AU634309B2

AU634309B2 - Condensation of metal vapours in a fluidized bed

Info

Publication number: AU634309B2
Application number: AU66391/90A
Authority: AU
Inventors: Rodney James Dry; Roger Leo Player
Original assignee: Mount Isa Mines Ltd
Current assignee: Mount Isa Mines Ltd; Commonwealth Scientific and Industrial Research Organization CSIRO
Priority date: 1989-11-08
Filing date: 1990-11-07
Publication date: 1993-02-18
Anticipated expiration: 2010-11-07
Also published as: AU6639190A

Description

TITLE: "CONDENSATION OF METAL VAPOURS IN A FLUIDIZED BED"

TECHNICAL FIELD

The present invention relates to the capture of metal vapours from a gas stream, for example zinc vapour from the offgas of a smelting furnace.

BACKGROUND

Traditional methods of extracting zinc from

sulphides involve roasting the concentrate to remove sulphur followed by reduction during which some or all of the zinc is removed as a fume. The zinc is then recovered from the fume by condensation.

Similar processes are used for the recovery of other relatively volatile metals from sulphides.

Although the invention will be herein described

primarily with reference to the recovery of zinc it will be understood to be applicable to other metals including tin, lead, magnesium, cadmium, manganese or the like which may be produced by pyrometallurgical means

involving fuming. The term "fume" as herein used refers primarily to a gas or vapour but includes a gas or vapour with entrained liquid and/or particulate matter.

Zinc blast furnaces, for example, generally use a lead splash condenser (e.g. such as described in British Patent 572961) to capture zinc vapour as zinc liquid and lower the activity of the condensed product. This approach involves relatively large capital investment and has high maintenance costs.

Another form of metal vapour capture currently in use relates to removal of zinc as fume from slag from lead production facilities, with subsequent oxidation. In that process, zinc vapour is fumed from the surface of a molten slag bath and is allowed to travel upwardly until it reaches a location well clear of turbulence associated with the bath surface. At that location an oxygen-containing gas such as air is admitted, and both temperature and oxygen potential are raised to the point where the zinc vapour is rapidly converted to zinc oxide. The zinc oxide has a very much lower vapour pressure than zinc metal and as a result precipitates from the gas stream as a fine powder. The zinc oxide is suitable for further purification by electrochemical methods, but must first be captured in suitable dust extraction equipment. This very fine dust is difficult to handle, and its capture once again requires large capital investment.

It has been found that by appropriate adjustment of temperature and oxygen potential in a furnace it is possible to achieve commercially significant direct separation of metal as a vapour from sulphide containing materials. However, in this case the furnace offgas contains sulphur dioxide together with the metal vapour and the sulphur dioxide reacts with molten metal in the splash condenser. Accordingly, it has hitherto been considered that the vapour would need to be oxidized prior to recovery.

THE INVENTION

An object of the present invention is to provide improved and/or more cost-effective means for recovery of a metal from a hot gas stream including the metal as a vapour, and more particularly for recovery of the metal in acceptable yield when there are species in the gas stream which are reactive with the metal.

The invention achieves that object by condensation of metal vapour directly from hot offgas on or in the vicinity of the comparatively cool surface of solid particles in a fluidized bed.

According to one aspect the invention consists in a method of recovery of a metal from a gas stream

containing the metal as a vapour, said method comprising the steps of:

(a) bringing the stream into direct contact with a fluidized bed of solid particles, the bed having a particulate loading of greater than 10 Kg/m³ and

(b) controlling the fluidized bed so as to include an inventory of particles having a combined mass, specific heat and temperature such as to reduce the temperature of the vapour in heat exchange

relationship with the bed to below the condensation temperature of the metal, whereby to condense the metal vapour.

Furnace offgas streams sometimes contain

particulate matter either as a result of entrainment in the furnace, or of introduction of abrasives to reduce accretion on downstream cooling surfaces to improve heat recovery.

However, in such cases the particulate density is low, that is to say not greater than 10 Kg/m³ and usually much less. In the present case the fluidized bed has a particulate loading which is preferably greater than 200 Kg/m³, more preferably greater than

400 Kg/m³ and as much as 1600 Kg/m3 or more.

Preferably, the thermal mass (mass multiplied by specific heat) of particles in the fluidized bed is such that if the bed were uncooled, the rate of temperature rise of the bed in heat exchange relationship with the furnace offgas would be less than 100°C/sec and

desirably less than 20°C/sec.

For preference the offgas stream has a temperature of above 960°C (more preferably above 1100°C) upon entry to the fluidized bed and the temperature of the gas leaving the bed is reduced to below about 500°C

(essentially to below the condensation temperature of the metal vapour and preferably to below the melting point of the metal to be condensed, e.g. to below 419°C in the case of zinc, to below 327°C in the case of lead, and to below 321°C in the case of tin) within a period of less than 1 second, more preferably less than 100 millisecs and desirably less than 1 millisecs.

Desirably the offgas stream is produced in a generally closed furnace containing a bath into which is injected gas via a submerged lance and the offgas is brought into contact with the fluidized bed particles without prior exposure to precooling devices.

The present invention aims at capturing metal vapour directly from hot offgas by condensation on, or in the vicinity of, the surface of cool particles in the fluidized bed. In preferred embodiments of the

invention there is direct coupling between the smelting furnace and the fluidized bed used for condensation - that is to say there is no significant attempt at heat recovery located between the furnace and the fluidized bed. Desirably, steps are taken to maintain or increase the offgas temperature relative to the bath

temperature. Further, the invention recovers a high proportion if not all of the metal as the elemental metal rather than as an oxide thereby avoiding the need subsequently to convert oxides to metal by

electrowinning.

In the case of zinc condensation it is well known that, thermodynamically, zinc vapour reverts to zinc oxides or sulphides as a typical zinc-laden smelter offgas containing oxygen and sulphur dioxide cools (e.g. from 1200°C to 500°C). The formation of such

non-metallic materials has been a major obstacle in the path of development of direct condensation processes in the past.

The current invention seeks to avoid that

difficulty by the rapidity of the quench action

involved. A thermodynamic driving force for the

formation of non-metallic compounds is allowed, but the actual formation of such compounds is inhibited by kinetic factors. It has been found that reaction rates for the formation of undesired (non-metallic) materials are such that, at low temperatures (e.g. 100°C - 350°C), formation is too slow to be significant in this context even though the thermodynamic driving force is strong. The quench action involves cooling the hot smelter offgas (e.g. at from 1200°C to 1300°C when containing sulphur dioxide) to a temperature below the melting point of the metal of the vapour very rapidly. The result is an effective system which avoids many of the difficulties and economic penalties associated with current technology.

The exact form of the fluidized bed system is believed to be not critical. It may be a shallow bubbling/spouted bed, or a circulating fluidized bed. Whichever form the system takes, it should be designed to maintain a significant inventory of cool particles in the vicinity of the hot gas entry point into the bed. The particles are selected to have a thermal mass that is large by comparison with that of the incoming gas and, in addition, to have a large effective surface area exposed to the gas for heat and mass transfer. The result is an extremely fast quench (of less than 1 second, typically less than 100 milliseconds and

desirably less than 1 millisecond) which prevents excessive formation of undesired materials e.g.

reversion to sulphur or oxygen compounds of the metal.

The nature of the particulate solid used is also not critical, though it is advantageous to use solids of the same material as that being condensed. With

appropriate seeding, it is possible to operate the condensation unit as a direct producer of particulate metal. The particle size range depends on the

application, but it is generally advantageous to operate with as fine a distribution as can be retained in the system. Maximum grain size in the bed is preferably less than 2mm, and more preferentially is less than 0.5mm. Energy balance constraints are such that heat removal from the fluidized bed is needed. This may be achieved in a number of ways such as by direct water addition or heat transfer to a cooling fluid. In the case of a cooling fluid, the layout of the required heat transfer area is a matter of engineering convenience and depends on the form of fluidized bed chosen.

DESCRIPTION OF PREFERRED EMBODIMENTS

Two embodiments in accordance with the current invention will now be described by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a bath-smelting furnace of a

non-recirculating form;

Figure 2 is a bath-smelting furnace of a

recirculating form.

The first of these schematically illustrates non-recirculating fluid bed condensation, and the second schematically illustrates circulating fluid bed (CFB) condensation.

Non-Recirculating Condensation

With reference to Figure 1, Furnace 101 is

maintained at an appropriate furnace temperature (e.g. bath temperature 1200°C) and oxygen potential by the introduction of an oxygen-containing gas via a lance 102 and ore/slag/coal feed 103. Metal vapour, for example zinc vapour in the case of zinc smelting, leaves liquid bath 104 and enters gas space 105 with furnace offgases, and from here flows into throat 106. The velocity in this throat is maintained at a value which prevents solids from falling downward from the fluidized bed into furnace 101.

The upwardly expanding duct above throat 106

contains cool (below 500°C and preferably below 419°C in the case of zinc, for example about 360°C) metal

particles in the form of a dense fluidized bed 107 defining a first zone. Average bed density is in the range of from 200 to 1600 Kg/m³ and preferably above 400 Kg/m³. A rapid quench is achieved as the hot

furnace gas at above 960°C, and preferably above 1100°C, comes into contact or at least into heat exchange

relationship with the particulate material, and material condenses on the surface of or in the vicinity of the particles. Particle diameters of less than 2 mm are preferred, and more preferably less than 0.5 mm diameter.

Cooled gas disengages from the solid in freeboard region 108, and leaves the system via outlet 109.

Cooling of fluidized bed 107 is achieved for

example by a combination cooling surface on the walls of the vessel, tube bundles within the vessel and/or water spray injection 110. A valved duct 111 is provided for draining solids rapidly from the system - this is used primarily for metal recovery, shutdown and in

emergencies. Recirculating Condensation

Identification numbers 201 to 206 identify parts of Figure 2 which correspond in function to parts 101 to 106 of Figure 1, and this system differs from that described previously only in the form of the fluidized bed used above throat 206.

In the CFB embodiment a vertical riser 207 is provided, and gas velocity in this duct (first zone) is such that average suspension densities in the range 10 to 400 Kgs. per cubic meter are maintained, preferably 200 to 400 Kgs/m³ in the first zone. These gas

velocities are in the range of from 2 to 15 m/s. Gas and entrained solids leaving the top of the riser enter a cyclone 208, and gas leaves the system via outlet

209. Solids captured in the cyclone are returned to the lower region of the riser via a suitable transfer device, a loopseal 210 being one such device.

The product metal may be removed continuously by means of one or more cyclones 208 from which gas free of particles is discharged at outlet 209. A proportion of solids recovered in the cyclone 208 are cooled at a second zone and directed back into the fluid bed while the remainder is collected as product via a valved duct 211.

Fluidized bed cooling is achieved via heat transfer surface, for example on the walls of the riser and/or the walls of the cyclone and/or tube bundles in the riser. Water spray-cooling 211 is optionally used.

Reducing the gas temperature to below 419°C ensures in the case of zinc, that the particles are below the melting point of the metal so that the particles grow directly from the vapour without formation of molten zinc. However, the process is not limited to use in zinc vapour condensation. In both illustrated

embodiments particles of fine slag, sand, or metal from previous operation, may be used to establish the initial fluid bed.

EXAMPLE

A recirculating fluid bed condenser with a riser of 200mm in diameter and 3m high was set up on a furnace in which zinc containing lead blast furnace slag could be melted at 1200°C and reduced with coal to form a source of metallic zinc vapour for the condenser. In addition, the gases were enriched with metallic zinc vapour by also feeding solid metallic zinc into the furnace, to be melted and evaporated. The blast furnace slag was fed at a rate of 140Kg per hour and the zinc metal fed at 26Kg per hour. This yielded a total zinc feed rate of 44Kg per hoμr. After the furnace was heated to

temperature and stabilised the circulating fluid bed condenser was fitted onto the furnace and operated for 40 minutes.

The recirculating fluid bed condenser was prepared for operation as follows. Cold air was drawn through the condenser at 250 Nm³/hr and crushed silica was added to form a

fluidised bed of density approximately 400Kg/m³. The recirculating rate of solids through the cyclone

assembly and return to the bed was 2 Kg/sec, and the particle size, after multiple recirculation through the bed was shown in Table 1. More than 90% of the

particles had a mean volume diameter of less than 0.5 mm. The condenser was then lowered into the furnace, air excluded, and gases containing zinc vapour drawn into the condenser. The bed temperature was held at 190°C by cooling water circulating in external jackets around the bed. A typical heat extraction rate into the cooling water was 68 kW. During the course of the test the temperature of the zinc vapour at the entry to the fluid bed ranged from 960 to 1100°C with an average temperature of 1030°C. Zinc metal condensed on the surface of the bed material.

Samples of the bed material were taken

periodically. The results are given in Table 2.

It was seen that the total zinc content of the bed material progressively increased up to 15% by weight, over the 40 minutes of the test. The metallic zinc content of the bed material was consistently 60% of the total zinc content; the remainder being zinc oxide and zinc sulphide. Thermodynamic calculations of the equilibrium distribution of zinc between the three phases, zinc metal vapour, zinc oxide and zinc sulphide, are given in Table 3 for the conditions of the test for a range of gas inlet temperatures to the condenser.

Under equilibrium conditions a metallic zinc f raction of about 60% would be obtained at an inlet vapour temperature of 880°C. This is about 150°C lower than the measured operating temperature of the vapour inlet to the condenser. This temperature difference may reflect the non-equilibrium nature of the condenser. Additionally in this test the yield of the metal was also depressed by air leakage into the bed via an unintentional leak in the loop seal 210. It is

anticipated that in practice a closer approach to equilibrium could be achieved with a correspondingly higher yield of zinc as zinc metal.

q

In practice of the invention it is desirable to control the oxygen potential in the vapour stream entering the bed. If the gas stream contains reactive species which are in chemical equilibrium with the metal, then the partial pressure of oxygen in the gas stream is controlled to promote the formation of the metal and inhibit

formation of reaction or reversion products. In the present example the oxygen partial pressure is controlled to inhibit the formation of zinc sulphides or oxides under chemical equilibrium conditions.

The optimum oxygen potential at the point of entry of the gas stream to the bed is typically significantly different from that which is the optimum in the smelting bath in the furnace.

Oxygen concentration in the vapour stream at the entrance to the bed can be controlled by introducing inert gases to the furnace, or by maintaining a positive

pressure to prevent air ingress, or by adjusting feeds to consume a predetermined ratio of oxygen, or by use of after burners located between the bath and the fluidized bed.

Likewise, the temperature of the vapour entering the fluidized bed may be controlled at an elevated level by use of after burners or other heaters and/or by furnace design to minimize heat loss prior to quenching.

The furnace gas stream might include as reactive species, by way of further example:

sulphur containing species such as sulphur dioxide, sulphur and hydrogen sulphide,

oxygen containing species such as sulphur dioxide, carbon dioxide and carbon monoxide and water vapour,

halogen containing species such as hydrogen chloride, hydrogen fluoride and chloride,

and NOX species.

Typically the gas stream will include a mixture of such species which will vary according to the source of the metal and of other furnace contents and operating conditions .

By way of example the calculated composition of the gas stream for a zinc smelter is shown in table 4.

In each case, the oxygen potential and temperature at entry to the bed which will result in optimum recovery of elemental metal can be determined based upon the teaching hereof by routine experiment.

Photomicrographic evidence indicates that a significant proportion of the metal vapour condenses in the vicinity of the fluidized bed particles rather than on the surface of bed silica particles. A proportion of the metal is thus recovered in the form of very fine metal particles (for example 0.2 micrometers or less) devoid of silica. Much of the metal found on the fluidized bed silica particles is believed to be deposited on the bed silica particles by impact with fine metal particles.

The metal particles and metal coated bed particles may be separated from the gas stream by conventional means including cyclones, bag house filters and the like.

Although the invention has been described primarily with reference to the recovery of metals from gas streams in which species reactive with the metal are present, the invention is also applicable for the recovery of metals in the absence of such species for example in metal

purification.

As will be appreciated by those of ordinary skill in the art from the teaching hereof, the invention may be conducted by other means without departing from the concept herein disclosed.

Claims

1. A method of recovery of a metal from a gas stream containing the metal as a vapour, said method comprising the steps of:

(a) bringing the stream into direct contact with a

fluidized bed of solid particles, the bed having a particulate loading of greater than 10 Kg/m³ and

(b) controlling the fluidized bed so as to include an

inventory of particles having a combined mass, specific heat and temperature such as to reduce the temperature of the vapour in heat exchange

2. A method according to Claim 1 wherein the gas stream is a furnace offgas stream and contains a species reactive with the metal vapour, said method further comprising the steps of:

(c) maintaining the offgas stream at a temperature

substantially equal to or greater than the furnace temperature prior to contact with the bed,

(d) controlling the fluidized bed so as to include an inventory of particles having a combined mass, specific heat and temperature such as to reduce the temperature of the offgas stream in heat exchange relationship with the bed to below the condensation temperature of the metal at a rate of change of temperature selected to minimize reaction between the vapour and the reactive species whereby to condense the metal vapour and

(e) separating the condensed metal from the remaining

stream.

3. A method according to Claim 1 or Claim 2 wherein the temperature of the gas leaving the bed is reduced to below the melting point of the metal.

4. A method according to any one of the preceding claims wherein the temperature at which the gas stream is brought into contact with the bed and the average temperature of particles in the bed is selected so as to optimize the recovery of the vapour as elemental metal.

5. A method according to any one of the preceding claims wherein the oxygen potential in the gas stream to be brought into contact with the bed is controlled so as to maximize the recovery of the vapour as elemental metal.

6. A method according to any one of the preceding claims wherein the particulate loading is greater than 200

Kg/m³.

7. A method according to any one of the preceding claims wherein the gas stream has a temperature greater than 960°C upon entry to the fluidized bed.

8. A method according to any one of the preceding claims wherein the gas stream has a temperature greater than 1100°C upon entry to the fluidized bed.

9. A method according to any one of the preceding claims, wherein the temperature of the gas stream upon leaving the bed is reduced to below 500°C.

10. A method according to any one of the preceding claims wherein the temperature of the gas stream upon leaving the bed is reduced to below 350°C.

11. A method according to any one of the preceding

claims, wherein the thermal mass of the bed in heat

exchange relationship with the gas stream is such that in the absence of cooling the rise in temperature of the bed would be less than 100°C/sec.

12. A method according to Claim 11 wherein the thermal mass of the bed in heat exchange relationship with the offgas stream is such that in the absence of cooling the rise in temperature of the bed would be less than 20°C/sec.

13. A method according to any of of the preceding claims, wherein the particles of the bed have an average grain size not exceeding 2mm in diameter.

14. A method according to any one of the preceding

claims, wherein 90% of the particles of the bed have a grain size not exceeding 0.5mm in diameter.

15. A method according to any one of the preceding

claims, wherein the metal vapour is zinc.

16. A method according to any one of the preceding claims wherein the particles of the bed are metal particles of the same metal as the metal of the vapour.

17. A method according to any one of claims 1 to 15 wherein the particles of the bed are silica.

18. A method according to any one of the preceding claims further comprising the step of continuously removing a proportion of the solids in the fluidized bed and

recovering the condensed metal therefrom.

19. A method according to any one of the preceding claims wherein the fluidized bed is a continuously recirculating fluidized bed.

20. A method according to any one of the preceding claims wherein the metal of the vapour is selected from the group consisting of zinc, lead, tin, magnesium, cadmium and manganese.

21. A method according to Claim 20 wherein the metal is zinc.

22. A method according to Claim 20 wherein the metal is tin.

23. A method according to Claim 20 wherein the metal is lead.

24. A method according to Claim 20 wherein the metal is magnesium.

25. A method according to any one of the preceding claims wherein the gas stream contains a plurality of metals. each as a vapour.

26. A method according to any one of claims 2 to 25 wherein the gas stream contains one or more reactive species selected from the group consisting of sulphur, oxygen, halogen or nitrogen containing species.

27. A method according to any one of claims 2 to 26 wherein the gas steam contains at least one reactive species selected from the group consisting of sulphur, sulphur dioxide and hydrogen sulphide.

28. A method according to any one of claims 2 to 26 wherein the gas stream contains at least one reactive species selected from the group consisting of carbon dioxide, carbon monoxide or water.

29. A method according to any one of claims 2 to 26 wherein the gas stream contains at least one reactive species selected from the group consisting of hydrogen halides and halogens.

30. A method according to any one of claims 2 to 29 further comprising the step of controlling the oxygen potential of the gas stream so as to inhibit reaction between the reactive species and the metal.

31. A method according to any one of the preceding claims further including the step of heating the gas stream so that the temperature at which the vapour is brought into contact with the bed is greater than the furnace

temperature.

32. A method according to any one of the preceding claims wherein the temperature of the gas stream in heat exchange relationship with the bed is reduced to below the

condensation temperature of the metal in less than 1 second.

33. A method according to Claim 33 wherein the

temperature of the gas stream in heat exchange relationship with the bed is reduced to below the

condensation temperature of the metal in less than 100 milliseconds.

34. A method according to Claim 32 wherein the

temperature of the gas stream in heat exchange

relationship with the bed is reduced to below the

condensation temperature of the metal in less than 1 millisecond.

35. A method according to Claim 1 substantially as herein described with reference to any one of the examples.

36. Apparatus for recovery of a metal from a hot gas stream containing the metal as a vapour, said apparatus comprising:

means for fluidizing a bed of solid particles having a particulate loading of greater than 10 kilograms per cubic meter;

means for bringing the gas stream into direct contact and heat exchange relationship with the fluidized bed at a first zone and

means for maintaining an inventory of particles of the fluidized bed in the first zone at a temperature below the condensation temperature of the metal vapour.

37. Apparatus according to Claim 36 wherein the fluidized bed is recirculated through the first zone.

38. Apparatus according to Claim 37 wherein particles of the fluidized bed are cooled at a second zone before recirculation to the first zone.

39. Apparatus according to any one of claims 36 to 38 in combination with a furnace operating at a temperature greater than 960°C to produce the hot gas stream.

40. Apparatus according to Claim 39 wherein the furnace is operating at a temperature greater than 1100°C.

41. Apparatus according to any one of claims 36 to 40 wherein the fluidized bed has a particulate loading of greater than 400 Kgs./m³.

42. Apparatus according to any one of claims 36 to 41 wherein the fluidized bed comprises silica particles.

43. Apparatus according to any one of claims 36 to 42 wherein the means for maintaining an inventory of cooled particles includes water spray injections.

44. Apparatus according to Claim 43 wherein the water is sprayed into the first zone.

45. Apparatus according to any one of claims 36 to 44 wherein an inventory of particles in the bed is maintained at below 500°C.

46. Apparatus according to any one of claims 38 to 45 further comprising means for maintaining the furnace offgas stream at above 960°C until it enters the fluidized bed.

47. Apparatus according to any one of claims 38 to 45 further comprising means for maintaining the furnace offgas stream at above 1100°C until it enters the fluid bed.

48. Apparatus according to any one of claims 36 to 47 further comprising means for separating a portion of the solids in the fluid bed from the remainder.

49. Apparatus according to any one of claims 38 to 45 including means for raising the temperature of the gas stream to above the furnace temperature.

50. Apparatus according to any one of claims 36 to 49 further including means to control the oxygen potential in the gas stream prior to entering the first zone.

51. Apparatus according to any one of claims 36 to 50 wherein the particles of the bed have a thermal mass such that if the bed were uncooled the rate of increase in temperature of the inventory of particles in the first zone would be less than 100°C/second.

52. Apparatus according to any one of claims 36 to 50 wherein the particles of the bed have a thermal mass such that if the bed were uncooled the rate of increase in the temperature of the inventory of particles in the first zone would be less than 20°C/second.