NO741691L - - Google Patents

Description

Procedure for precipitation and

separation of arsenic from

copper-containing solutions.

The invention relates to a method for the precipitation and separation of arsenic from arsenic- and copper-containing solutions, as they appear during the preparation of mining or process products in a hydrometallurgical manner. In addition to copper and arsenic, the solutions may contain arbitrary amounts of further non-ferrous metals and, as anions, chloride or sulphate ions or mixtures thereof.

Arsenic and copper-containing solutions appear during the hydrometallurgical preparation in numerous mining or processing methods. For example, the preparation of copper-containing, sulphidic flotation concentrates, especially so-called bulk

and middle concentrates, increasing in a hydrometallurgical manner. By oxidative pressure tilting or in combination with a sulphating roasting with

subsequent acid leaching results in solutions which, in addition to arsenic and copper, also contain zinc, iron and possibly further metals such as nickel, cobalt and the like. Furthermore, for example, the removal of disturbing contaminants from the aqueous solutions of zinc sulphate, which are to be added to a zinc electrolysis, is carried out, so that the solution is subjected to a zinc dust sedimentation. In this way, arsenic (As^O^) is added and a cementate is obtained, which, in addition to excess zinc, contains all zinc electropositive impurities, especially copper, nickel, lead and the like in metallic or arsenide form.

Such a cementate has, for example, the following analysis:

During the lead process treatment, a copper- and arsenic-containing intermediate product, which is referred to as "copper lick", appears during the removal of copper from the material lead. Depending on the method (either direct removal of the copper or additional mixing of sulphur), the analysis of the copper lick appears, for example, as follows:

Por transfer of the last-mentioned product in an acidic aqueous solution and extraction of the copper from this solution, a method is proposed in DOS 2,036,391 which can in principle be applied to similar materials such as e.g. above-mentioned cement sludge from zinc leaching. A method for arsenic removal resp. to the reduction of the As : Cu ratio is not stated in the said DOS. Consequently, the copper extracted by this method by reduction electrolysis is very impure and must be subjected to an additional refining electrolysis. Capital gains and electricity costs must therefore be applied twice, which makes the profitability of this copper production somewhat doubtful.

A method for separating copper and arsenic in a wet manner, which can be used for many purposes, was recently proposed by MONTECATINI EDISON (DOS 2,032,417). According to this method, arsenic- and copper-containing solutions are mixed with Fe^<+-> ions of arbitrary origin, as a strong reduction of copper to a univalent state occurs at the redox equilibrium, in which form the copper remains in solution in the chloride ion-containing solution as l_ CUCI2 _7 complex. By partial neutralization of. the solution with an arbitrary alkali precipitates at pH values from 3.0 - 3.5 the combined arsenic, iron and possibly, present sulphate ions of the solution as iron arsenate resp. iron(III) hydroxide resp. plaster. From the thus purified solution, the copper can be produced according to known methods, e.g. by cementing with iron as cement copper or by air oxidation as copper oxychloride.

This inherently elegant method has two significant limitations, which limit its applicability or makes it impossible for the very important area of electrolytic copper extraction: 1. The method is limited to chloride solutions, as in pure sulphate solutions no Cu<+-> compounds are stable, but are subject to disproportionation. Thereby, the method does not come into consideration for copper-containing solutions, which are to be added to a copper electrolysis. 2. In chloride solutions, the method is limited to such relatively low copper contents. For the complex formation of the copper (I) ion, which forms a decisive point in this method, considerably over-stoichiometric Cl - concentrations are necessary, as is also evident from the examples given. If the excess chloride is too low, copper (I) chloride (CuCl) precipitates out and is lost with the filtered iron and arsenic precipitate. The method is therefore limited to low copper concentrations, as for higher concentrations it is not brought into solution or cannot be kept in dissolution it. required chloride ion content. However, for economic reasons, practice is precisely interested in the most highly concentrated solutions possible.

In what follows, a wet metallurgical method is now described which does not have the aforementioned limitations and enables a practically quantitative separation of arsenic from the valuable metals, regardless of the As concentration of the solution, its addition and the type of anion.

The method according to the invention for the precipitation and separation of arsenic from arsenic and copper-containing solutions, which may also contain additional non-ferrous metals, in a metallurgical manner is characterized by treating the solution, unless the ions contained therein are already present in the highest degree of dignity with oxidizing agents to . achieving this condition while maintaining a pH value. equal to or less than 1, one relative to arsenic superstoichiometrically. quantity of iron is added in the form of an acid-soluble iron salt and brought into solution, post-oxidised if necessary and by the addition of alkali. preferably lime, a pH value of 1.9-2.0 is set in chloride-containing or of 2.7.-2.8 in pure sulphate-containing solutions, the precipitated mixture consisting of iron arsenate, excess iron hydroxide:, possibly gypsum and acid-insoluble residues of the material used are separated. from the mother liquor and this is worked up in a known manner to copper and any other metals still present.

The composition and origin of the solution is therefore arbitrary. It can therefore be a lye from the sulphating roasting of flotation concentrates, the chlorinating roasting of kisav fires, the oxidising solution of cementates. copper licks, copper lead and similar substances or about solutions for or from copper electrolysis. Any treatment of the solution with oxidizing agents, e.g. chlorine, oxygen, air or oxygen-air mixtures are necessary to ensure that arsenic is present in the pentavalent, iron in the trivalent valence state..If in exceptional cases sufficient iron is not already present, iron is added in the form of one of its acid-soluble salts, e.g.. Pe -(OH)-^, FeSO^^^O and lig- . nende. "Hereby, completely worthless, often even difficult-to-remove substances such as emery liquor, crystallized iron salts, e.g. from titanium white production or precipitation sludge from lye cleaning processes can be used, also mixed with valuable substances. If the iron in the mentioned substances is not yet in the highest, trivalent oxidation state , then post-oxidation with one of the mentioned oxidizing agents is necessary. The amount of iron added must stoichiometrically at least correspond to the amount of arsenic present in the solution, i.e. 56 kg.Fe/75 kg As. Appropriately, the stoichiometric excess is used of 1.5 - 3 Fe/lAs, i.e. 85 - 170 kg/Fe/75 kg As, in order to reduce the residual solubility of the arsenic in the subsequent precipitation. The precipitation of the arsenic is carried out by the addition of hydroxyl ion-releasing substances, such as alkali - or alkaline earth hydroxide or

-carbonate..For price and technological reasons, it is preferable to use lime, as the gypsum simultaneously precipitated from the sulfate ion-containing solution during the subsequent phase separation acts as a filter aid.. The precipitation of arsenic takes place according to the equation

Excess iron salt is precipitated at the same time according to the equation resp. in sulfate ion-containing solution according to the equation

as hydroxide or basic ferrous sulfate.

If the precipitation is carried out with lime, then gypsum is precipitated at the same time, corresponding to the equation

The pH value is to be observed precisely during the overall precipitation, as the position of the precipitation pH value depends on the type of anions. In chloride-containing solutions, the optimum value is exactly 1.9-2.0, in pure sulphate solutions exactly 2.7-2.8. If the specified pH values fall below, the arsenic content in solution remains unacceptably high. If they are exceeded, the arsenic precipitation is indeed complete, however, increasing amounts of copper are also precipitated with increasing pH value, namely according to the equations

and is lost with the precipitation sludge that goes to landfill. The formed precipitate contains iron arsenate, iron hydroxide or basic ferrous sulphate, gypsum and acid-insoluble residues, of. the material used and of the precipitating agent. It is separated from the mother liquor in the usual way by filter pressing, rotary filters and the like, and this is processed in a known way into the value carriers copper, zinc etc. contained therein.

The result of the precipitation procedure. according to the invention, where arsenic and trivalent iron are strongly precipitated, but copper practically not, is certainly surprising, as it cannot be deduced from the precipitation curve of. FeAsO^ and Cu-oxychloride resp. -hydroxide. Since the aforementioned curves intersect, there is strictly theoretically no pH range in which both components can be separated quantitatively. With the method according to the invention for setting a precisely defined pH value, a technologically satisfactory compromise is nevertheless achieved between the best possible As precipitation with minimal copper loss. This pH value is found in solutions containing chloride ions which may also contain other anions, Seks. also sulfate ions, at exactly 1.9-2.0. in pure sulfate ion-containing solutions at 2.7-2.8. This finding is in good agreement with the smaller complexation tendency of the SO2^ - ions, compared to Cl - ions.

Finally, reference must be made to the fact that precipitation sludge was formed. contains arsenic in a deposit-friendly form... The precipitated compound FeAsO^^HgO (Skorodite) has of all known arsenic compounds with

(Chuklantev, J. Anal. Ch. (Russian), 11 (1956) 529) the lowest arsenic solubility. In a work by G. Zintl in "Erzmetall", 26 (1973), 2, pages 141-147, the deposition safety of this compound is convincingly demonstrated.

The following examples shall explain the method according to the invention without limiting it.

Example 1.

10 kg of a cementate slurry with 37.77" Cu, 7.85% As and 10.458

Zn (in the dry state) was suspended in 50 liters of a dilute sulfuric acid (85 g/liter HgSO 4 ). The suspension was stirred at 60°C and elemental chlorine introduced at 800 litres/hour until the appearance of the chlorine potential of 1800 mV (measured with a thalamide electrode). The solution contained 76.5 g/litre Cu and 17.2 g/litre As. Then, with further stirring and addition of sulfuric acid, a total of 6 kg of a wet iron gypsum precipitation sludge (approx. 50% HgO, approx. 24% Fe (in dry weight), approx.

30% CaO (in dry weight). The suspension then contains 71.1 g/litre Cu, 14.0 g/litre As and 15.7 g/litre Fe, which corresponds to a stoichiometric ratio Fe:As = 1.5:1. The suspension was diluted with water in a ratio of 1:1 and precipitated with a milk of lime suspension until a pH value of exactly 2.0. The filtrate contained 35.2 g/liter Cu, 0.06 g/liter As and 0.26 g/liter Pe, the residue 1.19% Cu and 7.65% As. Taking into account the solvent volume or the amount of residue is thus As precipitated to more than $9% in the residue, while Cu remains in the solution to more than 91%.

Example 2.

1.5 kg of a cementate which in example 1 was suspended in 20 liters of a CuCl2 solution (38.8 g/liter Cu 2 +). Under the addition of dilute sulfuric acid and maintaining a pH value of approx. 0.5, the suspension was oxidized with air until the total copper was present in

bivalent form. The suspension contained 65.0 g/litre Cu and 5.4 g/litre As. This mixture was further mixed with sulfuric acid and iron gypsum precipitation sludge to a molar ratio of Fe:As = 2:1.

After precipitation with lime at pH = 2.0, the solution contained 63.6 g/liter Cu, 0.026 g/liter As and 0.185 g/liter Fe, the residue 0.71% Cu and 4.10% As.

Example 3.

10 kg of a copper lick with 56.7% Pb, 28.3% Cu, 1.45%

As, 4.45% Zn as well as Sb, Sn, Fe, Ni, Ag and other components were introduced into 50 liters of diluted sulfuric acid (17% strength) at 80°C. During intense stirring, technical oxygen (approx. 90%) was introduced to quantitatively dissolve the copper. The easily filterable residue contained the total lead as PbSO^ as well as the total content of Sb, Sn, Ag and part of the arsenic. The solution contained 99 g/litre Cu and 1.25 g/litre As. The solution was then mixed with approx. 0.5 kg of a ferric sulfate heptahydrate (FeSO^.7H20) originating from emery liquor and the iron oxidized to the trivalent stage. The Fe:As molar ratio was approx. 1.9:1. After precipitation with lime hydrate suspension at pH 2.75, the solution contained 94.3 g/litre Cu, 0.014 g/litre As, 0.11 g/litre Fe, which corresponds to an As separation of over 99%.

Claims (3)

1. Process for the precipitation and separation of arsenic from copper-containing solutions, which may also contain additional non-ferrous metals in a wet metallurgical manner, characterized by treating the solution unless the ions contained therein are already present in a higher rank with oxidizing agents to achieve this state while maintaining a pH value slightly less than 1, relative to the arsenic, one adds an overstoichiometric amount of iron in the form of an acid-soluble iron salt and brings it into solution, possibly post-oxidizing with the addition of alkali, preferably lime, setting a pH value of 1, 9-2.0 in chloride-containing resp. of 2.7-2.8 in pure sulphate-containing solutions, separates the precipitated mixture consisting of iron arsenate and excess iron hydroxide, possibly gypsum and acid-soluble residues of the material used, from the mother liquor and processes this in a known manner on copper and possibly still present other metals. 2. Method according to claim 1, characterized in that waste substances are used as iron salts, e.g. emery liquor, crystallized ferric sulfate heptahydrate or preferably trivalent ferric hydroxide from purification or precipitation processes. 3. Method according to claims 1 and 2. characterized by using chlorine, oxygen, air or an oxygen-air mixture.