NO781608L - PROCEDURE FOR HYDROMETALLURGIC TREATMENT TO REMOVE POLLUTIONS FROM A SOLUTION CONTAINING DISSOLVED METALS - Google Patents

Description

Method of.hydrometallurgical treatment for

removal of contaminants from a solution containing dissolved metals.

The invention concerns a method for hydrometallurgical treatment for the removal of metallic contaminants from a solution containing dissolved metals to be extracted, and in particular for the removal of iron-containing contaminants from a zinc sulfate solution.

In hydrometallurgy there are mainly three industrial methods for selectively removing iron in the form of an easily decantable and filterable residue from zinc sulphate solutions resulting from attack on oxidized zinc ores, especially roasted zinc ores.

Depending on the method used, this residue essentially consists of jarosite, goethite or hematite.

However, these methods have the disadvantage that such residues

are generally of no commercial or industrial interest and even raise significant storage problems from the point of view of environmental pollution.

Furthermore, these methods require expensive and towering facilities

in consideration of the large quantities of goods processed and the residence times, they involve.

One of the main purposes of the invention is to provide a very simple process which makes it possible to produce a residue which contains impurities to be removed, and without the need for significant further treatment is directly suitable for certain industrial applications or as starting materials in the manufacture of industrial products, especially building materials.

The invention also relates to the residue obtained, particularly used as filler or as an aggregate in cement or concrete.

Other details and distinctive features of the invention will be apparent from the following description of special non-limiting examples of the invention, where reference is made to the drawing. Fig. 1 is a flowchart for a first embodiment of the method according to the invention. Fig. 2 is a block core for a variant of the embodiment of fig. 1.

In the figures, the same reference numbers are used for identical or similar elements.

As mentioned, the invention generally relates to a method for hydrometallurgical treatment for the removal of metallic contaminants from a solution containing dissolved metals to be extracted in metallic form.

More specifically, it relates to a method for removing,

metallic impurities, especially iron, from a zinc sulphate solution to be subjected to electrolysis for the extraction of zinc in the metallic state. This solution is generally obtained by the attack of sulfuric acid on a residue of oxidized zinc ore, especially roasted zinc ore,

originating from a slope of the ore.

The method is characterized by adding a silicate to the aforementioned solution, e.g. a silicate of alkaline earth metal or zinc, so that dissolved silicic acid is formed in situ and metallic impurities are precipitated with a pH value between 1.5

and 4.5, forming a well filterable and easily washable, silicate-containing residue.

If a zinc silicate is used, the dissolution will begin

with kept at a pH value of no more than 1.5 to quickly dissolve this zinc silicate, after which a silicate of alkaline earth metal, preferably Ca, and/or lime is added and the solution's pH value is kept between 1.5 and 4.5 to effect precipitation of the pollutants while forming a solid silicate residue, primarily consisting of iron silicate.

In certain cases, oxidized zinc ore can also be used

to bring the pH value to the aforementioned value of no more than 1.5, as the formation of silicic acid then takes place by adding ay et jQrd--a,lkali; metal silicate < which then precipitates the aforementioned contaminants while forming the aforementioned solid silicate residue when the pH value is kept between 1.5 and 4.5. This can be done by using a sufficiently basic silicate and/or an addition of lime.

In other cases, you can use an alkaline earth metal silicate, especially Ca alone, as such silicates at a pH value between 1.5 and 4.5 constantly dissolve to a sufficient extent and thus make it possible to precipitate the ions that must removed/after the dissolution of these silicates.

The actual amount of dissolved silicic acid in the solution always remains extremely reduced, as the silicate, as it dissolves, reacts practically instantaneously with the iron and any other metal ions to be removed, to form a solid silicate residue, when the pH is kept between 1, 5 and 4.5.

The preferred limits regarding the precipitation in the solution are between 2.5 and 3.

In order to avoid an excessive excess of dissolved silicic acid after precipitation, it is of interest not to exceed the stoichiometric amount of added silicate in relation to the contaminants to be removed.

Fig. 1 shows more concretely the different stages of a particular embodiment of the method according to the invention applied to the extraction of zinc from an oxidized zinc ore.

This method consists in subjecting the ore 1 to an attack with sulfuric acid 2, termed "return acid", which essentially originates from an electrolysis of zinc sulphate to precipitate the zinc metal at the cathode. The electrolysis itself is not shown in the drawing.

The above attack is known and can occur in one or more stages. In the case of a single stage attack, as shown in fig. 1, an excess of ore is used to achieve an attack that ends with a neutral medium.

By decanting at II, a solution 3 containing zinc sulphate is separated, which after a purification phase not shown is sent back to the above-mentioned electrolysis from a residue, generally termed 'neutral residue' 4, which has a concentration of e.g. 200 - 400 g/l of solids, and which is subjected to an acid attack in heat at a temperature generally between 85 and 95°C with more concentrated acid to substantially dissolve the zinc ferrites.This acid attack on the neutral residue may occur in one or more stages in countercurrent .

In fig. 1, a two-stage attack is implied there. It consists of

to attack the aforementioned neutral residue 4 at III with more concen-

acid 10 and an addition of reflux acid 2, separate a solution 5 mixed with dissolved iron^by decantation at IV and subject the thickened portion 6 to a new attack at V, generally more acid than in the preceding step, with reflux -acid 2 supplemented with fresh, concentrated sulfuric acid 7, e.g. of 98%. The reaction product is then subjected to filtration at VI. The filter cake is treated with washing water 8, and a solid residue 9 is obtained, rich in lead and precious metals that may occur in the ore 1.

A part 10 of the filtrate forming the aforementioned more concentrated acid/ is recycled to the attack at III, while another part 11 diluted with the washing water 8 and at the same time also the solution 5 with a resulting acidity of 5 - 250 g/lH2S04, advantageously 30'- 60 g/l H2S04 and preferably 30-45 g/l H2S04 according to the invention is subjected to a neutralization at VII with a new quantity 12 of oxidized zinc ore, until a pH value is obtained which is at most equal to 1.5 and preferably amounts to between 0 .5 and 1.5.

During decantation, a residue 13 is separated at VIII, which

in consideration of the solubility of zinc ferrites, the attack on neutral residues is possibly added at III.

Dissolved silicon in the form of silicic acid is, according to up- . the invention formed at IX in the acidic solution formed by the outgoing (overflowing) fraction 14 from the aforementioned decantation by adding a sufficient basic silicate 15, e.g. a calcium silicate, possibly in the form of slag, while the pH value is kept at a value between 1.5 and 4.5 and preferably of the order of magnitude 2.5 - 3, whereby a silicate residue containing the contaminants, especially iron, is precipitated.

By carrying out this precipitation in an oxidizing environment, e.g. by introducing air or oxygen 16 and/or possibly an oxidizing agent 17, e.g. Mn 0„, Fe++ can also be precipitated which may be present in the solution, after oxidation to Fe

The duration of the attack at IX is generally 1-4 hours and depends significantly on the presence of Fe<++> ions.

The reaction product formed at IX is subjected to a decantation at X. The thickened part 18 is then subjected to a filtration at XI, followed by a treatment with wash water 19. The overflow 20 from the decantation at X as well as that with wash water

19 diluted filtrate 21 unites again with the solution 3, which, after a final purification known per se, is subjected to electrolysis for the extraction of zinc.

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

The filter cake 22 formed at XI forms a solid silicate residue which practically contains all impurities to be removed from the zinc sulphate solution to be subjected to the electrolysis, particularly iron, antimony, arsenic, aluminium, tin and germanium, which generally occur in a zinc ore.

Fig. 2 relates to a variant of the embodiment shown in fig. 1. The difference is that the above-described attack by IX takes place without prior separation of a residue originating from a neutralization VII.

In this variant, a sufficiently basic silicate of an alkaline earth metal, particularly calcium, in the form of a slag, or also a silicate-containing zinc ore, optionally supplemented with slag, zinc oxide and/or lime, is advantageously used in the only stages VII, IX, as that the contaminants are precipitated to form a solid silicate residue, which is then separated in the same way as in the embodiment in fig. 1.

In the following, reference will be made to a number of laboratory experiments which have been carried out to determine the chemical working conditions of the method according to the invention.

Example 1

300 ml gradient solution containing 10 g/l Fe, 150 g/l

Zn and 5.4 g/l Mg were added simultaneously with 29 g of slag in a reactor containing 100 ml of the same gradient solution without iron. The addition of the reagents was adjusted to maintain a pH value of 5 during the reaction. The reaction mixture at 70-80°C

was stirred with a propeller.

The hot mixture was filtered under vacuum. The filtrate

had the following composition: 149.8 g/l Zn, 7 g/l Mg, 0.05 g/l Al and 0.7 g/l Fe.

Example 2

300 ml gradient solution containing 10 g Fe, 150 g Zn

and 8.4 g of Mg and traces of Cu were added simultaneously with 26 g of slag in a reactor containing 100 ml of the same leach solution free of iron.

The addition of reagents was regulated to maintain a pH

value £.1.5 during the reaction, and the mixture was kept oxidizing by blowing in air. The whole thing was stirred at a temperature of 70-80°C.

The hot solution was filtered under vacuum. The filtrate

had the following composition: 149.8 g/l Zn, 8.3 g/l Mg and 0.1 g/l Al.

Example 3

300 ml of leach solution containing 10 g of Fe, 150 g of Zn and 11.4 g of Mg was added simultaneously with 28 g of slag in a reactor containing 100 ml of the leach solution free of iron. The addition of the reagents was regulated while maintaining a pH value > 1.5 during the reaction. A small amount of organic or inorganic oxidizing agent was added, and the reaction mixture was stirred at 70-80°C with a propeller. The hot mixture was filtered under vacuum,

and the filtrate had the following composition: 149.8 g/l Zn, 10.4 g/l Mg and 0.2 g/l Al.

These three examples make it possible to draw the following conclusions:

- practically all iron, both divalent and trivalent,

can be removed.

- the magnesium concentration is kept below a certain limit.

An embodiment of the method according to the invention

on an industrial scale and with continuous operation on the basis of the diagram in fig. 2 in combination with that in fig. 1, proceeds as follows:

19.5 tonnes/h roasted zinc ore with the following analysis:

was at I brought into contact with 79 m 3 /h return acid 2 with a concentration of 176 g/l fc^SO^.

The thickened residue at II containing zinc ferrites and with a solids concentration of 400 g/l is fed to the acid attack at III at a temperature of the order of 90-95°C and is here brought into contact with recycled acid 10, which makes up the majority of the filtrate from the filtration at VI, and with an addition of return acid 2, so that at III an average acidity of the order of 30 g/l is obtained. The thickened residue 6, which has been partly freed from the ferrites, is again treated at V with a more acidic solution with an acidity of 100-500 g/l, formed from return acid 2 and an addition 7 of fresh concentrated sulfuric acid of approx. 98%.

The residue 9 from this attack is practically freed from

all zinc, but contains all lead and any precious metals occurring in the treated ores.

The other part 11 of the filtrate, diluted with the washing water 8, is added to the overflow liquid 5, so that an acidic solution containing I^SO^i in an amount of the order of 30-40 g/l is obtained.

This solution, which besides zinc contains a strong concentration of dissolved iron and other metals which are harmful to the electrolysis, is then subjected to the simple treatment according to the invention for the precipitation of this iron and other harmful metals, while practically all the zinc is kept in solution. According to the second variant illustrated in fig. 2, calcium sulfate 12, 15 is added to this solution, while taking care to keep the pH value between 1.5 and 4.5, if necessary by adding other basic substances such as lime, and possibly an oxidizing agent 16; and also blows in oxidation air 17, which also favors the evaporation and thus the natural cooling of the solution, so the amount of dissolved CaSO present. is reduced.

The reaction with precipitation of Fe +++ is very fast and is practically finished after 1 hour. However, there is an interest in extending the reaction time at IX up to approximately 4 hours in order to oxidize the Fe<++> which e.g. can occur as a result of the presence of sulfur in the ore and slag, as the oxidation of Fe<++> to Fe<+++> is a relatively slow process. It is ensured that the pH value of the solution obtained at the end of the iron precipitation is close to the pH value of the solution from the neutral gradient at I, so that it becomes directly usable in the cleaning operation to which it is to be subjected before the electrolysis.

The relatively weak acidity of the order of pH = 3 - 4

of the solution to be filtered at XI, makes it possible to use

less expensive materials than in the known cleaning processes. The yield of the zinc extraction is of the order of 99.5%.

It was determined that the silicate residue obtained is very well filterable and allows intensive washing in countercurrent.

This residue forms a very stable and non-toxic product

and also has such properties that, in accordance with the invention, it can be used as filler, aggregate in cement and starting material for the production of building materials in block form.

This residue finds a particularly interesting application in cement production, since it contains iron in a form that allows easy handling on an industrial scale.

Furthermore, the fact that this product contains gypsum, particularly originating from the neutralization jned s.la,gg of the sulfuric acid in the acidic solution, silicic acid, which for the most part originates from the slag, but can also partly originate from the treated solution, as well as aluminum oxide, makes the use of the product is particularly useful in cement production as an addition to the starting materials before they are introduced into the kiln, or after passing through the kiln.

The invention thus also extends to the product which is made up of the dried or non-dried filter cake produced by the described method, as well as to its useful use, particularly in cement production and of course in any industry which can utilize a product containing the said components.

Finally, as can be seen from the above-mentioned laboratory experiments, it has been established that the magnesium concentration in the solution to be subjected to zinc electrolysis can be reduced, thanks to the simultaneous precipitation with silicate derivatives and especially iron silicate.

As already pointed out above, the concentration of silicic acid

in the environment is always very low and is practically constant during the entire shedding phase.

In view of the strong reactivity of the alkaline earth metal silicates and especially of the slag, it is not necessary to operate the reaction under varying conditions with regard to pH and thus work discontinuously. The silicic acid produced is immediately precipitated by the iron and at the same time takes with it other metals that are harmful to the later electrolysis, such as magnesium, also as pointed out above.

The first neutralization step up to a pH value of about 1.5 can advantageously take place using oxidized or silicified zinc ore, which further increases the value of the process.

If zinc oxide is used, the amount of final silicate residue is also considerably reduced.

Incidentally, the introduction of zinc silicate at this first stage can constitute a very practical means of extracting the zinc from such ores which are difficult to process by other methods specifically designed for silicate-containing ores.

Thus, it has been determined that the reaction to dissolve the zinc silicate, as long as the pH value of the environment does not exceed 1.5, is fast and complete, and the residue formed is very easily filterable. This is not generally the case in solutions with a higher pH value. E.g. any aluminum present in the solution at a pH value of the order of magnitude 3 will have a tendency to precipitate in the form of silicon-aluminium oxide on the surface of the grains of zinc silicate and inhibit its dissolution. It has thus been determined that it is not possible on an industrial scale to use a zinc silicate ore instead of a silicate of alkaline earth metals to effect precipitation of the metallic contaminants from solutions with a pH value above 1.5 without reducing the reaction rate and the zinc yield significantly.

This problem does not occur with slag with a high content of calcium silicate, thanks to the high reactivity of this silicate at pH values between 1.5 and 4.5.

Finally, it is of interest to note that the invention is not limited to the purification of zinc sulphate solutions originating from the leaching of zinc ores or to variants of the described method. Thus, other applications in hydrometallurgy could also be envisaged for the precipitation of harmful pollutants in the form of a silicate residue, and possibly with other variants of the method according to the invention than those described in particular above.

The conditions with regard to acidity during the precipitation of the silicate residue should e.g. adapted to the nature of the contaminants to be removed, and the nature of the metal to be extracted by electrolysis, or the acid used, or possibly adapted to another extraction technique to provide the most selective possible

separation.

Certain phases of the variants of the method illustrated in the drawing can be divided into two or more steps.

This would e.g. be the case for the attack at IX, where man

would be able to aim at, first significantly, forming silicic acid by adding basic substances, such as e.g. sufficiently basic silicates, and separating the basic substance which is incompletely attacked in this step, as well as in another step adding an excess of e.g. slag or lime in an oxidizing environment to ensure maximum precipitation of the iron.

Among the advantages the method according to the invention entails

for certain conventional stages preceding those with which the invention is concerned, e.g. in the treatment of residues originating from the leaching of zinc ore, a relatively significant purification for SO^ ions can also be mentioned, which facilitates the control of the sulfuric acid in the process as a whole and makes it possible to obtain concentrated solutions, taking advantage of the heat produced by the concentrated acid in dilution and during the attack.

As can be seen from the foregoing, the method according to the invention is above all useful for cleaning solutions containing a certain amount of iron to be removed.

The invention is actually based on that realization

that the iron plays a significant role in the chemical and precipitation phenomena that occur in the solution. Thus, the precipitation mechanism becomes completely different when silicon is removed from a silicate-containing zinc ore by dissolving this in a solution with a pH value below 1.5 and then precipitation of dissolved silicic acid, the pH value of this solution being brought to a value above 1. 5, while a relatively high temperature is maintained. In this case, the silicic acid hardens by polymerisation, while in the method according to the invention, one aims above all at the formation of iron silicate, which does not require a high temperature. It is particularly important to note that the completely unforeseen situation has been established that, when the magnesium content of the solution to be purified exceeds a certain limit, a partial precipitation of magnesium takes place together with the iron silicate formed. Thus, the presence of magnesium in the lime, which is used to bring the pH value above 1.5, hardly represents any disadvantage. j It will be advantageous in certain cases according to the invention

be useful to add a certain amount of iron ions to the aforementioned solution after its pH value has been brought above 1.5, e.g. if the content of silicate or magnesium in the ore is relatively high, in order to obtain a sufficient amount of iron silicate that is needed to obtain a good filterable and washable residue and a sufficient

precipitation of other contaminants occurring in the solution. The added amount of iron should preferably be largely stoichiometric in relation to silicic acid as a solvent or ex ist to be able to dissolve in the solution.

Claims (15)

1. Method for hydrometallurgical treatment for the removal of metallic contaminants from a solution containing metals to be extracted, in particular from a solution originating from an acid attack on ferrous residues, characterized in that it consists in adding a silicate to this solution, so that dissolved silicic acid is formed in situ, and the metallic impurities precipitate out at a pH value between 1.5 and 4.5, forming a solid silicate residue. 2. Method as stated in claim 1, characterized in that a silicate of alkaline earth metals, particularly of calcium, is used. 3. Procedure as stated in claim 2, characterized in that a silicate in the form of slag is used. 4. Method as specified in claim 2 or 3, characterized in that the solution is added, which is kept at a pH value between 1.5 and 4.5, an alkaline earth metal silicate, so that dissolved silicic acid is formed in situ which, as it is formed, can precipitate the aforementioned pollutants while forming the solid silicate residue. 5. Method as stated in one of claims 1 - 4, characterized in that a neutralizing agent such as lime, zinc oxide or alkaline earth metal silicate is used to bring or maintain the pH value between the above-mentioned limits and thus to precipitate the contaminants while forming a solid silicate residue. 6. Method as stated in one of the claims 1-5, characterized in that the pH value of the said solution is brought or kept between 2.5 and 3 for the precipitation of pollutants. 7. Method as stated in one of the claims 1 - 6, characterized in that the pollutants are precipitated in an oxide-free environment. 8. Method as stated in one of the claims 1 - 7, characterized in that a solution of zinc sulphate containing metallic impurities originating from an acidic attack on iron-containing residues, that a silicate is added to this solution, and that the above-mentioned pollutants precipitate out at a pH value between 1.5 and 4.5, forming a solid silicate residue. 9. Procedure as stated in claim 8, characterized in that the solution is neutralized using an oxidized and/or silicified zinc ore to a pH value of no more than 1.5 and preferably between 0.5 and 1.5 before a silicate of alkaline earth metals and/or lime is added to precipitate the contaminants in this solution at a pH value between 1.5 and 4.5, forming a solid silicate residue. 10. Method as stated in claim 9, characterized in that the residue from the aforementioned neutralization is separated and introduced in a phase for acid attack on the residue of oxidized zinc ore with the aim of forming the above-mentioned solution of zinc sulfate. 11. Method as stated in one of the preceding claims, characterized in that the precipitation of the pollutants is effected at a temperature between 40 and 85°C. 12. Method as stated in one of the preceding claims, characterized in that iron ions are added to the solution at a pH value between 1.5 and 4.5 in a largely stoichiometric amount to precipitate out in the form of iron silicate in the said residue. 13. Method of hydrometallurgical treatment for the extraction of pollutants from a solution containing dissolved metals, as described above or shown in the drawing. 14. Silicate residue obtained by precipitation of a process as described above. 15. Building material, such as filler and aggregate for cement or concrete, obtained from the above-mentioned silicate residue.