A process as claimed in Claim 10 or 11, wherein residue derived from the neutralisation is separated and subjected, together with the oxidised zinc ore, to an acid attack, whereby said solution of zinc sulphate is formed.

13. A process as claimed in any of Claims 1 to 12, wherein the solid silicate residue is caused to precipitate at a temperature between 40 and 85°C.

14. A process as claimed in any of Claims 1 to 13, wherein iron ions are added to the solution, at a pH between 1.5 and 4.5, in a substantially stoichiometrical amount so as to precipitate an iron silicate in said residue.

15. A process according to Claim 1, substantially as described herein with reference to Figure 1 or Figure 1 as modified by Figure 2 of the accompanying drawings.

16. A process according to Claim 1, substantially as - 18 hereinbefore described in any of the Examples.

17. A silicate residue obtained by a process according to any preceding claim.

18. Building material comprising a silicate residue according to Claim 17. F. R. KELLY & CO., AGENTS FOR THE APPLICANTS.