Process for producing electrolytic lead and elemental sulfur from galena

Abstract: The invention proposes a process for producing electrolytic lead and elemental sulfur from galena, characterized in that it comprises the following steps: (a) Galena is leached with an acidic aqueous solution of ferric fluoborate, with ferrous fluoborate, lead fluoborate and elemental sulfur being formed according to the reaction: 2 Fe(BF₄)₃ + PbS → 2 Fe(BF₄)₂ + Pb(BF₄)₂ + S (b) the solid residue, composed by elemental sulfur and galena gangue is filtered off; (c) the solution of ferrous fluoborate and lead fluoborate coming from the (a) step is sent to a diaphragm electolytic cell, wherein pure lead is deposited at the cathode and at the anode ferrous ion is oxidated to ferric ion; (d) the solution of ferric fluoborate regenerated at the anode in said (c) step is recycled to said (a) step of galena leaching. By operating under suitable conditions, lead can be selectively dissolved and separated from the other metals small amounts of which are contained in galena together with said lead. Sulfur produced by the reaction can be separated from the gangue by extraction with a solvent, or by flotation.

Description: It is well-known that lead is usually produced from galena by thermal way, by means of roasting processes. This production method involves a large number of problems, above all of environmental type, owing to the emission of considerable amounts of dusts containing lead and sulfur dioxide into atmosphere. Another problem which begins to constitute a considerable burden to the damage of the budgets of the industry of production of lead by extraction thereof from galena by the thermal way is the consequent production of sulfuric acid, which leads often to financial burdens and to general disadvantages. Owing to these reasons, most experts in this sector agree upon expecting that in a near future the production of lead by the hydrometallurgical process will become competitive with the present production by thermal way. In the relevant technical literature a very large number of papers exist, reporting about the studies and researches aiming at developing a hydrometallurgical process for producing lead from galena. Suffice it to say that the first investigations carried out by Bequerel and Marchese in order to obtain sulfur and lead from galena by electrolysis date back to the second half of the nineteenth century. The thermodynamic and kinematic parameters of the reaction of dissolution of lead in an electrolyte and production of elemental sulfur were subsequently investigated into greater details by a large number of researchers. The oxidating means which is by far the most studied one, is, still to-day, ferric chloride. With this reactant, during the past years the two most advanced processes for the hydrometallurgical processing of galena were developed, and precisely the Minimet Penarroya process and the U.S. Bureau of Mines (USBM) process. In both of these methods, a leaching of galena in an aqueous solution of ferric chloride with NaCl is carried out first, then the sulfur-containing residue is filtered off, and the so obtained lead chloride is electrolysed. It is in this latter operation that the two processes are different from each other, because according to Minimet process, the solution is submitted to electrolysis after being purified, with spongy lead being obtained, whilst according to USBM PbCl₂ is crystallized and is then submitted to electrolysis in a bath of molten chlorides. However, also the use of hydrometallurgy which chloride leaching is affected by drawbacks which derive from the specific characteristics of chloride ion, i.e.; low solubility of lead chloride in water; hence, need of adding such salts as NaCl or, in general, alkali and alkali-earth metal chlorides, which, as well-known, increase the solubility of the metals by forming the Cl₄- complex ion. The complex chloride, although is beneficial to the solubility of lead, causes the dissolving of nobler metals such as Bi, Ag and Cu, so that these latter are subdivided between the residue and the solution in a difficultly foreseeable way. Also in case such a contrivance is adopted, the leaching solutions can dissolve not more than 25-30 g of Pb⁺⁺ Per litre. Therefore a "galena/solution" ratio equal or lower than 1:20 is required. The electrolysis of lead chloride in an aqueous solution does not yield a compact deposit; on the contrary, lead is recovered as an incoherent sponge. The electrolytic cell must have a very complex structure in order to collect the product which falls to the bottom, and remains impregnated with electrolyte. In general, from the smelting of the lead sponge obtained from the electrolysis, a lead with a purity of 99.99% is not obtained, unless the electrolyte is submitted to a preliminary, laborious purification. the smelting of the lead sponge, owing to its high oxidability, is a delicate operation, to be carried out under a flux (NaOH), and causes the production of at least 5% of oxidation slags. The electrolysis of lead chloride in a molten electrolytic bath is much more complex, is not safe from the environmental viewpoint, consumes a larger amount of energy and the produced lead does not have a purity of 99.99%. According to the instant invention, the present Applicant has surprisingly found now that the problems which affect the prior art, as hereinabove briefly reminded, can be efficaciously overcome by means of a process for producing electrolytic lead and elemental sulfur from galena, characterized in that it comprises the following steps: (a) Galena is leached at pH <1 and at a minimal temperature of 80°C with an acidic aqueous solution of ferric fluoborate, with ferrous fluoborate, lead fluoborate and elemental sulfur being formed according to the reaction: 2 Fe(BF₄)₃ + PbS -> 2 Fe(BF₄)₂ + Pb(BF₄)₂ + S (b) the solid residue, composed by elemental sulfur and galena gangue is filtered off; (c) the solution of ferrous fluoborate and lead fluoborate coming from the (a) step is sent to a diaphragm electrolytic cell, wherein pure lead is deposited at the cathode and at the anode ferrous ion is oxidated to ferric ion; (d) the solution of ferric fluoborate regenerated at the anode in said (c) step is recycled to said (a) step of galena leaching. In order to better understand characteristics and advantages of the process according to the present invention, in the following a non-limitative, examplifying form of practical embodiment thereof is disclosed, by referring to the figure of the hereto attached drawing, which represents a block diagram of the same process. Galen, as a concentrate, is sent from 1 to a leaching vat 2, to which from 3 an aqueous solution of ferric fluoborate is fed. In 2 the following reaction takes then place: 2 Fe(BF₄)₃ + PbS -> 2 Fe(BF₄)₂ + Pb(BF₄)₂ + S The pH value is lower than 1, the temperature is preferably comprised within the range of from 80 to 100°C, the reaction time is comprised within the range of from 15 minutes to 4 hours, according to the type of galena and its granulometry. When the reaction is ended, the solution coming from 2 is filtered in 4, yielding an undissolved residue constituted by elemental sulfur and by the gangue of the ore used as the raw material, which generally contains Cu, Zn and precious metals. Such an undissolved residue can be advantageously recovered by being sent to a step 6 wherein sulfur is extracted with a solvent, such as carbon disulfide, or by flotation. The sulfur solution is then distilled in 7, and pure sulfur 8, not containing metals, is obtained. In the herein exemplified flow diagram the recycle of the condensates from the distillation step, to the step 6, is indicated by the reference numeral 9. From the extraction step 6 also a solid residue comprising the metals (Cu, Zn, precious metals) is recovered. From the filtration step 4, an aqueous solution of lead-(II) and iron-(II) fluoborates 11 is obtained and is sent to a diaphragm electrolytic cell 12, schematically shown and indicated by the reference numeral 13. In this latter at the cathode 14 pure lead 17 deposits, and the solution, partially deprived of lead, is sent through 15 to the anodic compartment 16, wherein the oxidation of ferrous fluoborate to ferric fluoborate takes place. The so regenerated solution of ferric ion is recycled from the electrolytic cell 12 to the leaching vat 2 wherein the attack of further galena takes place. A further example of practical embodiment of the present invention, in which particular reference to the amounts of the involved substances is made, is reported in the following. Also in this case, in no way said example should be construed as being limitative of the invention. Example 112 g of galena concentrate, having the composition: Pb 78 % Zn 1.1 % Cu 0.9 % Fe 2.2 % Mg 0.04 % S 14.97 % is gradually added with stirring to 1.6 litres of a solution containing 28 g/l of Fe⁺⁺ (as fluoborate), 32 g/l of Fe⁺⁺⁺ (as fluoborate), 20 g/l of Pb⁺⁺ (as fluoborate). After 4 hours of reaction at 105°C, the reaction solution is filtered and: 24 g of insoluble residue containing 23.3% of Pb, 4.5% of Zn and 3.9% of Cu; as well as 1.6 l of solution containing 86.5 g/l of Pb⁺⁺ (as fluoborate), 58 g/l of Fe⁺⁺ (as fluoborate), 2 g/l of Fe⁺⁺⁺ (as fluoborate), 0.11 g/l of Zn⁺⁺ (as fluoborate), 0.02 g/l of Cu⁺⁺ (as fluoborate), are obtained. The lead-containing solution, after being cemented with Pb powder in order to remove any copper traces, is circulated between the anodic compartment of a diaphragm electrolytic cell, the cathode of which is constituted by a stainless-steel plate and the anode is a grid of activated tantalum. Under the action of a direct current at 3.0 V, which maintains a current density of 300 A/m² at the cathode, the following processes take respectively place: at the cathode: Pb is deposited in a compact, smooth form; at the anode: Fe⁺⁺ is oxidated to Fe⁺⁺⁺. At the end of the process: 98 g of cathodic lead with a purity of 99.99% and 1.6 l of an electrolyte containing 24 g/l of Pb⁺⁺, 35 g/l of Fe⁺⁺⁺ and 25 g/l of Fe⁺⁺, are obtained. This electrolyte will be used for the successive operation of leaching of galena concentrate. The residue obtained from the leaching was treated with carbon disulfide in order to extract sulfur. After distilling off the solvent, 12.5 g of elemental sulfur without any traces of metals was obtained. From the above, as disclosed and exemplified, one can see how the process proposed according to the present invention makes it possible electrolytic lead and elemental sulfur to be produced simultaneously, without falling into the problems which affect the prior art, as hereinabove reminded. The solution of ferric fluoborate used in the (a) leaching step is capable of yielding a very soluble lead salt, very stable to the expected reaction temperatures. A further, important, feature of the process according to the present invention is the selectivity of the leaching in respect of the metals and of the precious metals which are contained in galena together with lead. According to the present invention, it has been seen that galena has a cementing effect as regards the nobler metals than lead (Cu, Ag, Bi, and so forth) in a fluoboric medium in the presence of Fe⁺⁺. In fact, the following reactions take place: PbS + Cu(BF₄)₂ -> Pb(BF₄)₂ + Cu + S⁰ PbS + 2 Ag(BF₄) -> Pb(BF₄)₂ + 2 Ag + S⁰ 3 PbS + 2 Bi(BF₄)₃ -> 3 Pb(BF₄)₂ + 2 Bi + 3 S⁰. Therefore, the purification of the solution takes place directly during the leaching step, if this latter is carried out with a slight excess of galena over the stoichiometric amount. The so producted lead has hence a purity of 99.99%. According to the process of the instant invention, lead can be deposited in compact form, even with low end lead contents in the solution at high values of current density, owing to the simultaneous presence of Fe(BF₄)₂, which exerts a beneficial action on the deposit. The deposit of pure lead at the cathode is of an extremely high quality. A further advantage of the process according to the instant finding is that one can operate in the electrolytic deposition step at a high value of cathodic current density, with a high deposition efficiency. A considerable advantage offered by the instant invention is that the fluoboric solution can be used again by being directly recycled from the electrolytic cell to the leaching step, without having to undergo a preliminary purification step. Thus, as one can realize from all of the features of the process as disclosed hereinabove, all of the problems which affect the processes known from the prior art are advantageously overcome.

Claims: Process for producing electrolytic lead and elemental sulfur from galena, characterized in that it comprises the following steps: (a) galena is leached at pH <1 and at a minimal temperature of 80°C with an acidic aqueous solution of ferric fluoborate, with ferrous fluoborate, lead fluoborate and elemental sulfur being formed according to the reaction: 2 Fe(BF₄)₃ + PbS -> 2 Fe(BF₄)₂ + Pb(BF₄)₂ + S (b) the solid residue, composed by elemental sulfur and galena gangue is filtered off; (c) the solution of ferrous fluoborate and lead fluoborate coming from the (a) step is sent to a diaphragm electrolytic cell, wherein pure lead is deposited at the cathode and at the anode ferrous ion is oxidated to ferric ion; (d) the solution of ferric fluoborate regenerated at the anode in said (c) step is recycled to said (a) step of galena leaching. Process according to claim 1, characterized in that the galena used as the raw material comprises minor amounts of one or more metal(s) such as Cu, Ag, Bi, and in said (a) step the following reactions take plae simultaneously: PbS + Cu(BF₄)₂ -> Pb(BF₄)₂ + Cu + S⁰ PbS + 2 Ag(BF₄) -> Pb(BF₄)₂ + 2 Ag + S⁰ 3 PbS + 2 Bi(BF₄)₃ -> 3 Pb(BF₄)₂ + 2 Bi + 3 S⁰. Process according to claim 1, characterized in that said solution of lead fluoborate and ferric fluoborate formed in the (a) step is treated with lead powder before being sent to said (c) step. Process according to claim 1, characterized in that said solid residue coming from the filtration according to the (b) step is submitted to a recovery treatment. Process according to claim 5, characterized in that said recovery consists in extracting sulfur with a solvent, in that way separating it from the gangue, with pure sulfur being subsequently obtained by means of the distillation of said solvent. Process according to claim 5, characterized in that said recovery is carried out by flotation.

Inventors: FRACCHIA PIERLUIGI, OLPER MARCO, FRACCHIA, PIERLUIGI, OLPER, MARCO
Applicants: BUS ENGITEC SERV AMBIENTALI, B.U.S. ENGITEC SERVIZI AMBIENTALI S.R.L.
Application year: 1990
Main classification: C25C1
IPC codes: C01B17, C25C1