WO1984001964A1

WO1984001964A1 - Process for selectively extracting lead from lead sulphides

Info

Publication number: WO1984001964A1
Application number: PCT/FR1982/000195
Authority: WO
Inventors: Patrick Noual; Michel Roulet
Original assignee: Extramet Sa
Priority date: 1982-11-18
Filing date: 1982-11-18
Publication date: 1984-05-24
Also published as: AU555840B2; AU9124882A

Abstract

Common lead sulphides and minerals and hutches containing lead sulphide are treated in order to extract metal lead. Lead sulphide is dissolved in an aqueous solution of fluosilicic acid, H2SiF6, also containing an oxidant. The oxygen carrier gas is bubbled therethrough, the mixture being maintained substantially at the atmospheric pressure and a temperature lower or equal to the boiling point of the acidulated aqueous solution and higher than about 50<o>. The solution lead is collected by any conventional method, particularly by electrolysis, the solution free of lead being recyclable for a new attack of lead sulphide. Application: selective extraction of lead, in the presence of precious metals or copper and iron, which are not dissolved in the conditions of the process.

Description

PROCESS FOR SELECTIVE LEAD EXTRACTION FROM LEAD SULPHIDES

The present invention relates to the extraction of lead from metallic sulphides such as common lead sulphides and ores and ore concentrates containing lead sulphide. It relates to a process which allows more particularly the selective extraction of lead (Pb) in the presence of other metals such as silver, copper or iron which may be in the state of sulfides with lead sulfide in this kind of minerals.

Pyrometallurgical treatment of lead sulphides is expensive, polluting and often requires the elimination of a by-product, sulfur dioxide.

To remedy the drawbacks of the pyrometallurgical process, and more particularly to pollution, so-called hydrometallurgical processes have been developed for oxidizing various sulphides in an aqueous medium, in hydrochloric solutions. These processes apply to most metals; They use a combination of different chlorinated leaching reagents, such as chlorine, hydrochloric acid, ferrous / ferric chloride, cupric chloride, manganic chloride, sodium chloride, calcium chloride. The reaction implemented consists of an oxidizing solution and it leads to a chlorinated salt solution from which the metal is then recovered.

However, these known hydrometallurgical processes are not suitable for extracting lead from metallic sulphides containing lead, in particular from lead sulphide ores. The difficulties encountered are mainly linked to a problem of solubility of lead in the medium, as well as to the recovery of the metal from the chloride form. We must generally deplore a large consumption of energy, the reduction of PbCl ₂ taking place either in pyrometallurgical form or by electrolysis with a high voltage.

In fact, no hydrometallurgical process is known which is applicable industrially to the treatment of sulphide ores or other metal sulphides with a view to the extraction of lead.

The present invention overcomes the drawbacks of known techniques thanks to a process for treating metal sulfides, which uses the hydrometallurgical route and which, however, is effective in ensuring the extraction of lead, selectively in the presence of the other metals which are usually found in lead sulphide ores.

The method according to the invention essentially comprises at least a first chemical etching step in which the metal sulphide is subjected to oxidation in an aqueous solution of fluosilicic acid. Preferably, it also comprises a second recovery stage in which the solution in the fluosilicon medium obtained is subjected to an electrolysis leading to the formation of lead metal at the cathode and of oxygen at the anode.

In the first step, it is also advantageous to use a redox oxidation torque with a potential of between 0.7 and 1.4 volts relative to the normal hydrogen electrode. This is the case in particular of the O ₂ / H ₂ O ₂ pair which appeared to be particularly favorable in the implementation of the invention. At this stage, the process according to the invention already makes it possible to obtain good solubility of the starting sulfide, at the same time as the oxidative attack of lead preferentially over the other metals generally found associated with lead in metallic sulfides. This effective and selective attack on lead has been proven by the experiments carried out by the applicants, while the previous knowledge in no way let him predict.

It will be noted that by the process of the invention, applied to lead, many of the disadvantages of conventional hydrometallurgical processes in a chlorinated medium are avoided, even in their application to other metals: variable and poorly controlled attack yields, lack of selectivity (especially in attack with ferric chloride), solution which crystallizes too easily, difficulties in controlling the redox potential, formation of sulphates which deposit on the sulphide particles and slow down the reaction.

Once the fluosilicic solution containing the oxidized lead in the form of dissolved salts has been obtained, from which the supernatant sulfur and any insoluble compounds are easily separated, the lead can be produced in the metallic state in various ways. We can use any treatment schemes known per se, such as case hardening or solvent extraction. However, the electrolytic route is preferred, in particular in accordance with the second step which has already been defined above. The fluosilicon medium, preferably in conjunction with the use of the redox couple O ₂ / H ₂ O ₂ , lends itself in a remarkable manner to the combination of oxidative chemical dissolution in an aqueous electrolyte with the recovery of lead metal by electrolysis , because it appeared especially advantageous to carry out this electrolysis also in a fluosilicon medium, directly on the solution obtained after the first step. While lead metal is thus produced, the same electrolysis operation makes it possible to recover the oxygen and the fluosilicic solution, which can be recycled in the first stage for dissolution. This avoids other drawbacks that hydrochloric media would present if one sought to subject a solution of conventional sulfide dissolution to electrolysis: uncertainty about an anodic reaction which is then of the type Fe ⁺⁺ / Fe ⁺⁺⁺ or Cl- / Cl ₂ , presence of iron in solution, with as a consequence a low cathodic yield, unless going to the production of electrolytic iron . The recovery of lead metal by electrolysis of the fluosilicic solution of dissolution has moreover the advantage that at the same time, by the same operation, one regenerates the oxygen and the acid solution, which one then has in general any interest to recycle at the first step, for dissolution. Overall, the invention thus allows the transformation of ores and metallic concentrates of lead sulphide at low cost, at atmospheric pressure, without the appearance of by-products and under optimum conditions of yield and selectivity.

The method according to the invention will now be described in more detail, under practical conditions of implementation, which are not, however, limiting. In general, one starts from an ore or concentrate based on lead sulphide, previously ground, to a particle size which can in particular be of the order of 50 to 500 microns and is suspended in a solution of fluosilicic acid. , of pH advantageously less than 2 or at most equal to 2, containing an oxidant. This oxidant can be hydrogen peroxide and / or oxygen. One can in particular initially use a solution of fluosilicic acid containing in addition hydrogen peroxide and then mix the slurry formed with the sulphide intimately with an oxygen-carrying gas, by bubbling this gas there, which can be pure oxygen or air, possibly enriched in oxygen. The mixture is maintained substantially at atmospheric pressure, and at a temperature less than or at most equal to the boiling temperature of the electrolyte, and generally greater than approximately 50 ° C. Preferably, the fluosilicic acid and the oxidant are present in the attack solution in amounts at least equal to the stoichiometric amounts corresponding to the reaction for dissolving lead sulfide, ie a molecule of fluosilicic acid and a molecule d hydrogen peroxide per molecule of lead sulfide to be dissolved. These theoretical quantities correspond to the reaction: PbS + 2H ⁺ + H ₂ O ₂ Pb ⁺⁺ + S + 2H ₂ O The actual consumption of hydrogen peroxide varies from one to two times the stoichiometric proportion, while for the acid la actual consumption remains close to the stoichiometric quantity.

In accordance with this reaction, the sulfur in lead sulfide is therefore transformed into elemental sulfur while the metal is dissolved in the form of Pb ⁺⁺ ions. In practice, sulfur floats on the solution; There is no appearance of passivating layers which would considerably slow down the reaction. In addition, the selectivity of the process results in the fact that for most ores, the other metals present, such as silver, copper, iron, zinc, or bismuth, are not or only very little. dissolved. The solid residue from the attack can also be treated with a view to recovering these metals, in particular by the hydrochloric route.

The fluosilicon solution containing the lead ions Pb ++ is then transferred to an electrolysis cell, which can in particular be a common type cell comprising a diaphragm for separation between the cathode and anode compartments and an insoluble anode. Electrolysis causes the following reactions: - at the cathode: Pb ++ + 2 e Pb - at the anode: H ₂ O 2 H ⁺ + ½ O ₂ + 2e.

The metal lead is thus recovered, which is deposited on the cathode, and, in parallel, the oxygen which is released at the anode. The pH conditions linked to the implementation of the attack of the first stage are directly favorable, without there being any need to intervene between the two operations, and on the other hand, it is easy to adjust the current density so as to best favor the above reactions, to the detriment of the concurrent anodic reaction which leads to the formation of lead oxide. The oxygen recovered at the anode is generally recycled in the dissolution step, as is the fluosilicic solution which is regenerated by electrolysis substantially to its original acidity. The only reagent consumed by the process as a whole is then the oxidant (hydrogen peroxide more often than not, which is used in excess for the dissolution of lead sulfide. The numerical examples which follow illustrate the implementation of the process according to invention, purely by way of indication and in no way limiting: Example 1: In an attack solution, stirred by magnetic stirrer, in which air is bubbled, the ore of finely ground lead sulphide is dispersed to a smaller particle size at 100 microns The attack solution is an aqueous solution of fluosilicic acid having the following composition:

H ₂ SiF ₆ 100 g / l Pb ++ 50 g / l (in the form of ground PbS) H ₂ O ₂ 0.25 mol / l Temperature of the solution: 70 ° C.

The kinetics of dissolution are such that after one hour, a proportion of 40% of the lead sulfide is already dissolved. After filtering the solution containing lead in the dissolved state, it is electrolysed in a diaphragm cell, with platinized titanium anode, to obtain lead in the metallic state. The conditions applied for electrolysis are as follows: Terminal voltage: 2.0 volts Current density: 250 A / m ² Electrolysis efficiency: around 100%.

The consumption of electrical energy is 0.5 kWh per kilogram of lead obtained. Example 2:

It is the attack on a galena containing a significant proportion of sulphides other than lead sulphide. The conditions of the operation are those indicated in Example 1.

To quantify the selective nature of the dissolution, the partition coefficient defined for each element is determined as follows: if A is the ratio of the quantity of the element considered to the quantity of lead in the ore, and if B is the ratio of the quantity of the same dissolved element to the quantity of dissolved lead, so in the solution, the partition coefficient is:

K = A / B.

In the table below, we noted for each element, its proportion Q in average weight percent in the ore, the ratios A and B defined above and the partition coefficient K:

Element Q (% wt) A B K

Pb 56 1 1 1

Cu 4 0.071 0.12 10 ^-3 590 Fe 8 0.142 0.028 5

Zn 4 0.071 9.5 10 ^-3 7.5

Ag 0.3 5.3 10 ^-3 0 100

Bi 0.2 3.5 10 ^-3 3 10 ^-4 12 These figures demonstrate the high selectivity of the process. The dissolution of metals other than lead is very weak and that of silver is zero.

Claims

1. A method of treating metal sulfides with a view to extracting the lead therefrom, characterized in that it comprises at least a first step in which the metallic sulfide is subjected to oxidation in an aqueous solution of fluosilicic acid, the lead passing so in solution.

2. Method according to claim 1, characterized in that the oxidation is ensured by a redox cut with potential between 0.7 and 1.4 volts relative to the normal hydrogen electrode.

3. Method according to claim 2, characterized in that the redox couple is O ₂ / H ₂ O ₂ .

4. Method according to claim 1, characterized in that the fluosilicic acid solution is mixed with the ground metal sulphide and added with hydrogen peroxide and in that an oxygen-carrying gas is bubbled through the mixture.

5. Method according to any one of claims 1 to 4, characterized in that the fluosilicon solution is then subjected to a second treatment step in which the lead metal is recovered by electrolysis.

6. Method according to claim 5, characterized in that one also recovers the oxygen released at the anode to recycle it in the first step.

7. Method according to claim 5 or 6, characterized in that the fluosilicic solution regenerated by electrolysis is recycled to serve for dissolution in the first step.

8. Method according to any one of claims 1 to 7, characterized in that the solution has a pH of less than 2 or at most equal to 2.

9. Method according to any one of claims 1 to 8, applied to the treatment of sulphide lead ores or concentrates of such ores.

10. Lead produced from metal sulphides by the process according to any one of claims 1 to 8.