ABSORPTION OF ZINC VAPOUR IN MOLTEN LEAD

Abstract: A process for absorbing zinc vapour in molten lead is characterized in that a gas containing zinc vapour is contacted with and then separated from a flowing stream of molten lead in a cyclone.

Description: ABSORPTION OF ZTNC VAPOUR TN MOLTEN EAP This invention relates to an improved apparatus and process for absorbing zinc vapour into molten lead. Gases containing zinc vapour are commonly generated in zinc smelting processes (for example, the Imperial Smelting Process (ISP) ) in slag fuming; and in the treatment of zinc-containing dusts and residues . Existing industrial processes for recovering zinc from gases containing zinc vapour are essentially of three kinds, of which the abovementioned ISP process is one . The ISP process uses rotors or impellers to splash lead from a molten pool into the zinc-laden gas stream. In an alternative ISP process, zinc is used as the condensing medium rather than lead. The so-called SKF process uses molten lead or molten zinc in the form of a spray or curtain as cooling metal or medium towards which the gas stream containing zinc vapour is directed. SUBSTITUTE SHEET References relevant to the processes mentioned include D Temple, "Zinc-lead blast furnace - key developments", 1980 extractive metallurgy lecture to AIME, Metallurgical Transactions B vol. 2B, pp 343-352; GB 1,010,436 (Imperial Smelting); and GB 2,122,648 (SKF) . The ISP process suffers from accretions at the mouth of the condenser and in the condenser/absorption chamber causing frequent stoppages of the furnace operation. These accretions form on surfaces that are below the temperature where solid ZnO forms by the reaction Zn + CO2 —> ZnO + CO. This reaction is called the reversion reaction and the temperature at which it occurs the 'reversion temperature' . The SKF process is free of this problem only because of the highly reduced gas entering the condenser. Both processes suffer from shortcomings such as listed below:- - Build-up of dross within the condenser/absorber - Poor efficiency - The need for a large cooling and liquation circuit which is expensive to build and maintain - Large carryover of lead droplets in the off-gas stream leading to lower zinc recoveries. Further shortcomings of the existing process technologies and how they are overcome by the present invention will be described below. In a principal aspect the invention provides a process for absorbing zinc vapour in molten lead characterised in that a gas containing zinc vapour is contacted with and then separated from a flowing stream of molten lead in a cyclone. ' 3 - Preferably the cyclone contact stage is preceded by a stage in which the molten lea d is introduced into and contacted with the stream of gas containing zinc vapour in a mixing chamber. In a more preferred embodiment, the apparatus of the invention comprises a refractory lined cross ¬ over or off-take with an outlet at the bottom which opens into the mixing chamber that joins a cyclone. Lead is introduced into the chamber by a lead spray directed into the gas stream. This spray produces a dispersion of lead droplets within the gas stream. Lead may be also introduced by additional sprays that completely wet the walls of both the vertical section before the cyclone and also within the cyclone itself. The vertical chamber before the cyclone may also house one or more banks of static mixing elements. These elements not only serve to mix the lead droplets and gas together but also break up the lead droplets. This action causes a high degree of shear and a large contacting area as well as turbulence in both phases. Mixing columns housing static mixing elements are known, and reference may be made by way of example to one such apparatus described in US Patent 4,744,928 to Sulzer Brothers Limited of Switzerland. The static mixing elements in that design are disposed within the chamber in a manner that deflects the flow of fluid impinging thereon and thereby promotes efficient mixing of gases and/or liquids passing tftrough the chamber. Provided the static mixing elements promote efficient mixing, their precise configuration is not critical to the present invention. SUBSTITUTE SHEET 4" The molten lead now containing the absorbed zinc is passed into a system for recovery of the latter as well as for recirculation of the molten lead for renewed absorption. The off-gas is passed to a conventional gas cleaning system. In the accompanying drawings: Fig. 1 is a diagrammatic representation of an exemplary preferred embodiment of the process according to the invention; Fig. la is a diagrammatic representation of an alternative exemplary preferred embodiment of the process according to the invention; and Fig. 2 is a more detailed illustration of a preferred form of the components 2 ( in part ) , 3 ar i d 4 of Fig. 1. The construction and operation of the apparatus will be better understood by reference to Fig. 1 of the accompanying drawing. In this figure 1 represents zinc-laden gas from the smelting or slag fuming operation, and 2 represents a refractory lined off-take. A burner 3, called the 'transition burner', is provided to maintain the temperature of the lower region of the refractory above the Zn-ZnO reversion temperature. A lead spray ( alternatively, a bank of lead sprays) 4 direct (s) molten lead to a vertical chamber 5 which may contain mixing elements 5a to enhance the contact between the zinc-laden gas and the molten lead. A cyclone 6 serves both to contact and to separate the gas and the lead, the latter passing to the gas-cleaning system 7. The zinc-rich lead stream 8 is passed to the pump sump 9 provided with pump 10 conveying lead back through the absorption system , A zinc-lead stream is pumped from the sump 3 via l i ne 11 to a lead cooler 12 and returned to the sump, and via line 11a to lead sprays 4. Numerals 13 and 14 represent a cooling water inlet and outlet T TE SHEtT respectively. A small portion of the zinc-lead is passed via line 16 to the liquation pot 17, which is provided with cooling water inlet 18 and outlet 19 respectively. Numeral 20 represents the zinc product and 21 a launder for returning liquated lead to the pump sum 9. Turning to Fig. la, line 16 of Fig. 1 is replaced by line 16a from pump 10 direct to liquation pot 17 and bypassing lead cooler 12. The items illustrated in Fig. 2 provide an example of a preferred transition burner and lead spray assembly. Process gas, indicated by numeral 39, enters at the top and flows downward through the assembly. A fuel such as propane is precombusted with o ygen. The hot gas is introduced tangentially into a toroid 30 penetrating a circumferential offtake body 38. The toroid 30 serves two purposes. Firstly, it evenly distributes the gas before it exits the burner and, secondly, it serves to heat the offtake body 38. Numeral 30a indicates an exit port ' for hot gas into the central open space defined by the offtake body 38, the gas exiting as shown by arrow 33. Upper and lower circumferential mains, 31 and 32 respectively, are shown for supply of streams of lead or zinc-lead in streams indicated by arrows 34 and 35 respectively. Numeral 36 indicates the presence of baffles to remove the swirl from stream 34 before it is deflected downwardly and towards the centre of the open space. A circumferential truncated cone 37 extends downwardly into the stream of gas 39 and forms part of exit ports 30a. SUBSTITUTE SHEET The surface temperature of the inside of the toroid 30 is maintained at around 1500°C. Furthermore, the burner is run to give a good exit gas velocity ( 18 m/s) of highly reducing gas ( CO/COz ■ 10 ) . As a fine control on the temperature, and to achieve good velocities without the burner getting too hot, nitrogen is also introduced into the burner. The offtake body 38 is heated by the burner otherwise its surface would fall below the reversion temperature. The lower part of the offtake body 38 is directly above the region where lead is sprayed into the absorber. Consequently this lower part loses heat by radiation to the lead. The gases exiting from the exit port 30a serve primarily to stop zinc from diffusing to the top lip of the lead spray causing an accretion. This top lip will always be held below the reversion temperature because of the lead in the spray. The shape of cone 37 was found to be necessary to give protection against diffusion of process gas onto the cold lip of the lead spray. The high turbulence of the process gas greatly enhances the possibility of diffusion. The top lead spray 34 is designed to introduce lead to the centre of the process gas stream. Lead may be introduced tangentially into a main 31 surrounding the spray. The swirl introduced to the lead by the tangential inlet is removed by baffles so that the lead is introduced radially but inclined downwardly into the process gas stream. Lead or zinc-lead is introduced tangentially into a main 32 surrounding the spray. The lead maintains its high swirl and as it exits the spray it flattens itself against the walls.. The swirl is sufficient to give a uniform coating down the mixer column. SUBSTITUTE SHEET A s we ll as introducing lead or zinc-lead to t h e centre of the gas stream the top spray 3 4 is nee d e d to contain t h e highly swirled bottom spray 3 5 . W ithout this containment, lead from the bottom spray would flush upwards. The spray system is designed so that splash upwards onto t h e refractory areas or upward movement from the b ottom spray is substantially non-existent. If splash or upward movement occurs, the refractory is cooled below its reversion temperature and accretion forms. The outlets of the top and bottom sprays are designed to b e close together so that there are no unwetted areas of steelwork. The principal benefits achievable by pre f erre d em b o d iments of the present invention ( designate d ' P asminco') are demonstrated vis-a-vis t h e c h aracteristics of existing technologies in Table 1 below. SUBSTITUTE SHEiΞT Characteristic Pasminco ≤KE ZSΣ. Reasons Contactor size Of the order of Large Large Pasminco design of the absorber 50% reduction in provides for much more shear and volume over ISP, turbulent contact between gas and 30% over SKF liquid. Settler size Of the order of Pasminco settler uses a cyclone 6 75% reduction to separate the lead droplets out over others of the gas, the other technologies rely on gravity settling. Liquation/ 50% reduction in Large Large Pasminco design separates the cooling size plant floor area liquation/cooling functions into two different devices. The Pasminco cooler treats only highly turbulent, high temperature zinc- Cϋ I lead while the liquation pot 03 treats only a small proportion of _ _ the zinc-lead in a quiescent bath. - _ The ISP design relies on a C quiescent open launder where all the lead is brought to the m H liquation temperature. t Growth of Eliminated N.A. due Frequent The transition burner on the curtain to high cause of Pasminco design keeps the entry to accretions at 00 shutdowns the condenser above the reversion the condenser/ content temperature. The transition absorber inlet region between the hot and cold zones is entirely occupied by a flame or a lead spray making reversion impossible. In the SKF design reversion would occur here if the gas mixture was subject to reversion. In the ISP design the curtain forms in this area. Characteristic Pasminco. 2ΣE. ISΣ. Reasons. Build up of Eliminated Severe Severe Both SKF and ISP rely on dross dross within the build-ups build-ups removal by dross being sucked down condenser/ cause cause with the lead into the pump sump. absorber shutdowns shutdowns This is not very positive and dross builds up. Pasminco has no internal lead pool and dross must exit with the lead. Zinc recovery Of the order of 80-90% 90% The more efficient contacting of efficiency 95% or better the Pasminco device produces a higher recovery for the same reasons that permit a smaller size IΛ device . The lower amount of dross c make and lower lead droplet 09 carryover both contribute to H increased efficiency. C Lead droplet Very Low High High The cyclone action of Pasminco's H carryover design eliminates the lead droplet carryover. The other technologies rely on gravity separation. tn _Z rri Furnace dust Some Nil Nil The cyclone action of Pasminco's rn collection design also collects some of the furnace dust . Weight Low Medium High - Pasminco does not have an internal 500 t lead pool, neither does it incorporate a large launder to cool the lead. Power Low Low High - The ISP design has a high power Consumption 700 hp consumption due to the power required to turn the rotors. It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to above.

Claims: CLAIMS : 1. A process for absorbing zinc vapour in molten lead characterised in that a gas containing zinc vapour is contacted with and then separated from a flowing stream of molten lead in a cyclone. 2. A process as claimed in claim 1, wherein the cyclone contact stage is preceded by a stage in which the molten lead is introduced into and contacted with the stream of gas containing zinc vapour in a mixing chamber. 3. A process as claimed in claim 2, wherein molten lead is also sprayed onto the walls of the mixing chamber to prevent reversion on the cold interior surfaces of the chamber. 4. A process as claimed in claims 2 and 3, wherein cold surfaces in the general region where lead is introduced are heated above the Zn-ZnO reversion temperature. 5. A process as claimed in claim 4, wherein a buffer layer of hot reducing gas is inserted between the gas containing zinc and the cold, lead-covered surfaces. 6. A process as claimed in any one of claims 2 to 5, wherein one or more banks of static mixing elements are provided in the mixing chamber to promote mixing between the zinc and the molten lead. 7. A process as claimed in any one of claims 2 to 6, wherein the zinc-rich lead stream from the cyclone is passed to a reservoir of zinc-lead, part of the zinc- lead being cooled and part being recycled to the mixing chamber before the cyclone. SUBSTITUTE SHEET 8. A process as claimed in claim 7, wherein a portion of the cooled zinc-lead is withdrawn to a zinc separation stage. 9. A process as claimed in claim 8, wherein the portion of the cooled stream withdrawn to the zinc separation stage is a relatively small proportion of the mass flow of zinc-lead ' in the circuit. 10. A process as claimed in claim 7, wherein a portion of the reservoir of zinc-lead is withdrawn without cooling to a zinc separation stage. 11. A process as claimed in any one of claims 8 to 10, wherein the zinc separation stage comprises liquation means. 12. A process as claimed in claim 1, substantially as described with reference to the accompanying drawings. 13. An apparatus for absorbing zinc vapour in molten lead, which comprises a cyclone adapted to receive a stream of molten lead or lead alloy together with a zinc-laden gaseous stream, and wherein the two streams may be contacted so as to promote absorption of the zinc into the molten lead, the gas then being separated from the zinc-rich lead. 14. An apparatus as claimed in claim 13 in combination with a vertical mixing chamber adapted to receive the two streams prior to their entry into the cyclone. 15. An apparatus as claimed in claim 14 which comprises spray means within the mixing chamber adapted to spray the internal walls of the chamber as well. as inject a spray of molten lead or lead alloy into the interior space of the chamber. 16. An apparatus as claimed in claim 14 or claim 15, wherein a re ractory-lined cross-over or offtake for receiving gas containing zinc vapour is provided having an outlet in its bottom opening into the vertical mixing chamber. 17. An apparatus as claimed in claim 16, wherein a transition burner is provided in the general region of the cross-over or offtake whereby the temperature of the refractory material, particularly i n its lower region, can be maintained above the Zn-ZnO transition point and a buffer layer of hot reducing gas can be inserted between the gas containing zinc and the cold, lead-covered surfaces in that region of the apparatus. 18. An apparatus as claimed in claim 13, substantially as described with reference to the accompanying drawings. 19. A zinc recovery unit which comprises an apparatus as claimed in any of claims 13 to 17 in combination with means for recovery of zinc from the zinc-rich lead stream and means to return a portion of the zinc-lead to said apparatus. 20. A zinc recovery unit as claimed in claim 19 which comprises in addition, means to cool a portion of the zinc-lead. SUBSTITUTE SHEET 21. A zinc recovery unit which comprises an apparatus as claimed in any one of claims 13 to 17 in combination with: (1) means to convey the zinc-rich stream to (2) a sump for zinc-lead; (3) means to return a portion of the zinc-lead to said apparatus; (4) means to convey a portion of the zinc-lead to ( 5) cooling means, ( 6 ) means to convey a portion of cooled zinc-lead to (7) a liquation pot; and ( 8 ) means to return zinc-depleted lead from said liquation pot to said sump. 2 A zinc recovery unit which comprises an apparatus as claimed in any of of claims 13 to 17 in combination with: ( 1 ) means to convey the zinc-rich stream to ( 2 ) a sump for zinc-lead; ( 3 ) means to return a portion of the zinc-lead to said apparatus; (4) means to convey a portion of the zinc-lead to ( 5 ) cooling means; (6 ) means to return the cooled zinc-lead to said sump; ( 7 ) means to convey a portion of the zinc-lead to ( 8 ) a liquation pot; and ( 9 ) means to return zinc-depleted lead from said liquation pot to said sump. E 23. A zinc recovery unit which comprises a zinc absorption apparatus in combination with: (1) means to convey the zinc-rich stream to (2) a sump for zinc-lead; (3) means to return a portion of the zinc-lead to said apparatus; (4) means to convey a portion of the zinc-lead to (5) cooling means; (6) means to convey a portion of the cooled zinc-lead to (7) a liquation pot; and (8) means to return , zinc-depleted lead from said liquation pot to said sump. 24. A zinc recovery unit which comprises a zinc absorption apparatus in combination with: (1) means to convey the zinc-rich stream to (2) a sump for zinc-lead; (3) means to return a portion of the zinc-lead to said apparatus; (4) means to convey a portion of the zinc-lead to (5) cooling means; (6) means to return the cooled zinc-lead to said sump; (7) means to convey a portion of the zinc-lead to (8) a liquation pot; ' and (9) means to return • zinc-depleted lead from said liquation pot to said sump. 25. A transition burner/spray assembly for use with or in apparatus as claimed in any one- of claims 13 to 17 which comprises a circumferential insulated offtake in combination with: SUBSTITUTE SHEET i - (1) a burner to heat said offtake and supply hot reducing gas to the injection point of (2) spray means located in use below said offtake, - said spray means being adapted to inject lead or lead-zinc radially and downwardly into a stream of zinc-laden gas flowing through said offtake.

Inventors: HOSCHKE MARK IAN, HOSCHKE, MARK, IAN
Applicants: PASMINCO AUSTRALIA LTD, PASMINCO AUSTRALIA LIMITED
Application year: 1990
Main classification: C22B19
IPC codes: C22B19