Zinc oxide residue recycling method and equipment

Abstract

The present invention relates to a method for recycling zinc oxide residue and related equipment. According to the method, zinc oxide residue is granulated to produce a particle size d 80 The particles are fed to a roaster and heat treated in a fluidized bed at temperatures ranging from 500 to 1,200°C, preferably 800 to 1,100°C to produce cinders. The zinc oxide residues are processed in kilns, submerged lance furnaces, iron reduction furnaces, galvanizing and / or recycling processes, in particular from the recycling of iron, copper, lead, nickel and / or electronic scrap, to produce particles of size d. 80 is less than 100 μm, preferably with a particle size d 80 Zinc oxide dust smaller than 75 μm and / or zinc oxide residues are obtained from ash and / or dross generated in the production of lead and zinc castings, zamak, zinc oxide ash, catalysts, Zn melting castings and / or zinc slag.

Description

Detailed Description

[0001] The present invention relates to a method for recycling zinc oxide residue and related equipment, and relates to a method for granulating zinc oxide residue to produce zinc oxide particles having a particle size d 80 The resulting particles are fed into a roaster and heat-treated in a fluidized bed at a temperature in the range of 500 to 1200°C, preferably 800 to 1100°C, to produce cinders.

[0002] As waste is increasingly recognized as a resource with broader potential, recycling of waste materials is becoming increasingly widespread. This is particularly true for zinc. While total zinc production has declined in recent years, the proportion of this metal obtained from secondary sources has become significant. Currently, it is estimated that 25% by weight of the total zinc production worldwide is obtained from secondary sources. Aside from economic considerations, the main environmental benefits achieved include (i) raw material savings, thereby reducing the need to further exploit and deplete natural resources; (ii) avoidance of final waste disposal and thus reducing the potential environmental pollution load; and (iii) various potential energy savings, often reducing energy use by 40% to 85% and reducing the carbon footprint on the environment.

[0003] It is also important to understand that today it is possible to produce metals that meet specifications and are indistinguishable from the same metals extracted from raw ores.

[0004] Therefore, for a variety of reasons, there is a strong and tangible argument for zinc recycling to become even more important due to economic and environmental incentives.

[0005] On the other hand, typical zinc-containing residues are flue dusts, such as those obtained from the final product of purification after removing soot, welt oxides and / or halogens from electric arc furnaces in steel recycling production, from top submerged lance processes such as the Ausmelt or Isasmelt process, from iron reduction residues, from galvanizing, from copper or electronic scrap recycling processes, from lead recycling processes, or from nickel recycling processes. The zinc content in the soot dusts is in the range of 40-80 wt. %, typically 60-70 wt. % zinc.

[0006] On the other hand, zinc dross and zinc oxide-containing residues from lead and zinc casting, ash and dross from the zamak production process, zinc oxide-containing ash, catalysts and zinc slag can be sources of zinc-containing residues to be recycled. The zinc content in dross or sludge materials is 80-99%.

[0007] Typical reactor types used in the roasting process are fluidized-bed reactors, rotary kilns, or multi-hearth furnaces. In fluidized-bed reactors, gases and at least small particles of roasted concentrate (cinder) are recovered from the top of the roaster and fed into at least one separator to separate the solid particles. The at least one gas-solid separator may be configured as a parallel or series-connected cyclone unit, an evaporative cooler, and / or a waste heat boiler (referred to as a combined cooler). Cooling of the gas-solid mixture is particularly important, as an electrostatic precipitator (ESP) is expected to follow the separator. The use of a waste heat boiler has the added benefit of generating saturated or superheated steam, which can be used for internal use or to generate electricity. While fluidized-bed reactors offer the significant advantage of very high heat transfer rates and mass transfer rates, the need for post-processing of the recovered fluidizing gases and contained particles reduces overall profitability.

[0008] State-of-the-art technology ensures a high torrefaction rate and a low carbon and / or sulfur content by recycling the particles from the gas-solid separator and increasing their residence time. However, a very small percentage of particles with a fairly small diameter, especially below 10 μm, are too small to be selectively separated in the separator and returned to the reactor, but are instead removed with the gas stream and transferred to subsequent process steps.

[0009] Large amounts of soot also cause deposits in waste heat boilers, which is one of the reasons for frequent shutdowns. Furthermore, extensive cleaning can also damage the boiler's steam tube bundles.

[0010] Furthermore, current methods using concentrates have a low amount of fine particles, especially particles with a particle size of less than 10 μm, which means that generally less than 10% by weight of the zinc obtained in the recycling process has a very small average particle size. Therefore, for the reasons mentioned above, the use of fluidized bed reactors has hitherto been practically impossible.

[0011] Therefore, the underlying rationale behind the present invention is to use a fluidized bed reactor to roast the recycled materials.

[0012] The above object is solved by a method using the features of claim 1.

[0013] Such a method relates to the recycling of zinc oxide residues. These zinc oxide residues are reconstituted in a particle size d 80 The soot particles are those with a particle size of less than 100 μm, preferably less than 75 μm, which are produced in kilns, submerged lance furnaces, iron reduction furnaces, galvanizing and / or recycling processes, in particular in the recycling of iron, copper, lead, nickel and / or electronic scrap. And / or zinc oxide residues are those produced in lead and zinc casting, ash and / or dross from the production of zamak, zinc oxide ash, catalysts, zinc and / or zinc slag melting and casting. The roasting in a fluidized bed granulates these zinc oxide residues to a particle size d of 0.3-5 mm. 80The particles are then fed to a roaster and heat-treated at a temperature in the range of 500 to 1,200°C.

[0014] Here, the particle size distribution d 80 means that at least 80% of the particles contained therein have a particle size less than a given value, which is particularly true for measurements obtained by sieve analysis, photoanalysis or optical counting.

[0015] The roasting and post-treatment are known and are described in detail, for example, in PCT Publication WO 2018 / 162089. However, complete granulation of the residue allows the use of a fluidized bed reactor during roasting, which benefits from the excellent mass and heat transfer.

[0016] This method according to the present invention is particularly important for residues from the Welz process. The so-called Welz process is a pyrometallurgical process in which zinc, cadmium, and lead are volatilized under reducing conditions. The Welz process is carried out in a long, slightly inclined, refractory-lined rotary kiln (Welzkiln). The name Welz comes from the German verb "Waelzen," which describes the rolling motion of the materials fed into the kiln. In the 21st century, the Welz process is more widely used than ever before.

[0017] Typical feed materials in the Welz process are, for example, steelmaking dust from Zn / Pb-containing electric arc furnaces, neutral leach residues from zinc smelting furnaces, or other Zn-containing materials. Such feed materials are agglomerated before being fed into the Welz kiln to minimize the amount of so-called carryover contamination that affects the quality of the Welz oxide.

[0018] Depending on the basicity of the feed material, modifiers such as sand or limestone may need to be added to maintain optimum Welzkiln operation during charging. Additionally, coke breeze is added as a reducing agent when granulating below 10 mm.

[0019] The feed mixture is slowly moved downward by the kiln's rotation and heated by the exhaust gas flow exiting the kiln against the material flow. After drying and preheating, the charge enters the reduction zone, where the iron and zinc oxides are reduced to metal. The zinc is vaporized in a fluidized bed at temperatures up to 1200°C. The charge residence time is 5-10 hours, depending on the size and fill of the kiln (typically 20 vol%). Zinc fumes and carbon monoxide generated by the charge are combusted in the freeboard section by air entering at the discharge end of the kiln.

[0020] Because zinc oxide is produced in the gas phase, it leaves the kiln in a highly finely divided state via the hot exhaust gases, which can cause damage to the downstream torrefaction fluidized-bed reactor. Other volatilized metals, such as lead and cadmium, and some of the kiln feed material (carry-over contaminants), are also carried by the exhaust gases. As the soot-laden gas passes through large settlers, the coarse particles settle in the settler, which then sends them to a surface or evaporative cooler and finally to a baghouse or electrostatic precipitator, where the Welz oxide is collected. The so-called pre-oxide, consisting of the soot and material that flows back into the kiln from the settler, is recycled to the kiln inlet.

[0021] The Welz slag is discharged by gravity from the bottom of the kiln at approximately 1100°C and falls through a downspout into a wet slag extractor. After cooling, the slag is separated and graded in a magnetic separator, and the unburned coke is recovered.

[0022] The typical composition of the product obtained in the Welz kiln by the Welz process is shown in Table 1.

[0023] Table 1: Chemical analysis of typical crude Welz oxides at European Welz facilities

[0024] [Table 1]

[0025] Due to the large volumes involved, the value-adding steps of further processing the zinc residues obtained in the Welz process are of particular importance.

[0026] For other sources, in particular for zinc oxide residues resulting from the melting and casting of zinc and / or zinc oxide, the zinc oxide residues are 80 The zinc oxide residue is milled to a particle size d of less than 100 μm, preferably less than 75 μm, before being granulated to particles of 0.3 to 5 mm, preferably 0.5 to 2 mm. 80 To achieve the more uniform composition and density required for fluidized bed technology, milling and re-granulation are required.

[0027] The recycling process is particularly relevant from an environmental and economic point of view for all residues with a relatively high zinc content. In particular, such residues include dust with a zinc content of 40 to 80% by weight, preferably 60 to 70% by weight, or dross or sludge material with a zinc content of 80 to 99% by weight. It is of course also possible to carry out the process according to the invention with a mixture of dust and dross and / or sludge, or the zinc oxide residue can be a mixture of zinc dust and dross or sludge.

[0028] Generally, zinc oxide residues contain halogens, carbonates, sulfides, and / or sulfates, which must be removed. In particular, the halogens Cl and F must be removed together with the exhaust gas from the roaster to avoid their high concentration in the downstream hydroelectric power plant. Therefore, preferably in the case of a fluidized bed roaster, the washing and filtering of the soot prior to entering the roaster can be simplified or even omitted.

[0029] Additionally or alternatively, in many embodiments, the residue contains recyclable lead.

[0030] The zinc oxide residue may also contain at least one element from the list consisting of cadmium copper, arsenic, silver, platinum group elements, and silica, which may also be recycled from the roaster.

[0031] Furthermore, it is possible to mix in additive materials containing zinc and / or sulfur before and / or during and / or after granulation. By mixing in zinc, impurities can be diluted, and it is particularly preferred that the total amount of metals other than zinc is less than 15% by weight. By using this dilution method, even residues with a very high impurity content can be easily recycled. Typical sources of materials to be mixed in are zinc concentrate, zinc dust (particle size d 80 particles less than 60 μm), zinc oxide, sulfur-containing residues, soot dust obtained from electrostatic precipitators and / or soot dust obtained from cyclone devices.

[0032] The addition of sulfur-containing materials increases the combustible mass and therefore provides an additional source of energy.

[0033] Each mixing step offers the following advantages, among others: pre-mixing ensures a very uniform composition of the particles obtained by granulation, whereas direct mixing during granulation requires an additional pre-preparation step, reducing capital expenditures and operational maintenance costs. However, in both cases, for better granulation, it is recommended to grind the concentrate to an average particle size d before mixing prior to granulation or before mixing directly during granulation. 80 It is preferable to make the thickness less than 2 mm.

[0034] On the other hand, addition to the roaster feed or directly in the roaster leads to low granulation throughput, allowing the roaster to be designed on a small scale.

[0035] Looking more specifically at granulation, it is also preferred to mix water with the zinc oxide residue before and / or during granulation, which results in better bonding between the resulting particles.

[0036] Additionally or alternatively, sulfuric acid can be mixed with the zinc oxide residue prior to and / or during granulation, which also enhances binding during granulation.

[0037] In this case, it is particularly preferred that the added sulfuric acid is obtained in a later zinc processing stage, i.e., a hydrometallurgical process. The hydrometallurgical process generally includes the steps of neutral leaching, thermal acid leaching, refining, and electrowinning. The acid is primarily extracted in electrowinning (spent acid). The concentration of the added sulfuric acid is usually less than 35% by weight, preferably less than 30% by weight, and more preferably between 2 and 30% by weight. Most preferably, the concentration of the acid recycled from electrowinning is between 12 and 18% by weight, preferably between 14 and 16.5% by weight, while the concentration of the acid obtained by wet gas cleaning is between 5 and 35% by weight.

[0038] This has the advantage that not only the precious metals but also the sulfur content can be recovered, and furthermore, sulfuric acid can be avoided from processes unrelated to acid contamination or its concentration, thereby eliminating the need for wastewater treatment or reducing the total wastewater treatment flow.

[0039] As mentioned above, it is also preferable to increase the sulfur content of the particles obtained by granulation. In this case, the sulfur content of the particles is 6 to 35 wt. % of the sulfide sulfur (dry basis), more preferably 8 to 30 wt. % of the sulfide sulfur, and even more preferably 9 to 20 wt. % of the sulfide sulfur (dry basis). The most suitable sulfur content to realize the autothermal process in the roasting stage, or at least to reduce the energy requirements, is more than 10±0.5 wt. % of the sulfide sulfur (dry basis).

[0040] In another preferred embodiment of the present invention, roasting is a continuous process while granulation operates in a batch mode, which has the advantage that the pellets have the same residence time rather than the same average residence time for all pellets, resulting in much higher quality pellets in terms of particle size, especially a smaller particle size range, and particle stabilization.

[0041] In contrast, another advantageous example is that the entire method is a continuous process that can be easily controlled.

[0042] The present invention also provides the following claims: 12 The present invention relates to the equipment described in claim 1 to claim 2. 11 This includes, in particular, the device designs for the method options mentioned above.

[0043] Such a facility for recycling zinc oxide residues involves pelletizing the zinc oxide residues to a particle size of d 80 is less than 100 μm, preferably d 80 The plant comprises at least one granulator for reducing the size of the granules to particles smaller than 75 μm, and a roaster constructed as a fluidized bed reactor for heat treating the particles in the fluidized bed at a temperature of 500-1200°C to produce cinders. The plant further comprises at least one device for submerged lance, iron reduction, galvanization, recycling processes, in particular for recycling iron, copper, lead, nickel and / or electronic scrap, lead and zinc casting, and / or for the production of zamak, zinc oxide ash, catalysts and / or zinc slag.

[0044] In a preferred embodiment, it is anticipated that a high intensity mixer will be installed above the granulator to mix water, sulfuric acid, zinc-containing material and / or sulfur-containing material, which will achieve a homogeneous particle composition with very good particle stability.

[0045] In another preferred embodiment, it is anticipated that a reservoir for the particles resulting from granulation will be provided, which will allow for batch operation of the granulation for the reasons discussed above, and for continuous feeding into the fluidized bed for continuous operation of the roaster.

[0046] Further objects, features, advantages and possible applications of the present invention can be gleaned from the accompanying drawings and the description of the embodiments that follow. All features described and / or shown, whether included in the individual claims or as background matter, form the subject of the present invention in themselves or in any combination. [Brief explanation of the drawings]

[0047] [Figure 1] 1 shows a schematic diagram of a reactor system of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0048] In Figure 1, at least one apparatus 1 for producing zinc oxide residue is provided. Such apparatus 1 is constructed as at least one apparatus for use in submerged lance, iron reduction, galvanization, recycling processes, in particular the recycling of iron, copper, lead, nickel and / or electronic scrap, lead and zinc casting, and / or the production of zamak, zinc oxide ash, catalysts and / or zinc slag, with or without post-treatment equipment. Preferably, apparatus 1 represents the above-mentioned welter kiln and subsequent cooling and separation units. However, there is no need to directly link the production of zinc oxide residue with its recovery. In many cases, the residue is a transport to the roasting process.

[0049] According to the invention, the obtained zinc oxide residue is at least partially transferred via conduit 2 to a feed preparation system FPS. Such an FPS optionally comprises at least one preparation feed 10 in which the zinc oxide residue can be mixed with other solid materials, for example zinc concentrate and / or sulfur-containing materials, which are added via conduit 11.

[0050] From there, the zinc oxide residue is transferred via conduit 12 to granulator 20, or directly into device 20 without mixing (not shown). Granulator 20 is preferably constructed as an intensive mixer. Granulator 20 is used to increase the particle size of the feed material. Granulation homogeneously distributes impurities to reduce the risk of sticking or sintering. Preferably, water and / or sulfuric acid is added via conduit 21 to improve the granule quality, especially its stability. The source of sulfuric acid is preferably a process step (not shown) in the downstream zinc production. Optimally, spent acid obtained from electrowinning or wet gas cleaning is used (H2SO4 content preferably 14-18 wt%).

[0051] Optionally, granulator 20 is operated in a batch mode. In this case, it is anticipated that at least one reservoir 30 is provided to store the particles obtained by granulation and fed via conduit 22. This allows for continuous operation of a downstream fluidized bed reactor 40, where the particles are roasted. Since the particles are fed to fluidized bed reactor 40 via conduit 31, it is anticipated that conduit 3 may be provided as an option, which mixes the materials branched off from conduit 2 and feeds the combined stream to fluidized bed reactor 40 via conduit 41. It is also possible to add other materials, such as zinc concentrate, to conduit 41 or to a stand-alone feeder for fluidized bed reactor 40.

[0052] A fluidizing gas, often air, flows from below into the fluidized bed reactor 40 through conduit 42 to form a fluidized bed from which a stream of solid particles is withdrawn through conduit 43, while the fluidizing gas entrains at least a portion of the particles from the bed and exits the fluidized bed reactor 40 through conduit 44.

[0053] The gas-solids stream is fed via conduit 44 into a heat exchanger, often referred to as a waste heat boiler, where a portion of the solids are further removed via conduit 52. The cooled gas stream is fed via conduit 51 to at least one cyclone 60, where the remaining solids are largely separated from the gas stream and collected via conduits 62 and 73. The gas stream is fed via conduit 61 to an electrostatic precipitator 70, where the remaining particles are removed via conduit 72. These particles may be mixed with the stream in conduit 73. Any combination of the streams in conduits 41, 43, 52, 62, and 72 may be mixed. Additionally, particles removed from any of the gas-solid separators may be recycled to the fluidized bed reactor 40.

[0054] Solid particles removed directly from the fluidized bed via conduit 43 are sent to heat exchanger 80, and optionally particles recovered from heat exchanger 50 can also be introduced via conduit 52. This solids stream is recovered via conduit 81. Conduits 81 and 73 can be combined to transport the solids stream to storage or to an acid leach site. [Example]

[0055] As can be seen from the data shown in Table 2, the present invention using granulation can significantly reduce soot and its associated drawbacks.

[0056] Table 2: Comparison of processes with and without granulation

[0057] [Table 2]

[0058] Test 1 shows entrained dust in fluidized bed roasting without granulation, and Test 2 uses a feed of the same composition and nearly the same mass flow rate. The results show a reduction in entrained dust of over 50%. [Explanation of symbols]

[0059] 1. Zinc oxide residue generator 2 conduit 3 Bypass conduit 10 Preparation supply section 11, 12 Conduit 20 Granulation equipment 21, 22 conduit 30 Storage 31 Conduit 40 Fluidized bed reactor 41~44 Conduit 50 heat exchanger 51, 52 Conduit 60 Cyclone Device 61, 62 Conduit 70 Electrostatic Precipitator 71~73 Conduit 80 heat exchanger 81 Conduit

Claims

1. The zinc oxide residue is granulated to a particle size of d 80 1. A method for recycling zinc oxide residue, comprising the steps of: preparing zinc oxide particles having a particle size of 0.3 to 5 mm, feeding the particles to a roaster and heat treating them in a fluidized bed at a temperature in the range of 500 to 1,200°C to produce cinders; mixing the zinc oxide residue with sulfuric acid before and / or during granulation; carrying out the granulation batchwise while roasting the particles in the roaster in a continuous process; wherein the zinc oxide residue is soot obtained from an electric arc furnace and / or a Weltz process; and / or the zinc oxide residue is obtained from the melting and casting of zinc and / or zinc oxide and is crushed to a particle size d before granulation. 80 A recycling method characterized by reducing the particles to less than 100 μm.

2. 2. The method of claim 1, wherein the zinc oxide residue has a particle size d 80 granulating the powder to particles of 0.5 to 2 mm and / or heat treating the particles at a temperature in the range of 800 to 1,100°C.

3. 3. The method according to claim 1 or 2, wherein the zinc oxide residue is obtained from the melting and casting of zinc and / or zinc oxide and is ground to a particle size d 80 The method of claim 1, wherein the particle size is less than 75 μm.

4. A method according to any one of claims 1 to 3, characterized in that the zinc oxide residue is zinc dust having a zinc content of 40 to 80% by weight, or the zinc oxide is dross or sludge material having a zinc content of 80 to 99% by weight, or the zinc oxide residue is a mixture of zinc dust and dross or sludge.

5. 5. The method according to claim 1, wherein the zinc oxide residue contains halogens, carbonates, sulfides and / or sulfates, and / or the zinc oxide residue contains lead.

6. 5. The method of claim 4, wherein the zinc oxide residue further contains at least one element selected from the list consisting of cadmium, copper, arsenic, silver, PGM, Pb, and silica.

7. 7. The method according to claim 1, wherein zinc concentrate, zinc dust, zinc oxide, sulfur-containing residual dust obtained from an electrostatic precipitator and / or soot dust obtained from a cyclone device are mixed with the zinc oxide residue before and / or during and / or after granulation.

8. 8. The method according to any one of claims 1 to 7, characterized in that the zinc oxide residue is mixed with water before and / or during granulation.

9. 9. A method according to claim 7 or 8, characterized in that the sulphuric acid is obtained in a zinc treatment stage subsequent to the method.

10. 10. The method of claim 9, wherein the subsequent zinc treatment step is an electrowinning step.

11. 11. The method according to claims 8 to 10, wherein the sulfur content in the particles obtained by the granulation is 0 to 35% by weight.

12. The zinc oxide residue is granulated to a particle size of d 80 1. A zinc oxide residue recycling plant comprising: at least one granulator for granulating zinc oxide residue into particles having a size of 0.3 to 5 mm; a roaster constructed as a fluidized bed reactor for heat treating the particles in the fluidized bed at a temperature in the range of 500 to 1200°C to produce cinders; means for mixing the zinc oxide residue with sulfuric acid before and / or during granulation; and at least one storehouse for particles obtained in the granulator, which is operated in batches and is used for continuous feeding into the fluidized bed reactor, wherein the at least one device as a source of zinc oxide residue is an electric arc furnace, a weltz kiln, and / or a device constructed for melting and casting zinc and / or zinc oxide.

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