WO2017129816A1

WO2017129816A1 - Method for the carbothermal reduction of zinc oxide to zinc

Info

Publication number: WO2017129816A1
Application number: PCT/EP2017/051926
Authority: WO
Inventors: Ulrich Bech
Original assignee: Ulrich Bech
Priority date: 2016-01-29
Filing date: 2017-01-30
Publication date: 2017-08-03
Also published as: CN108699626A

Abstract

The invention relates to a method for the carbothermal reduction of zinc oxide to zinc, comprising the following steps: feeding zinc oxide and coal into a furnace as reactants, reducing the zinc oxide to liquid zinc melt (20) by using the coal as a heating agent and as a reactant, and tapping the liquid zinc melt (20) from the furnace as a product. Heat is supplied for the endothermic reaction by the condensation of zinc vapor (24) blown into the furnace, the zinc vapor acting as a heating vapor.

Description

Process for the carbothermic reduction of zinc oxide to zinc

Field of the invention

The invention relates to a process for the carbothermal reduction of zinc oxide to zinc and to a rotary kiln for its conversion,

State of the art

From the state of the art, the novel method of reducing zinc oxide is known as a traditional process which uses the introduced carbon as chemical

Reagent and also used as a heating agent of the endothermic reaction. The necessary temperature for the endothermic reaction is above 1000 ° C, if an intensive, efficient production is to be achieved. If instead of e.g. Almost pure charcoal cost-effective but contaminated boiler coal is to be used, then the specific consumption is high and the resulting exhaust gases when fired with atmospheric oxygen must be cleaned consuming. Unfortunately, it is a reality that such smelters are often moved to locations where the emissions controls are less stringent.

In order to reduce the high specific consumption of coal, several research institutes tried to integrate concentrated solar energy into the heating process. However, a successful production technique has never been developed.

On abe of the invention

From the disadvantages of the described prior art, the object initiating the present invention results in proposing a process for the carbothermic zinc oxide reduction, which uses concentrated solar energy in order to reduce the high coal consumption. In addition, the process should run reliably despite the high temperatures.

description

The solution of the problem is achieved in a method for carbothermic reduction of zinc oxide to zinc in that the heat for the endothermic Reaction by the condensation of a in the furnaces, eingehlasenen zinc vapor, which acts as heating steam takes place. The zinc vapor has the advantage that it can not only give off the enthalpy of steam as a heating medium, but also gives off the enthalpy of condensation, which is released during the transition to the liquid state of aggregation. The molten zinc mixes with the molten zinc in the furnace to the final product. The prerequisite for this method is the presence of a sufficient source of energy, which can evaporate liquid zinc. A rotary kiln is preferably used as the furnace, although a shaft furnace or a roasting furnace would also be suitable for implementing the method.

In a particularly preferred embodiment of the invention, a portion of the molten zinc is recycled as the zinc vapor. This makes it possible to realize a closed heating circuit in which the molten zinc is evaporated and in the. Oven is condensed again.

The invention is characterized, preferably characterized in that the part of the molten zinc, which is recycled as .Zinkdampf, in. A cooling circuit of a high-temperature receiver for collecting concentrated radiation, in particular solar radiation, is evaporated. This has the advantage that the cooling circuit of the receiver, which in any case must dissipate heat, can be used for heating the partial stream of the molten zinc. This creates a particularly environmentally friendly process, as the solar radiation can be used as a heating medium instead of fossil fuels. As concentrated radiation and X-rays or gamma rays are conceivable.

In another embodiment of the invention, the molten zinc is indirectly vaporized by contacting the molten zinc with a heat exchanger which is part of the liquid metal refrigeration cycle of the high temperature refrigerator. Indirect heating allows the molten metal to remain spatially separate. The liquid metal cooling circuit therefore does not necessarily contain zinc.

In a further preferred embodiment of the invention, the method is carried out in a rotary kiln. As a result, the process can be carried out continuously and is therefore economical. It is inherent to the rotary kiln that all educt or product streams are easy to dispense or to be removed. It has proved to be useful when the supply of a portion of the reactants. by pellets containing this part premixed and compressed. The pellets have an optimized composition for zinc oxide reduction. By changing the composition, the process can be controlled. Other reactants, which may be hot steam and atmospheric oxygen, must be added separately from the pellets in the rotary kiln.

Conveniently, charcoal with a high proportion of slag can be used as coal. The disadvantage of the resulting slags can be compensated by the fact that the slags in the cement industry are recyclable.

It proves to be advantageous if synthesis gases are produced in the furnace by reaction of the carbon monoxide formed during the reduction of zinc oxide with steam. Synthesis gas can be used to produce valuable raw materials for the chemical industry. Examples are hydrogen and methanol. Gaseous educts and products can be easily made in a rotary kiln by providing a stuffing box.

In a further embodiment of the invention, the reactants are introduced into the furnace in an introduction tube, which inlet tube is preheated countercurrently by the exiting product gases. The heat of the product gases can therefore be used particularly economically to preheat the pellets fed in the feed tube. This can save heating energy.

It proves to be advantageous if the steam for controlling the synthesis gas process is abandoned in different temperatures. This allows the bou- douard equilibrium between carbon monoxide and carbon dioxide to be adjusted.

The gas process can also be controlled by the partial recirculation of product gases.

Conveniently, the generated slag, which can be used in the cement industry, cleaned by short-term heating of residual zinc. This provides clean products that are not contaminated by other products.

The invention is also preferably characterized in that tin is added to the part of the molten zinc from which zinc vapor is recycled. With a composition of 80% tin and 20% zinc, a hutectic at 200 ° C form. As a result, the molten zinc in the cooling circuit can be kept liquid up to 200 ° C. Also, when solidification of the melt, for example, if maintenance work is necessary, the solidified alloy can quickly melt again. Zinc vapor can be produced according to type, as tin has a higher boiling point. As already stated above, it is preferred if the composition of the reactants controls the synthesis gas process and the energy balance of the process. The process can therefore be planned in advance through the pellet composition and can run largely automatically in the rotary kiln, without having to set process parameters afterwards during operation. Further reactants can be hot steam and atmospheric oxygen, by the addition of which the synthesis gas process can be controlled.

In a further particularly preferred embodiment of the invention, the recirculated part of the molten zinc is evaporated in a memory by metal vapor directly o- indirectly, which memory with an electric heater is additionally heated in addition to the Metalldampl ^*. The heating has the advantage that zinc can also be evaporated in cooled stores, for example by inductive heating. Also, the zinc vapor can be heated by the heater. The heater can be heated, for example, with inexpensive night stream. The heating, preferably inductively heated heating pipes, can also prevent solidification of the molten zinc and can therefore keep the zinc liquid. This works especially well if it is a zinc-tin alloy with a eutectic. The molten zinc may also be melted from zinc ores, which may contain, for example, iron, cadmium, arsenic, calcium, sodium, potassium and other metals. The evaporation in the store acts as a distillation in which the zinc vapor evaporates very pure and zinc from the. is separated from other metals.

In a further preferred embodiment of the invention, the temperature of the zinc vapor is regulated by the addition or withdrawal of zinc vapor and / or molten metal in the store. The temperature of the recycled zinc vapor can therefore be set in the memory exactly to the required temperature. Another aspect of the invention relates to a rotary kiln for implementing the method described. The rotary kiln is characterized by the fact that in the area the second end of a return pipe is arranged, through which the recycled zinc vapor can be fed into the reaction space. The rotary kiln has the advantage that the inventive method can run continuously therein. One. Another advantage is that the rotary kiln. as a Gegenstrom.- heat exchanger acts. The gas stream flows from the second to the first end and flows against the pellets / zinc stream. The process in the rotary kiln can be operated particularly economically, since there is heat transfer from the gas stream to the solid / liquid stream over the entire length of the rotary kiln.

In a particularly preferred embodiment of the rotary straw according to the invention, at least the inner wall of the rotary tube, which comes into contact with the zinc vapor, is lined with non-oxidic ceramic baubles. As a result, a "Ofenfras" due to the oxygen affinity of the zinc vapor, ie a destruction of conventional furnace linings, can be prevented.

Conveniently, the ceramic components of free inner lining of the rotary tube is lined with fireclay. The more expensive Keramikbeschichtun is thus used only at the points to the lining of the rotary tube, where it is necessary through, the contact with zinc vapor. In general, the lining can also act as wear protection and thermal insulation. In particular, a lining of the insertion tube and the return tube acts as protection against mechanical and chemical destruction. It proves to be advantageous if the region of the introduction tube, which projects into the reaction space, is surrounded by a spiral passage. This improves the heat transfer at the first end of the rotary kiln from the product gases to the pellets.

It proves to be particularly advantageous if internal graphite bodies are provided in the return pipe which are inductively heatable. This allows the control of the zinc vapor overheating in the return pipe.

Further advantages and features will become apparent from the following description of an embodiment of the invention with reference to the schematic representations. It does not show in full-scale faithful representation: a longitudinal section through a rotary kiln, in which a method for the carbothermal reduction of zinc oxide to zinc is carried out.

In Figure 1, a longitudinal section through a rotary kiln furnace is shown, which is generally designated by the reference numeral 11. The rotary kiln 11 comprises a rotary tube 13, which is rotatable about a rotation axis 15. The rotary tube 13 is shown interrupted in the middle to. not to have to represent the entire length of the rotary tube 13 and to improve the clarity. The rotary kiln 11 comprises a first end 17. At the first end 17, an introduction tube 19 projects into the rotary tube 13, through which the reactants are introduced into the reaction space 21 of the rotary tube 13. Opposite the first end 17 is a second end 23. At the second end 23, the product can be cut off in the form of a molten metal. in the rotary kiln 11 are given as reactants zinc oxide and carbon. Since it is the reduction of Ziiikoxid an endothermic reaction, heat must be supplied to the end of the reaction. The products formed are liquid zinc as a melt and carbon monoxide: ZnO + C- * Zn + CO

Since a refrigeration cycle of a high temperature receiver for capturing concentrated solar radiation is a sufficient source of heat to vaporize liquid zinc, vaporous zinc may be provided as a heating medium to attenuate the above-described reduction reaction. There is even enough heat energy to produce superheated zinc vapor.

According to the invention, part of the liquid zinc or the molten zinc 20 produced can be returned to the reaction space 21 as zinc vapor 24. After heating and evaporation by the energy transfer from an energy source above the evaporation temperature of the molten zinc, the zinc vapor can be blown into the reaction space 21. become. In the reaction chamber 21, the condensation takes place and heats the starting materials of the reaction accordingly. Condensation regions 26 are shown in FIG. 1 as clouds. This immediately causes an intense heat transfer to the reactants, which accelerates the reaction - in comparison to the heat transfer in a conventional hot gas heating. This is The fact that the zinc vapor changes the state of aggregation and the enthalpy of condensation is released during the transition to liquid zinc, whereby the endothermic reaction is driven.

The regulation of the superheated zinc vapor 24 for heating the zone in which the endothermic zinc oxide reaction is to take place takes place, inter alia, by means of a return pipe 25 or a heating pipe. The return pipe 25 preferably protrudes at the second end 23 into the reaction chamber 21. The zinc vapor 24 is blown into a zone of the rotary kiln 11, preferably with non-oxide ceramic elements 27 or those with high. A1203 / MgO portion are equipped or lined. As a result, the high oxygen affinity of the zinc vapor 24 in the rotary tube 13 can be endured and so-called "oven-baked" can be avoided The feedstocks can be discharged by pellets 29, which are introduced into the introduction tube 19. The pellets 29 contain the reactants zinc oxide and carbon The pellets 29 are preheated in countercurrent to the effluent synthesis gas 31. For improved heat transfer, a metallic spiral passage 33 may be connected to the introduction tube 19 if the composition of the pellets 29 of zinc oxide and coke allow them to remain particulate and do not stick to the spiral duct 33. If, for cost and availability reasons, inferior boiler coal instead of coke is used, sintering processes with partial liquid of the raw coal can take place in the warm-up process Such a mixture can also serve to produce a slag 35, the starting materials for the cement Industri If such starting products are used, the spiral passage 33 is dispensed with and the rotary tube 13 is simply lengthened for the purpose of expanding the heat transfer. The entire introduction and gas reaction zone of the rotary tube 13 may be lined with low-cost fireclay 37, which may also serve as a wear protection 39 in case of a corresponding leakage, in particular if the insertion tube 19 is lined with it.

The rotary kiln has reacted with a gas reaction zone 41 in which CO produced in accordance with the Boudouard equilibrium reacts with the gases evolved from the crude coal and with the carbon offered. This reaction process can by introducing water vapor at different temperatures and possibly by Teückückführun of Synthesis gases are controlled. The necessary stuffing box feed 43 for introduction of the water vapor is state of the art for the outlet temperature, as well as the possible upper flow of synthesis gas in methanol and its suction 45.

The heat generation by condensation of superheated zinc vapor can also take place from such heat buffers (51), which are already cooled to the extent that zinc from the circulation can not be sufficiently evaporated. These can then be electrically reheated with cheap night-time electricity, as long as such production makes economic sense due to demand (e.g., surplus from nocturnal wind energy). Conversely, condensate arising in such thermal buffers (51) for the partial recycling of the zinc can be kept liquid up to 200 ° C, e.g. Tin is supplied, since an alloy having a composition of 80% tin and 20% zinc at 200 ° C has a eutectic. With the residual melt, e.g. With the heating pipes placed on the floor, the evaporation can be restarted, since only zinc vapor is produced (the tin boiling point is greater than 700 ° C). The heating pipes can also be heated inductively by internal graphite body. Suitable in the return tube 25 of the rotary kiln 11 placed inductors can be used for the control of Zn-Dampfüberhitzu n when graphite sleeves are installed.

The structure of a chain of latches is also shown in FIG. The molten zinc 20 may be passed via a first regulation 47 into a production buffer 49 or into an inductively heatable recycling buffer 51. The two latches 49, 51 are connected to one another by a melt transfer line 53 in order to be able to control the melt concentration. The return buffer 51 is equipped with an inductive heater 55, which can be used in addition to the heating by the high-temperature reeeiver 57. This allows the zinc vapor to overheat. It is also advantageous if tin is present as a boil-retardant in the melt, as a result of which overheated zinc vapor is obtainable. Via an inlet 59, melt can be added to the production buffer 49 and melt can be removed from the production buffer via the outlet 61 49 are removed. The melt can be supplied via the outlet 61 to the first circulating buffer 63 of the high-temperature receiver 57. Out the high tempera ture receiver 57 can melt back and metal vapor in the first circulation buffer 63 occur. To control the Schmelzenkonzentra tion and the control of steam overheating a second circulation buffer 65 is provided. The circulation latches 63, 65 are sealed by a first and second regulator 67, 69 in the vapor phase and in the liquid phase.

The superheated steam can via a third controller 71 on the production buffer 49 and. the return buffer 51 are distributed. The superheated heating steam is fed to the rotary kiln 11 via the heating steam outlet 73.

A supply line 75 may be electrically heated. The provision of a plurality of latches 49,51,63,65 allows precise control of the superheated heating steam 24. The latches are connected to each other in the vapor phase and in the Fiüssigphase, whereby the melt concentration and the Heizdampf properties, in particular its temperature, regulate to let.

Legend:

11 rotary kiln

13 rotary tube

15 axis of rotation

17 First end

19 insertion tube

20 liquid, zinc melt

21 reaction space

23 Second end

24 zinc vapor

25 return pipe

26 condensation areas

27 ceramic elements

29 pellets

31 synthesis gas

33 spiral course

35 slag

37 fireclay

39 Wear

41 gas reaction zone

43 stuffing hopper feeder

45 methanol suction

47 First regulation

49 production buffers 51 feedback cache

53 melt transfer

55 Inductive heating

57 high temperature receiver

59 melt inlet

61 Melt outlet

63 First circulating buffer

65 Second Circulation Buffer

67 First controller

69 Second controller

71 Third controller

73 heating steam is extinguished

75 supply line

Claims

claims

1, A process for the carbothermic reduction of zinc oxide to zinc comprising the following process steps

Zinc oxide and coal in an oven as reactants, reduction of the zinc oxide to liquid molten zinc (20) by using the coal as a heating agent and as a reactant and

Parting off the liquid molten zinc (20) from the furnace as a product, characterized

Since the heat input for the endotlierme reaction by the condensation of a blown into the furnace zinc vapor (24), which acts as heating steam takes place.

2. The method according to claim 1, characterized in that a part of the molten zinc (20) is recycled as the zinc vapor (24).

3 .. The method of claim 2, characterized in that the part of the molten zinc (20), which is returned as zinc vapor (24), in a cooling circuit of a high-temperature Reeeivers (57) for collecting concentrated radiation, in particular solar radiation is evaporated.

4. The method according to claim 3, characterized in that the molten zinc is evaporated indirectly by the molten zinc is brought into contact with a heat exchanger, which is part of the liquid metal cooling circuit of the high tempera ture receiver.

5. The method according to any one of the preceding claims, characterized in that the method is carried out in a rotary kiln (13).

6. The method according to any one of the preceding claims, characterized in that the supply of a portion of the reactants by pellets (29) takes place, which contain this part before mixed and compressed.

7. The method according to any one of the preceding »claims, characterized in that charcoal with high Sc paint share is used as coal.

8. The method according to any one of the preceding claims, characterized in that in the. Furnace by reaction of during the reduction of zinc oxide resulting carbon monoxide produced with steam synthesis gas.

9. The method according to claim 8, characterized in that the steam for controlling the synthesis gas process is abandoned in different temperatures,

10, Method according to one of the preceding claims, characterized in that the reactants in an insertion tube (19) are introduced into the furnace, which F.inführrohr (19) in. Countercurrent is preheated by the exiting product gases.

11, method according to any one of claims 7 to 10, characterized in that the generated slag (35), which can be applied in. The cement industry is cleaned by short-term heating of residual zinc,

12, Method according to one of claims 2 to 11, characterized in that the part of the molten zinc (20), from which the zinc vapor (24) is recycled, is added with tin.

13, Method according to one of claims 6 to 12, characterized in that is controlled by the composition of the reactants of the synthesis gas process and the energy balance of the method,

14. The method according to any one of claims 3 to 13, characterized in that the recirculating part of the molten zinc (20) in one. Memory (51) is vaporized directly or indirectly by metal vapor, which memory (51) in addition to the metal vapor with an electric heater (55) is additionally heated.

15, according to claim 14, characterized in that the temperature of the zinc vapor (24) by the addition or withdrawal of zinc vapor and / or

Molten metal in the memory (51) is regulated.

16, rotary kiln (11) for implementing the method according to the preceding claims, comprising a rotary tube (13) forming a reaction space (21),

an axis of rotation (15) about which the rotary tube (13) is rotatable,

a first end (17) having an insertion tube (19) through which the landing can be abandoned and;

a second end (23), opposite the first end, on which the

Product (20) can be torn off,

characterized,

a return pipe (25) is arranged in the area of the second end (23) through which the recirculated zinc vapor (24) can be introduced into the reaction space (21).

17. Rotary kiln according to claim 16, characterized in that at least the Imienwandung of the rotary tube (13) which comes into contact with the zinc vapor (24) is lined with non-oxide ceramic components (27).

18. Rotary kiln according to claim 17, characterized in that the parts of the ceramic tube (27) free imienwandung of the rotary tube (13) is lined with fireclay (37),

19. Rotary kiln according to one of the preceding claims, characterized in that the region of the introduction tube (19) which projects into the reaction space (21) is surrounded by a spiral passage (33).

20. Rotary kiln according to one of the preceding claims, characterized in that in the return tube (25) inner graphite bodies are provided, which are inductively heated.