Process For Treatment Of Mattes And Sulphurated Nickel Concentrates

Abstract

Nickel containing concentrates, particularly nickel matte, containing by weight 55 to 75 percent nickel, 10 to 40 percent sulphur and a minor proportion of secondary metals in powder or pulp form are subjected to an oxidizing lixiviation with nitric acid preferably in the presence of oxygen, nitrous vapors formed being regenerated to nitric acid and recycled. The solution from the oxidizing lixiviation is sulphated to substantially replace nitrate by sulphate; secondary metals, in particular iron, copper and cobalt are removed and pure hydrated nickel sulphate is crystallized from the solution. Preferably nickel hydroxide and carbonate are precipitated from the mother liquor from the crystallization of nickel sulphate and are used in the removal of iron and cobalt.

Description

This invention relates to a process for the hydrometallurgical treatment of sulphurated nickel concentrates, especially the nickel mattes obtained after conventional pyrometallurgical treatment of ores, with a view to recovering from them not only the nickel but also other secondary metals of commercial significance, more particularly cobalt and copper which are usually associated with nickel in concentrates of this kind.

Sulphurated nickel concentrates to which the treatment according to the invention is applicable include those which generally contain from 55 to 75 percent of nickel and from 10 to 40 percent of sulphur, these percentages being given by weight, like all those which will follow in the present specification.

Examples of mattes capable of undergoing the treatment according to the invention include the mattes produced by the pyrometallurgical process worked at the DONIAMBO factory of LE NICKEL, NOUMEA (New Caledonia), emanating from the sulfurizing fusion in a low-shaft furnace of a nickel oxide ore called garnierite. The crude matter thus obtained is refined in converters of the BESSEMER type wherein the iron is burnt out by an air blast and scorified by the silica introduced during blasting. The mattes in question generally contain from 22 to 28 percent of sulphur, from 71 to 73 percent of nickel, from 1.5 to 5 percent of iron and from 2 to 5 percent of cobalt, in addition to small quantities of other elements including copper, manganese, aluminum, magnesium and silica.

Examples of hydrometallurgically sulphurized concentrates lending themselves to the treatment according to the invention include those obtained by precipitation with hydrogen sulfide from sulphuric solutions which have been used for the lixiviation of laterites, for example, the sulphurized concentrates manufactured in the Cuban MOA-BAY works. A material of this kind usually has the following composition on a dry weight basis:

nickel 52 to 68% cobalt 5 to 6% copper 0.5 to 1% iron 1 to 1.5% zinc 1 to 1.5% sulphur 30 to 38%

The other impurities consist of manganese, magnesia, alkaline-earth metals, silica, in a total quantity not exceeding 1 percent.

In addition, the initial products capable of being treated in accordance with the invention are not necessarily intermediate products obtained by an initial concentration of ores. For example, they can be mixtures emanating by precipitation with hydrogen sulfide from impure nickel solutions as are obtained in processes for regenerating nickel-plating baths or hydrogenation catalysts, or even materials emanating from the pyrometallurgical treatment of slags with sulphur with a view to concentrating the metals present in them.

It has long been known to subject sulphurized concentrates of the aforementioned type to an acid or basic lixiviation to dissolve the metals, generally in the form of salts, to enable them to be subsequently extracted in pure form. There are two general methods of treatment in common use, namely ammonical lixiviation in which a solution of ammonia and ammonium carbonate is used as the lixiviating agent, and sulphuric acid lixiviation. Both these methods have the disadvantage that the initial concentrate has to be subjected to an attack which is as long as it is difficult and which in addition usually has to be carried out under pressure.

The ammonical lixiviation method also has the disadvantage of converting the sulphur present into complexes which are all the more difficult to separate if they consist of thionates or polythionates. A second stage of oxidation under pressure or of hydrolysis by prolonged boiling is necessary for the reconversion of these complexes. In addition, the metals are extracted in the form of amines and have to be collected in ammonia recovery and distillation systems which use up a considerable amount of energy.

Apart from difficulties arising out of the use of sulphuric acid in dilute form under pressure, the sulphuric lixiviation process does not lead to high yields and the lixiviation stages have to be increased in number if it is desired to obtain economically acceptable yields.

The present invention provides an improved process for the hydrometallurgical treatment of mattes and concentrates of the aforementioned type which obviates the disadvantages affecting conventional processes, is easier and less expensive to carry into operation and gives a higher yield.

The process according to the invention comprises the steps of:

i. subjecting the finely comminuted nickel containing concentrate to an oxidizing lixiviation with nitric acid, nitrous vapours formed in this stage being regenerated into nitric acid which is recycled;

ii. subjecting the solution produced from the oxidizing lixiviation stage to hydration or sulphation to substantially replace nitrate in the solution by sulphate;

iii. subjecting the solution produced from the hydration or sulphation stage to successive treatment stages in which the secondary metals are separated to leave a solution with nickel as substantially the only remaining cation; and

iv. recovering nickel from the residual solution by crystallization in the form of pure hydrated nickel sulphate.

If necessary the nickel containing concentrate is subjected to an initial physical treatment to convert it to the finely comminuted form suitable for the oxidizing lixiviation.

The most important aspect of the invention is the oxidizing lixiviation with nitric acid in which the nitric acid performs the simple function of an oxygen-transfer agent. This stage is preferably carried out in the presence of a free-oxygen containing gas. The main advantage of this method is embodied in the fact that the oxidation reactions involving the nitric acid are exothermic with the result that the process only requires a minimum supply of energy from outside so that it is particularly economic to operate.

The nitric lixiviation is preferably carried out at atmospheric pressure with stirring in air at a temperature below the boiling temperature of nitric acid. All these features contribute to the economy of the process according to the invention from a practical point of view.

Any conventional methods may be used for the subsequent separation of the secondary metals, in particular iron, copper and cobalt. For example, the cobalt may be precipitated in the form of cobalt (III) hydroxide by treatment with an alkali metal or alkaline-earth metal hypochlorites or with chlorine in the presence of nickel (II) hydroxide or nickel carbonate. According to one important aspect of the invention, however, the secondary metals are preferably separated by operations which do not totally reduce the nickel content of the sulphated brine, namely separation of the iron by the action of nickel (II) carbonate obtained from the mother liquors, separation of the copper by cementation through the addition of nickel powder, and separation of the cobalt by the addition of nickel (III) hydroxide obtained by treating the residual mother liquors. Thus the process according to the invention may be referred to as a "loop" process in which the nickel which is not recovered in the form of pure sulphate is continuously recycled, which is a factor of economic significance.

So far as the preparatory physical treatment is concerned, to transform solid products into a suspension, for example mattes, the starting material is initially crushed to convert it into a relatively fine powder. Crushing should not be over done because on the one hand the oxidizing lixiviation with nitric acid is extremely effective and on the other hand introduction of an excessively fine material into the nitric acid gives rise to the formation of a foam or froth which interferes with the normal operation of any installation. It has been found that the best results are obtained with a powder 80 percent of whose grains are between 100 and 200 microns in size.

In the case of concentrates obtained by wet precipitation, the preparatory phase merely comprises dispersion in water to convert them into a homogeneous pulp which is fluent enough to be continuously introduced into the reaction medium.

The accompanying drawing shows a flow-sheet illustrating the process according to the invention in its preferred embodiment in which the nickel from the residual mother liquors is recycled in the form of nickel carbonate and nickel (III) hydroxide.

As shown in the flow-sheet following the initial physical treatment, the process comprises the following stages:

introduction of the finely comminuted concentrate into a nitric acid solution with cyclic recovery of the nitrous vapours produced and their reconversion into nitric acid by oxidation in air,

adjustment of the sulphur: metal ratio to obtain a true sulphate brine by addition of sulphuric acid,

separation by filtration of any insoluble materials resulting from these treatments,

precipitation of iron with nickel carbonate and separation by filtration of the ferric carbonate formed

separation of the copper by cementation with powdered nickel and recovery by filtration of the copper precipitated,

elimination of the cobalt by double decomposition with nickel (III) or nickel carbonate converting the cobalt sulphate into insoluble cobalt (III) hydroxide or cobalt carbonate and separating these precipitates by filtration,

separation of pure nickel in sulphate form by crystallization after concentration of the brine; the mother liquor containing traces of nitrate which have been formed are themselves converted by double decomposition into either nickel (II) carbonate and nickel (III) carbonate or into nickel (III) hydroxide which is returned to the circuit.

The compounds to be treated are introduced into a nitric acid solution whose concentration may vary within certain limits, although the best results are obtained with solutions whose concentration is around 50 percent.

The reactor is kept at a temperature above 60.degree. C., and below the boiling point of nitric acid, preferably 80.degree. C. A vigorous current of air bubbles through the reaction medium being necessary to accelerate elimination of the nitrous gases. The nitrous gases are introduced into an absorption and oxidation tower which reconverts them into a nitric acid solution which is returned to the circuit. This installation is similar to that used in nitric acid manufacturing plants.

The main chemical reactions which underlie these operations are as follows:

1. M.sup.+.sup.+S + 4HNO.sub.3 .fwdarw.M.sup.+.sup.+SO.sub.4 + 2N.sub.2 O.sub.3 .uparw.+ 2H.sub.2 O

2. 3N.sub.2 O.sub.3 + H.sub.2 O.fwdarw.2HNO.sub.3 + 4NO

3. 4NO + O.sub.2 .fwdarw.2N.sub.2 O.sub.3

and the cycle begins again.

The brine resulting from this treatment is subjected to a hydration treatment and, optionally to a, sulphuration treatment. This treatment comprises adding the sulphuric acid required to neutralize the basic sulphates if an initial material unsaturated with sulphur is used, for example the nickel mattes from New Caledonia. The effect of the sulphuric acid is instantaneous when the temperature is around the boiling point. This sulphation treatment also has a secondary function, namely to displace the nitrates because they are not very stable at the temperature at which this treatment is carried out.

Assuming that a nickel matte has the approximate formula Ni.sub.3 S.sub.2 (26.7 percent of sulphur), nitric oxidation in the presence with air will lead to a product of the formula:

2NiSO.sub.4.sup.. Ni(OH).sub.2.

This material is converted into the neutral sulphate by the following reaction:

2NiSO.sub.4.sup.. Ni(OH).sub.2 + H.sub.2 SO.sub.4 .fwdarw.3NiSO.sub.4 + 2H.sub.2 O

On the other hand, a small proportion of this subsulphide is converted into nickel nitrate Ni(NO.sub.3).sub.2. Under the operating conditions selected, this gives rise to a displacement in accordance with the simplified reaction:

Ni(HO.sub.3).sub.2 Ni(NO.sub. H.sub.2 SO.sub.4 .fwdarw.NiSO.sub.4 + 2HNO.sub.3.

The solution which results from this treatment and which is substantially denitrified is subjected to a filtration whose function it is to eliminate the small amount of residual sulphur, that proportion of the iron hydroxide already precipitated and above all insoluble refractory components such as silica in particular.

The filtrate is then delivered into a tank where its pH-value is adjusted with a view to precipitating the ferric iron. This operation is best carried out by adding nickel carbonate and the iron which was in the ferric state precipitates completely at about pH 4.

The reaction can be formulated as follows:

Fe.sub.2 (SO.sub.4).sub.3 + 3NiCO.sub.3 + 3H.sub.2 O.fwdarw.3NiSO.sub.4 + 3CO.sub.2 + 2Fe(OH).sub.3 .uparw.

or

Fe.sub.2 (SO.sub.4).sub.3 + 3NiCO.sub.3 .fwdarw.3NiSO.sub.4 + Fe.sub.2 (CO.sub.3).sub.3 .uparw.

The ferric compound thus formed is then separated by filtration. The resulting solution is then treated with nickel (III) hydroxide to precipitate the cobalt (III) hydroxide in accordance with the following reaction:

CoSO.sub.4 + Ni(OH).sub.3 .fwdarw.NiSO.sub.4 + Co(OH).sub.3 .uparw.

In the preferred embodiment of the invention in which nickel (II) carbonate is used for precipitating the iron and nickel (III) hydroxide for precipitating the cobalt, filtration of the cobalt (III) hydroxide leaves a saline solution whose cation is solely nickel whilst the anions consist of a mixture of nitrate and sulphate in which it is substantially the sulphate component which predominates. The final purification stage comprises a selective crystallization following evaporation of the brine. In effect, the nickel sulphate remains much more insoluble than the corresponding nitrate and almost all the nickel is readily separated in the form of a pure hydrated sulphate. The mother liquors which contain all the nitrate and a little sulphate are then recovered in the form of nickel carbonate and hydroxide which are returned to the circuit and which are used to precipitate the iron and the cobalt, respectively. The reactions involved are as follows:

1. Ni(NO.sub.3).sub.2 + Na.sub.2 CO.sub.3 .fwdarw.2NaNO.sub.3 + NiCO.sub.3 .uparw.

The insoluble nickel carbonate is filtered and washed until the sodium ions have been removed.

2. 2Ni(NO.sub.3).sub.2 + NaOCl+4NaOH + H.sub.2 O.fwdarw.2Ni(OH).sub.3 + NaCl + 4NaNO.sub.3

The nickel (III) hydroxide is filtered and washed in boiling water until the chlorine and sodium ions have disappeared. It is then used to precipitate the cobalt. Thus it can be seen that the cycle is completely in the form of a "loop". Naturally the nickel sulphate obtained in a perfectly pure form can be converted into pure nickel by conventional processes.

The invention is illustrated by the following Examples:

EXAMPLE 1

The apparatus used consists of a three-liter-capacity glass reactor equipped with a stirrer and a thermometer and also surmounted by a condenser which opens into a column packed with Raschig rings comprising several stages separated by nozzles for the introduction of air. The reactor is also provided with means for introducing the reactants. The reactor itself is placed in a thermostatically controlled cabinet by means of which the reacting mixture can be heated and cooled.

The packed column is continuously washed with a recycled solution, the downward circulation of the liquid being ensured by means of a pump. 919.5 g of 10.95N nitric acid of specific gravity 1.30 are introduced into the reactor. After the acid has been introduced, 20 liters of air per hour are bubbled through it. The temperature is raised to 80.degree. C. and 300 g of nickel matte are introduced over a period of 1 hour and 40 minutes, the temperature being kept at 90.degree. C. throughout the entire operation. The matte treated has the following weight analysis:

Ni 71.88% Co 2.48% Fe 5.20% Cu 0.028% Mn 0.005% Insoluble components such as metal silicates and silica 0.243% S 20.10%

granulometry:

eighty percent of the grains are unable to pass through a 100.mu. screen while all of them pass through a 200.mu. screen.

After the reactants have been introduced, stirring is continued and the temperature kept at 90.degree. C. over a period of 21/2 hours. Eight hundred and ninety ml of a brownish-colored sludge-like suspension are obtained. Over a period of 1 hours and 45 minutes 367.7 g of 35N sulphuric acid with a specific gravity of 1.86 are added to this suspension. The temperature is increased to 102.degree. C. and deep red vapours escape in abundance towards the nitric acid regeneration column.

On completion of this operation, 461 g of water are added and the suspension is filtered in a Buchner funnel. During this operation the temperature is not reduced below 95.degree. C. The residue collected on the filter is washed repeatedly with 200 g of boiling water and all the filtrates are collected and subjected to analysis. They weigh 1,970 g, have a specific gravity of 1.349 and a volume of 1,670 ml. After their volume has been increased to 2 liters by the addition of water at 20.degree. C., their composition is as follows:

Ni 104.16 g/l Co 3.325 g/l Fe 7.625 g/l Cu 0.054 g/l NO.sub.3 1.03 (equivalent/ liter)

The solid residue weighs 13 g and corresponds to the following weight analysis:

S 62.8% Fe 0.82% Ni 0.43% SiO.sub.2 5.58%

a portion of the preceding solution (1815 ml) is neutralized to pH 5.9 by the addition of 230 g of nickel carbonate weighed dry but dispersed in 694 g of water. Accordingly the iron is precipitated out as Fe(OH).sub.3 and removed by filtration. This operation is carried out at a temperature of 50.degree. to 60.degree. C. with vigorous stirring. It is completed by the addition of 99.6 mg of ultra-fine nickel powder (98.7 percent pure) in order to precipitate the copper. In addition the oxidation-reduction potential is adjusted by the addition of 8.25 ml of N/50 KMnO.sub.4 so as to eliminate every risk of leaving iron or manganese in solution.

The dispersion obtained is filtered in a Buchner funnel at room temperature. There is obtained on the one hand a clear green solution of nickel and cobalt in the form of sulfates weighing 2,500 g with a specific gravity of 1.210 and on the other hand a precipitate weighing 791 g. The precipitate is washed with 1,576 g of boiling water and is then filtered which leads to two fractions:

i. a liquid fraction with a volume of 1985 ml containing 9.816 g/liter of nickel; and

ii. a solid fraction weighing 654.5 g and containing on a dry basis 2.28 percent of nickel, water content 55.3 percent.

The concentrated solution emanating from the first filtration is subjected for a period of 45 minutes at 95.degree. C. to vigorous stirring in the presence of 240 g of nickel (III) hydrate pulp containing 6.57 percent of nickel. During this operation, the pH-value is kept at about 4 by the addition of 300 ml of normal sulphuric acid. Following filtration in a Buchner and repeated washing of the precipitate with boiling water, 3,994 ml of a pure green liquor corresponding to the following weight analysis are finally obtained.:

Ni 70.37 g/l Co 0.025 g/l Fe <0.001 g/l Cu <0.001 g/l Mn immeasurable traces Insoluble component less than 0.001 g/l HNO.sub.3 0.225 equivalent/liter

The final precipitate weighs 568 g and contains 3.45 percent of nickel.

It was found that the Ni:Fe ratio initially 13.8 exceeds 70,000 in the purified liquor, while the Ni:Co ratio which was 28.75 almost reaches 3,000.

On the other hand, it was found that the other impurities such as copper, manganese and silica were eliminated.

If a complete reckoning is made, allowing for samples taken, it will be found that 212 g of nickel in matte form, 125.7 g of nickel in carbonate form, and 17.3 g of nickel in hydroxide form, making a total of 355 g, were introduced. Of this total 295 g were found in the completely purified solution, 9.5 g in the final cobalt (III) hydroxide precipitate, 7.25 g in the ferric residues and 21.65 g in the liquors used to wash the ferric residues.

Finally, the matte conversion yield during the oxidizing lixiviation is remarkably high at 99.9 percent.

EXAMPLE 2

On this occasion, a large laboratory apparatus in the form of a 100-liter capacity glass reactor is used. The treatment is carried out on 15.25 kg of a sulphurated compound emanating from the hydrometallurgical recovery of laterites having the following weight analysis:

Ni 51.050% Co 3.404% Fe 2.458% Cu 0.0035% S total 30.174%

of which S in the form of

sulphide 28.52% Mn 0.270% SiO.sub.2 0.55% Ca 0.13% Mg 0.055% Al.sub.2 O.sub.3 2.28% Cr 0.095% H.sub.2 O 1.71%

oxidation is carried out in 31/2 hours with 49.42 liters of exactly 11N commercial nitric acid. Air is bubbled through the reactor at a rate of 6 cubic meters per hour whilst its contents are stirred with a 3-blade propellor agitator turning at 600 r.p.m. The temperature at the beginning of introduction of the matte was 54.degree. C. It rises quickly, i.e., in less than 15 minutes, to around 75.degree. C. and is kept at this value throughout the introduction period. After all the sulphurated compound has been introduced, the contents of the reactor are boiled for 30 minutes, followed by the addition of 30 liters of boiling water to dilute the reaction product. The residue weighing 1,290 g is filtered. It has the following weight analysis:

H.sub.2 O 51.65% Ni 0.040% Co 0.002% S total 37.23% Fe 0.625% Cu 0.001% Mg 0.225% Mn nil Al.sub.2 O.sub.3 0.14% SiO.sub.2 6.32%

the solution with a volume of 71.25 liters contains 101.06 g/l of Ni + Co and less than 40 g/liter of nitric acid.

The 55 liters of washing liquors of the precipitate contain 8.3 g/liter of Ni + Co. It can be seen by analyzing the operation that all the nickel and cobalt have been completely dissolved whilst aluminum, iron and magnesium have not been dissolved. As in the preceding Example, the solution is subjected to a treatment, in which it is deprived of iron, copper and cobalt before being concentrated and crystallized. Crystallization is carried out by evaporation and most of the nickel is obtained in the form of crystalline sulphate. The crystals have the following weight analysis:

Ni 21.11% Co 0.011% Fe 0.001% Cu 0.002% Mg 0.028% Ca 0.18% SiO.sub.2 0.015% Al.sub.2 O.sub.3 0.007% HNO.sub.3 0.03% Mn--Cr--Pb nil

By contrast the mother liquors have the following analysis:

Ni 157.37 g/l Co 0.014 g/l Fe 0.01 g/l Cu 0.001 g/l Mg 0.043 g/l Ca 2.42 g/l SiO.sub.2 0.21 g/l Al.sub.2 O.sub.3 0.049 g/l HNO.sub.3 145.03 g/l

These mother liquors can be converted by precipitation with sodium carbonate, caustic soda and hypochlorite into carbonate and nickelous and nickelic hydroxide available for an operation of the same type, these reactants being used to eliminate the iron and the cobalt.

EXAMPLE 3

4.3 Kg per hour of a nickel matte having the following weight analysis:

Ni + Co (Combined) 74.98% Co alone 1.6% Fe 1.01% S 22.00%

and 12 liters per hour of 50.7 percent recycled nitric acid, are continuously introduced into a 100-liter capacity reactor situated at the head of a series of apparatus arranged in a cascade and consisting of components specially suited to each operation, more particularly: oxidation, sulphation and dentrification, hydration, elimination or iron and manganese, elimination of copper and cobalt, evaporation and crystallization.

In addition, air is continuously bubbled through the reaction mixture at a rate of 6 cubic meters per hour whilst the contents of the reactor are vigorously stirred by means of a propellor stirrer with four blades each 5 cm in diameter, turning at 650 r.p.m.

The temperature recording curves indicate that the temperature varies between 80.degree. and 90.degree. C.

The gases are delivered to the bottom of a three stage tower filled with Raschig rings in which the washing liquid circulates downwards, the admission of air required for reoxidizing the nitrous vapours being ensured by inlets arranged at intervals along the tower which is placed under a slightly reduced pressure. Ten to eleven liters per hour of a 44-46 percent nitric acid solution are taken from this nitric acid regeneration system and returned to the reactor after the addition of enough fresh concentrated acid to restore the original concentration.

The brine which results from the oxidation regularly overflows into the second reactor in which concentrated sulphuric acid (66.degree. B) is introduced at a rate of 1.5 liters per hour by means of a metering pump. The temperature is regulated to 105.degree. C. The reddish fumes escaping from this reactor are delivered to a main collector leading to the nitric acid regeneration tower.

The highly concentrated solution thus obtained is hydrated by the introduction of 10 liters per hour of cold water and brought to the boil in a third reactor before being separated in a continuous centrifuge.

An average of 23 liters per hour of solution containing 137 g/liter of Ni + Co and 320 g/liter of residue containing 19.8% of Ni + Co are collected at this stage.

The yield from the treatment is thus above 98 percent while the recovery of nitric acid varies between 70 and 85 percent.

The iron is removed as follows: the solution is adjusted to pH 5.5 by the addition of a basic nickel carbonate pulp and is then filtered in a press. The amount of iron left in the solution is less than 5 ml per litre. The filter cake which is rich in nickel is used for the initial removal of iron from the solution to be worked up this treatment being carried out at pH 4 and at a temperature of 60.degree. C.

By an average taken over 1 hour's operation 23 liters of solution containing 137 g/liter of Ni + Co and 769 g of nickel carbonate pulp containing 13% of Ni, i.e. 3,249 g/hour of nickel and cobalt are introduced, while 26.2 liters of iron-free solution containing 123 g/liter of Ni + Co and 307 g of ferric residues containing 5.54 percent of Ni + Co are removed.

It is obvious that these ferric residues, poor in nickel, can be completely deprived of nickel by reintroducing them into the sulphate solution in a third treatment stage with a view to eliminating the iron.

Finally, the iron-free solution is subjected in two reactors whose temperature is regulated to 100.degree. C. to intensive stirring by means of turbine impeller rotating at 2,200 r.p.m. in the presence of trivalent nickel hydroxide.

The first reactor receives the cake emanating from the centrifuging of the reaction product issuing from the second reactor, while a large excess of the oxidizing reactant is introduced into the second reactor. The following distribution is observed, calculated on an average of 1 hour's operation:

incoming solution 26.2 liters containing 123 g/litre of Ni + Co and 905 g of trivalent nickel hydroxide pulp containing 10.97 percent of nickel;

outgoing solution 28 liters containing 115 g/liter of nickel with less than 10 mg/liter of residual cobalt and 598 g of filter press residues containing 5.85 percent of nickel and 11.54 percent of cobalt.

The solution is concentrated and crystallized in a continuous-cycle crystallizer/evaporator. The crystal paste is centrifuged and the crystallizate is clarified by spraying it with water.

An average of 12.8 kg/hour of crystals with the following analysis are recovered:

Ni + Co (combined) 20.73% Co less than 0.005% Fe less than 0.001% No.sub.3 less than 0.01%

and 4.2 liters of mother liquor containing 132.8 g/liter of Ni + Co and 6.2% of NO.sub.3.

The nickel present in this mother liquor is precipitated with sodium carbonate and sodium hydroxide in the presence of hypochlorite to form the reactants required for removing the iron and the cobalt.

The invention is not limited to the examples described above and in particular although the presence of free oxygen during the oxidizing lixiviation phase is preferred it is not indispensable and the presence of nitric acid alone is entirely sufficient to convert the sulphides into sulphates.

Claims

We claim

1. A process for hydrometallurgically treating a nickel concentrate containing from 55 to 75 weight percent of nickel, from 10 to 40 weight percent of sulphur and minor proportions of secondary metals, which process comprises the steps of:

i. subjecting said concentrate in comminuted form to an oxidizing lixiviation by means of nitric acid at a temperature lower than boiling point, said lixiviation being carried out, while stirring, at the atmospheric pressure and in the presence of air with such an amount of nitric acid that it forms a suspension containing between 45 and 67 percent by weight of said concentrate, the nitrous vapours formed during said oxidizing lixiviation being regenerated into nitric acid and recycled,

ii. subjecting said suspension to a sulphation by the addition of enough sulphuric acid to produce, after filtration, a solution of nickel in the form of a true sulphate,

iii. subjecting said solution to successive treatment stages to remove said secondary metals as follows:

a. treating said solution with nickel carbonate at a pH between 4 and 6 to precipitate the iron in the form of ferric hydroxide and ferric carbonate which are separated by filtration from said solution,

b. treating said solution with nickel powder to remove the copper by cementation,

c. treating said solution with nickel (III) hydroxide at a temperature of from 80.degree. C. to boiling point over a period of from 15 minutes to 2 hours to precipitate the cobalt in the form of cobalt hydroxide which is separated from said solution by filtration,

iv. subjecting the solution which results from said steps iii and which contains nickel as substantially the only remaining cation, to an evaporation-crystallization treatment, whereby pure hydrated nickel sulphate crystallizes from a remaining mother liquor solution,

v. separating said pure crystalline hydrated nickel sulphate from said mother liquor, and

vi. treating said mother liquor with sodium carbonate to precipitate nickel carbonate which is recycled to said iron separation stage iiia and treating said mother liquor with sodium hypochlorite and caustic soda to precipitate nickel hydroxide which is recycled to said cobalt separation stage iiic.

2. A process as claimed in claim 1 in which said nickel concentrate is nickel matte crushed to a powder of which 80% by weight of the particles are between 100 and 200 microns in size.

3. A process as claimed in claim 1 in which said nickel containing concentrate is a dispersed pulp produced by a wet precipitation treatment.