A METHOD FOR RECOVERING LEAD FROM A LEAD-CHLORIDE RAW MATERIAL

Abstract: A process for recovering lead from lead chloride raw materials obtained particularly hydrometallurgically, by reducing said raw materials with a reducing gas. The process is suitably effected with hydrogen or ammonia in a column reactor (1) in counterflow, liquid lead chloride being contacted with the reducing gas, suitably at a temperature between 900 and 970`C. The column reactor (1) may comprise a plurality of reactors (22, 26, 30) coupled in series. Formed lead metal (5) is tapped-off from the bottom (15, 35, 36, 37) of respective columns. Non-reacted lead chloride (6) and optionally purified process gases can be recirculated (8, 9, 10, 7 and 19, 16 respectively).

Description: A METHOD FOR RECOVERING LEAD FROM A LEAD-CHLORIDE RAW MATERIAL Since time immemorial lead has been produced primarily by the pyro- etallurgical treatment of lead sulphide. The pyrometal urgical treatment of lead sulphide in the manufacture of lead is still today the most common method used and normally comprises a * sintering pro¬ cess followed by reduction and smelting processes in a shaft furnace. Even though this technique can be considered in present times to be a well developed technique, it has still been unable to satisfy those requirements placed today on emission levels, energy consumption and economy. During recent years many companies and institutions have been en¬ gaged in research in an attempt to find ways and means in which lead can be manufactured economically while, at the same time, satisfying demands with respect to the care and protection of the environment, this research resulting in the proposal of new processes, which are often hydro etallurgical processes. In general, the proposed pro¬ cesses include the leaching of lead sulphides in a chloride environ¬ ment during simultaneous oxidation. Subsequent to the solution being purified from contaminants, it contains lead chloride in a dissolved form. The lead chloride can be reduced to metal form directly, by electrolysis, although the lead thus obtained is porous and does not adhere satisfactory to the surface of the cathode, and hence there is obtained a lead-metal product which is difficult to remove from the electrolysis cell. Another proposed method of reduction is one which is based on the concept of crystallizing lead chloride from the purified solution, whereafter said lead chloride is subjected to a smelt-electrolysis process, lead metal and chlorine gas being formed. Up to the present day, however, this method has not been shown to be sufficiently economical . It has also been proposed to reduce lead chloride with a hydrogen gas, by blowing said gas from beneath through a layer of molten lead, upon which a layer of molten lead chloride lies. The gaseous lead chloride departing with the gas at the reduction temperature is condensed by supplying cold, moist lead chloride from above. The reaction gas containing hydrogen and hydrogen chloride is combusted above the salt bed of lead chloride, in order to provide the process with the necessary heat supply. The heat economy of the process and the manner in which the hydrogen gas is utilized in said process is so poor, however, that hitherto the method cannot be justified com¬ mercially; neither is it probable that the method will be commerci- ally-justifiable in the future. It has now been surprisingly found that lead can be recovered from lead chloride raw materials on a commercially viable scale by reduc¬ tion with a reducing gas. The process is based on the concept of bringing liquid lead chloride in a column reactor into contact, in counterflow, with a stream of reducing gas; and recirculating the non-reduced lead chloride, optionally subsequent to adjusting the temperature. The lead reduced in the process is separated in a bottom container and removed. Suitably, the temperature of the lead chloride is maintained within a range of between 900 and 970°C, the lower temperature limit being determined by the fact that beneath this temperature, the equi- librium of the reaction for forming metallic lead from lead chloride at normal process pressures is displaced to the left, and hence the reaction yield of the lead chloride is very low and, as a result thereof, the major part of the reaction product must be recirculated. On the other hand, in the case of temperatures exceeding about 900°C, the reaction equilibrium is displaced to the right and con¬ siderably higher reaction yields can be achieved; the higher the temperature selected the greater the yield of the reaction. However, at the upper temperature limit, about 970 C, a substantial part of the lead chloride will as a vapour accompany the residual gas and formed hydrogen chloride gas, owing to the increasing vapour pressure of the lead chloride, and said lead chloride must be con- densed from the process gases in order not to be lost in the process. Optimation of the temperature within the preferred range, and also in special circumstances outside said range, is determined by the specific conditions prevailing in the choosen plant. If an elevated pressure is applied, an elevated temperature can thus be choosen, therewith to influence the reaction yield positively, without any appreciable vaporization of lead chloride and losses to the resi¬ dual gases. On the other hand, it may be advantageous in certain cases to use a low process pressure, thereby enabling low reaction temperatures to be used, primarily with respect to the construction of the apparatus used. The reduction gases used may be reducing gases, such as hydrogen, gaseous carbon compounds, such as hydro¬ carbons, carbon monoxide and the l ke, and ammonia or mixtures thereof. Among those reduction gases which can conceivably be used, ammonia and hydrogen are preferred. The use of hydrogen, however, is particularly preferred. The process particularly relates to a reduction process in a column reactor provided with packings or intermediate bottoms, in which reactor hydrogen or some other reducing gas is contacted with liquid lead chloride in counterflow. The temperature in the bottom con¬ tainer is maintained at a level above the melting point of lead chloride, 501°C, and suitably beneath its boiling point, 954°C. Lead melts at 327.5°C and boils at a temperature immediately above 1700°C. The process is carried out continuously, by continuously conveying molten lead chloride up to the upper part of the column . reactor, suitable means being provided for heating the lead chloride melt to a temperature above 900 C during the continuous trans- port of said melt. The heated chloride is then caused to pass down¬ wardly through the reactor and is contacted with and reacts with hydrogen, or some other reducing gas, in counterflow, said gas being supplied continuously to the lower part of the reactor. The metallic lead forme -by the reaction is collected in the lower part of the reactor, from where the lead is tapped-off continuously, or at suitable intervals, while non-reduced lead chloride is re- circulated in the process and returned to the upper part of the column reactor. Raw material in the form of molten lead chloride is also supplied continuously during the process, or at uniform inter¬ vals of time. The process can also be carried out in a line of column reactors coupled in series, in which molten lead chloride is transferred from the bottom container of the first column reactor to the upper part of a second column reactor, where said molten lead chloride is treated with reducing gas at elevated temperature. The lead chloride can then be transferred, at further elevated temperature, to a third column reactor, and to further reactors, and from there returned to the upper stage of the first column reactor. In this method of pro¬ cedure, the reaction temperatures in the latter stages can be choosen so as to obtain a high reaction yield, without the corresponding risk of losses of lead chloride in vapour form. This first stage in the line of series-coupled reactors is preferably effected at a temperature which provides an acceptable low loss of lead chloride to the process gas. The following formulae are indicating the reac- tions between lead chloride and hydrogen, used as the reducing gas, in the column reactor: PbC1 2(l) + H 2(g) Pb + 2HC1 (9) (1) PbCl Pb + 2HC1 2(g) + Mg) (g) (2) Lead chloride melts at a temperature of about 500°C, and has a boiling point at atmospheric pressure of about 954°C. Thermodynamic- ally, the conditions of equilibrium for the reactions can be seen from Figure 1, where G in kcal/mole are given for the reduction of solid, liquid and gaseous lead chloride with hydrogen. The relationship G= +40060 + 16.18 T log T - 83.74 T, is applicable for formula (1), where T is given in Kelvin; from which it will be seen that the process can be effected at temperatures higher than 903°C (1176 K) with gas activities equal to 1. OiVFI A corresponding relationship ' A G = -5740 - 14.33 T log T + 47.48 T is applicable to formula (2), which illustrates that the reaction with gaseous lead chloride is ther odynamically possible, irrespect¬ ive of the reaction temperatures. Upon closer analysis, it will be seen from the above formulae that the process should be carried out with a surplus of liquid lead chloride in respect to available hydrogen. Corresponding thermo¬ dynamic conditions also apply to ammonia, where the reducing com- ponent is hydrogen, but exists in nascent form and is thus more re¬ active. The invention will now be more closely described with reference to the accompanying drawings, in which Figure 1, as beforementioned, illustrates the thermodynamic conditions for said process when re¬ ducing lead chloride with hydrogen, and Figure 2 illustrates an apparatus for reducing lead chloride:- in accordance with the inven¬ tion, said apparatus including a column reactor, and Figure 3 illustrates an arrangement according to the invention comprising three column reactors in series. Thus, in Figure 2 there is illustrated a column reactor 1 provided with a column part 2 having packings 3 arranged therein, and a bottom container 4 in which, in continuous operation, there is located a lower lead-layer 5 and an upper layer 6 of non-reduced, liquid lead chloride. Arranged above the column is a liquid lead chloride distributing means 7. Lead chloride is supplied from the bottom container 4 to a pump 9, via a line 8, said lead chloride being conveyed from the pump 9 to the distributing means 7 via a line 10. Arranged in the line 10 is a means 11 for heating the lead chloride with a heating coil 12 and a supply line 13 for raw material, viz. liquid lead chloride. The bottom container may also be provided with a heating means 14, and a device for tapping-off reduced lead 15. Reduction gas is supplied to the column reactor through a line 16, and departs from said reactor with reaction gases, through a line 17. The reaction gases are cooled in a cooler 18 in a manner such as to condense the gaseous lead chloride entrained with said gases, where- after the reaction gases, which comprise hydrogen chloride and non- reacted reduction gas, depart through a line 19. The reaction gases are recirculated, suitably totally or partially and optionally after absorbing hydrogen chloride therefrom by washing said gases in aqueous . solution and returned, to said column via a feed-back line (not ≤hown) to said line 16. The cooler is provided with cooling coils 20 and a line 21 through which condensed lead chloride is re¬ turned to the reactor. Figure 3 illustrates an embodiment of the invention comprising three column reactors coupled in series, liquid lead chloride, having a temperature of between 900 - 970°C, being supplied to a first reactor 22 through a line 23. Lead and lead chloride are separated into two separate layers in a bottom container 24 of the column reactor 22. Lead chloride is passed through a line 25 to a second column reactor 26, subsequent to having passed a heating means 27. Liquid lead and lead chloride are separated in a bottom container 28 of the second reactor 26. Lead chloride is introduced to a third column reactor 30 through a line 29 which incorporates a heating means 31. Lead and lead chloride are separated from one another in a bottom container 32 of the third column reactor 30, said lead chloride being returned to the upper part of the first column reactor 22 through a line 33 having a heating means 34 incorporated therein. Lead is taken from the column reactors 22, 26 and 30 through lines 35, 36 and 37 respectively. Reaction gas is supplied to the bottom container 32 of the third column reactor 30 through a line 38, and passes from the upper part of said reactor to the bottom container 28 of the second reactor 26 through a line 39, from where it is passed from the upper part of said second column reactor 26 to the bottom container 24 of said first column reactor 22 through a line 40. Reaction gases are removed from the reactor through a line 41, incorporating a cooling means 42, and a line 43, said cooling gases OΛ ' .PI being recirculated, suitably subsequent to washing said gases to absorb hydrogen chloride contained therein, to the bottom container 32 of the column reactor 30, through a line 38, to form part of the reduction gas. Lead chloride condensed in the cooler means 42 is re- turned to the first column reactor 22 via a line 44.

Claims: CLAIMS: 1. A process for recovering lead from a lead chloride raw material, by reduction with a reducing gas, characterized by bringing liquid lead chloride into counterflow contact, in a column reactor, with a stream of reducing gas; and by recirculating non-reduced lead chloride, optionally subsequent to modifying the temperature. 2. A process according to claim 1, wherein said reduction is effected at a temperature of between 900 and 970°C, preferably at a temperature of between 930 and 950°C. 3. A process according to claim 1 or 2, wherein said reducing gas is ammonia or hydrogen gas. 4. A process according to claims 1-3, wherein non-reduced liquid lead chloride is transferred from a first column reactor to one or more further reactors located one after the other; and wherein said lead chloride is contacted in counterflow with reducing gas, where¬ after said non-reduced lead chloride is returned to the upper part of said first column reactor. 5. A process according to claims 1-4, wherein outgoing reaction gases are recirculated in the process, either totally or partially, optionally subsequent to washing said gases with an aqueous solution to remove hydrogen chloride therein by absorption. GlmmTl

Inventors: BAECK E
Applicants: BAECK E, BOLIDEN AB
Application year: 1979
Main classification: C22B13
IPC codes: C22B13