JP2017128808A - Method for recovering zinc from zinc-containing waste substance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for recovering zinc from a waste substance containing zinc, applicable to an actual electric furnace dust or zinc chloride-based waste substance, which are complex system, mainly containing oxide containing many impurities.SOLUTION: A method has a temperature increasing process (S1) for heating chloride gas and the waste substance to a predetermined temperature in advance, a selective chlorination process (S2) for conducting a selective chlorination reaction of a zinc component, which is an endothermic reaction, by reacting the chloride gas and the waste substance of which the temperature is increased to the predetermined temperature to produce coarse zinc chloride steam, a molten salt producing process (S3) for concentrating the coarse zinc chloride steam to obtain coarse zinc chloride and then purifying the coarse zinc chloride to obtain a molten salt, and a molten salt electrolysis process (S4) for electrolyzing the molten salt as an electrolysis bath to precipitate zinc.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for recovering zinc from zinc-containing waste, and more particularly, to a method for recovering zinc from waste containing zinc components such as electric furnace dust and zinc chloride-based waste.

  Zinc is a highly chemically active metal that reacts with oxygen, chlorine, sulfur and many other substances. The melting point is 419 ° C. and the boiling point is 906 ° C., both of which are low melting metals that are easy to melt and evaporate, and sulfides occupy most of the ores.

  In the usual smelting process, sulfide ore is first roasted into oxides, and this is dissolved in sulfuric acid to make zinc sulfate, and an aqueous electrolysis method is obtained by electrolysis, and the roasted oxides are reduced by carbon. There is an ISP (Imperial Melting Process) method in which the metal zinc vapor obtained in this way is rapidly cooled with a liquid lead splash condenser to obtain a liquid metal. The former requires thorough removal of impurities in the aqueous solution before electrolysis. Since the latter contains lead and cadmium in the smelted metal zinc, further rectification is required, and the product purity is slightly inferior due to the high energy consumption type.

  Although zinc is an inexpensive metal at present, it is a rare resource with less abundance in the crust than copper and nickel. Closure of a prominent mine that produces a large amount is also expected, and recovery is highly important because it is expected that the amount will be insufficient and the price will rise in the medium to long term.

  Although zinc as a metal has applications as an additive element of a die-cast alloy or an aluminum alloy, a negative electrode of a battery, etc., the greatest use is for rust-proof plating of steel products. In any application, recovery of zinc as it is is not performed, and the recovery rate is very low compared to other metals. This is mainly because zinc acts as a sacrificial anode for rust prevention of iron and the like and is destined to diffuse into the environment.

  Among them, the only recovery route is electric furnace dust generated from an electric furnace that melts iron scrap to make steel. Electric furnace dust is a valuable zinc resource that generates about 2% of steel produced in the electric furnace and contains 20-30% zinc. This is equivalent to or more than iron, and contains several percent of chlorine, and also contains heavy metals such as lead and cadmium, and is considered hazardous industrial waste. Table 1 shows an example of composition analysis of electric furnace dust by fluorescent X-ray analysis. It is mainly composed of zinc and iron oxides, and contains chlorine, lead, and alkali metals.

  Most of the zinc contained in this electric furnace dust is contained in the form of oxides, but most of it is in a form that is hardly soluble in sulfuric acid called zinc ferrite (iron zincate), which is a compound with iron oxide. Since it contains, the aqueous solution electrolysis method which is the present smelting method cannot be applied as it is. Since it contains chlorine and contains a lot of iron, it is difficult to apply to the ISP method.

  For this reason, electric furnace dust is once heated together with carbonaceous materials in a rotary kiln, called the Welsh method (Waelz method), and both zinc and iron are once carbon-reduced. Zinc is evaporated as metal vapor using the difference in vapor pressure and cooled. A method is generally used in which crude zinc oxide is produced by, for example, recovering zinc oxide that has been reoxidized and then sent to an existing zinc smelting process.

  However, in the Welsh process, chlorine, lead, cadmium, and the like are also transferred to the gas phase. This necessitates an additional process that is not present in the ore processing smelting process, resulting in technical difficulties and increased costs.

  In this way, even though electric furnace dust is a valuable zinc resource, it is treated as industrial waste because it contains chlorine and heavy metals, and the Wales method, which is currently the only treatment facility, is also expensive and expanded. There is no momentum. For this reason, it is difficult to treat the entire amount of the generated electric furnace dust, and a certain amount is disposed in landfill after being subjected to chemical treatment.

  Under such circumstances, Patent Document 1 has proposed a zinc production method using electric furnace dust that is fundamentally different from the conventional method. This zinc production method selectively chlorinates zinc content in zinc oxide or zinc ferrite by reacting electric furnace dust with chlorination gas (chlorine or a mixed gas of chlorine and oxygen or a mixed gas of chlorine and air) at a high temperature. The zinc chloride is taken out in the gaseous state. At this time, most of the free iron oxide and iron content in zinc ferrite are separated as oxide residues. Next, a reducing agent is added to the obtained zinc chloride or molten salt mixed with alkali chloride to reduce and remove noble elements from zinc, and finally the obtained molten salt is electrolyzed. Metal zinc and chlorine are obtained. Zinc chloride is a substance that has a melting point of 320 ° C. and a boiling point of 727 ° C. and is easily melted and easily evaporated.

  In the molten salt electrolysis, when an attempt is made to obtain a metal in a liquid state, the interface between the electrolytic bath and the cathode metal is usually horizontal, so that an enormous area is required without taking an example of aluminum electrolysis. In order to avoid this, a plurality of electrodes are arranged vertically so that one side of each electrode plate is a positive electrode and the opposite side is a negative electrode, which is called a bipolar electrode (as an entire electrolytic cell) Also called a multipolar electrolytic cell.)

  Patent Document 2 discloses a technique related to the structure of an electrolytic cell using zinc chloride as a molten salt, but this technique is characterized in that the bipolar electrode described in the previous section is used.

International Publication No. 2015/030235 JP 2014-25134 A

“Recycling of zinc from electric furnace dust via chloride” Resources and materials 2013 (Sapporo) Lecture document C5-3, p487-488 “Development of Zinc Recovery Method from Electric Furnace Dust Using Selective Chlorination”, Graduate School of Engineering, Tohoku University, press release, March 5, 2014

  Whether the oxide or chloride is stable depending on the metal is determined by the position of the oblique line in FIG. 7 described later. Zinc is more stable in chloride than iron, and zinc oxide is preferentially salified over iron oxide. This is the principle of the selective chlorination method, but it was not always clear whether it could be applied to zinc oxide in zinc ferrite, which is a stable compound.

  Therefore, as shown in Non-Patent Document 1, as a result of experiments using synthetic zinc ferrite, it became clear that zinc ferrite generates zinc chloride by reaction with chloride gas as well as a mixture of zinc oxide and iron oxide. It has been confirmed that the presence of zinc as zinc ferrite does not interfere with the selective chlorination method. However, the techniques disclosed in Patent Document 1 and Non-Patent Document 1 are based on chemical and crystallography synthesized at Tohoku University, focusing on the fact that zinc ferrite is a major component of electric furnace dust and is difficult to treat. The recovery of zinc from electric furnace dust was devised based on the results of a chlorination experiment using a laboratory-scale apparatus using only zinc ferrite, which is almost pure and simple (Non-patent Document 1, Patent Document 1). [0049] etc.).

  Therefore, there are other problems to be solved in order to apply to actual electric furnace dust which is a complex system containing many impurities. For example, in the chlorination step (Patent Document 1), the reaction of converting zinc ferrite or zinc oxide contained in electric furnace dust into zinc chloride with chloride gas (when M is zinc in the formula (1) described later) is an endothermic reaction. Therefore, there arises a problem that a continuous reaction cannot be maintained due to a decrease in the reaction rate. In this regard, the technique disclosed in Patent Document 1 is based on a laboratory level experimental result in which a small amount of reactant (only zinc ferrite) is performed in a furnace having a large heat capacity. Was not considered at all.

  The present invention is based on the invention based on the experiment using the synthetic zinc ferrite described in Patent Document 1, and has been made by finding a new problem that has not been solved in Patent Document 1, and includes many impurities. It is an object of the present invention to provide a method for recovering zinc from waste containing zinc, which can be applied to actual electric furnace dust and zinc chloride waste that are complex systems including the above.

  The method for recovering zinc from zinc-containing waste according to the present invention is a method for recovering zinc from waste containing zinc, and a temperature raising step of heating each of the chloride gas and the waste to a predetermined temperature in advance. And reacting the chlorinated gas heated to a predetermined temperature with the waste, thereby selectively performing a selective chlorination reaction of zinc as an endothermic reaction to generate crude zinc chloride vapor. A step of condensing the crude zinc chloride vapor to obtain the crude zinc chloride, and then purifying the crude zinc chloride to obtain a molten salt; And a molten salt electrolysis step for precipitating the material.

  According to the present invention, it can be applied to actual electric furnace dust and zinc chloride waste that are complex systems containing many impurities, and it is possible to efficiently recover high-purity zinc from zinc-containing waste. it can.

It is process drawing which shows one embodiment of this invention. It is the figure which showed typically one Embodiment of the flow which collects zinc from electric furnace dust by applying this invention. It is a schematic diagram of the electric heater which is an example of the heating apparatus of a chloride gas, (a) is a perspective view, (b) is bb sectional drawing, (c) is cc sectional drawing, Figure (a) The thick arrow in (b) indicates the flow of chloride gas. It is a schematic diagram which shows an example of the electrolytic bath which implement | achieves a molten salt electrolysis process. It is a schematic diagram of the apparatus for conveying the molten salt as an electrolytic bath accommodated in the low position side bathtub to the high position side bathtub. It is a schematic diagram which shows an example of the laboratory selective chlorination apparatus provided with the heating function. It is an oxygen-chlorine potential diagram of zinc and iron at 1200 K (927 ° C.).

  The present invention is an invention devised based on the results of analyzing various phenomena in an experiment using an actual electric furnace dust while being based on the invention based on the experiment using the synthetic zinc ferrite described in Patent Document 1. is there. That is, this is a method for realizing recovery of zinc from waste such as actual electric furnace dust, which is a complex system containing many impurities, which is difficult to realize with the technology disclosed in Patent Document 1.

  Hereinafter, after touching the thermodynamic principle of selective chlorination assumed by the present invention, the characteristics and properties of electric furnace dust, which is one of the wastes to be recovered by the present invention, will be described. Each process (S1-S4) is demonstrated.

FIG. 7 is an oxygen-chlorine potential diagram of zinc and iron at 1200 K (927 ° C.). The vertical axis in the figure is the logarithmic value of the oxygen partial pressure in the atmosphere, and the horizontal axis is the logarithmic value of the chlorine partial pressure. The combination of both values determines whether the metal element is in the oxide, chloride or metal state. That is, the oxide is stable in the region where the oxygen partial pressure is high and the chlorine partial pressure is low (upper left in the figure), and the chloride is stable in the opposite region (lower right in the figure). In a region where both partial pressures are low (lower left side in the figure), the metal state is obtained. The conversion of oxide and chloride is represented by the chemical reaction formula (1). M represents a metal element.
2MO + 2Cl ₂ = 2MCl ₂ + O ₂ (1)

  In the middle of FIG. 7, in the region having a width from the upper right to the lower left, the chloride is stable for zinc and the oxide for iron is stable. By controlling the atmosphere in this region, zinc oxide is converted to zinc chloride. That is, in the formula (1), when M is zinc, the reaction proceeds to the right side, whereas with iron, the reaction proceeds to the left side. Therefore, the iron oxide remains without being salified. This is the principle of the selective chlorination method. The “field” and “+” marks in the upper right of the figure are the atmospheres in which 50% and 80% of air is mixed with chlorine while maintaining the pressure of the whole gas at 1 atm. Even if it is not carried out, chlorination gas by simple mixing is sufficiently within the range of selective chlorination. If the chlorination gas is pure chlorine, iron chlorination is also a concern, but oxygen is by-produced by the reaction in which M in formula (1) is zinc, so that it does not fall outside the range of selective chlorination. In addition, unless the chlorine concentration is extremely low (about 1% or less), the reaction proceeds sufficiently.

  With respect to other metals, the same lines as in FIG. 7 can be drawn and considered. Aluminum, silicon, titanium, and the like are more stable in oxide than iron, exist as an oxide in electric furnace dust, and remain as an oxide even in a chloride atmosphere. On the other hand, metals that are somewhat more stable in chloride than zinc include lead, cadmium, calcium, and the like, which exist both as chloride and oxide. Extreme ones are alkali metals, in which the chlorides are very stable and most exist as chlorides. That is, the chlorine originally contained in the electric furnace dust is preferentially combined with the alkali metal, and the surplus chlorine is still combined with lead, cadmium, calcium and the like depending on the type of the electric furnace dust.

  FIG. 1 is a process diagram showing an embodiment of the present invention, and FIG. 2 is a diagram schematically showing an embodiment of a flow for recovering metallic zinc from electric furnace dust by applying the present invention. In the following, a method of recovering zinc mainly using electric furnace dust as a raw material will be described, but the same steps can be applied to zinc-containing waste other than electric furnace dust.

<Washing process> As mentioned above, the actual electric furnace dust contains alkali metals such as sodium and potassium as chlorides, but these do not evaporate even at temperatures above the boiling point of zinc chloride, and leave them alone. The zinc chloride produced in the selective chlorination step (S2) described later is absorbed, and a melt having a low melting point and a low vapor pressure is formed. The melt is pre-removed not only to wet the electric furnace dust, which is a powder, but to close the gap, impede the permeability of chloride gas and reduce the yield of zinc chloride, and to solidify at low temperatures and adhere to the residue. It is preferable to do.

  Therefore, as a pretreatment prior to the temperature raising step (S1), selective chlorination is carried out by washing the alkali metal mainly contained as a chloride in waste such as electric furnace dust and other water-soluble compounds with water. The problem of the formation of a low melting point melt in the step (S2) is solved. This is obtained by considering the phenomenon that actually occurs in the selective chlorination method, and in Patent Document 1, it was not studied because reagent grade synthetic zinc ferrite was used instead of actual electric furnace dust.

  Specifically, by utilizing the fact that sodium chloride and potassium chloride, which are contained in the electric furnace dust and interfere with selective chlorination, are water-soluble, by adding water to the electric furnace dust and washing it, a part of these components can be obtained. Dissolve or remove all (water washing process). The solubility of lead chloride in water is small, but the solubility of alkali chloride, calcium chloride, and cadmium chloride is large. Therefore, as long as they are chlorides, they can be removed by washing with water. By passing through such a water washing step, it is possible to prevent the formation of a low melting point melt in the selective chlorination step and to prevent troubles that impede the permeability of the chloride gas by the low melting point melt. Moreover, since the electric furnace dust to be processed is reduced by several percent, an effect of reducing the load of the selective chlorination step can be obtained. In FIG. 2, the water washing apparatus 11 is described as a means for realizing the water washing step.

  Here, an experimental example of the water washing step will be described. Since the actual electric furnace dust contains some moisture, a drying process was performed to evaluate the amount of moisture prior to the water washing step. In the case of electric furnace dust used in the experiment, the weight was reduced by about 4% when dried at 150 ° C. Then, by washing with 2-3 times the amount of water, about 7% of the original electric furnace dust was reduced. About 7% of this is soluble salts such as alkali chlorides. Most of the solute components in the washing water used for washing are alkali chlorides and the like, but the washing water after washing contained about 50 ppm of lead and cadmium at the maximum.

  Then, based on the consideration about the solubility product of the lead and cadmium salts, by treating the wash water with sodium carbonate and calcium hydroxide under the condition of pH = 11-12, both are regulated to 0.1 ppm drainage. It was possible to reduce to below the value. The drainage is weakly alkaline. This was neutralized with exhaust gas or inorganic acid in the selective chlorination step (S2), and it was confirmed that it could be discharged. In FIG. 2, the decontamination apparatus 12 is described as means for realizing these decontamination processes.

  In FIG. 2, the electric furnace dust is washed with water by the water washing device 11, and the aqueous chloride solution containing alkali chloride, calcium chloride, etc. discharged from the water washing device 11 is decontaminated by the decontamination device 12 and discharged. This is schematically shown. The water-washed electric furnace dust is conveyed to the heating device 13 that realizes the next temperature raising step (S1).

<Temperature raising step (S1)> Since a normal combustion reaction or the like is an exothermic reaction, the reaction temperature is maintained even when a low-temperature reactant is charged, but in the selective chlorination step (S2) described later. Selective chlorination reaction of zinc content, that is, reaction to convert zinc ferrite or zinc oxide contained in electric furnace dust into zinc chloride by chlorination gas (chlorine, chlorine-oxygen mixed gas or chlorine-air mixed gas) Is an endothermic reaction. Therefore, if the heat is not replenished from the outside, the temperature will decrease, and a sustained reaction cannot be maintained due to a decrease in the reaction rate, so both the electric furnace dust and chloride gas, which are reactants, are heated in advance. It will be necessary.

  Therefore, in the present invention, each of the chloride gas and the waste is heated in advance to a predetermined temperature, and in the selective chlorination step (S2) described later, the chloride gas and the waste that have been heated to a predetermined temperature. In this way, a selective zinc chloride reaction, which is an endothermic reaction, is continuously carried out to produce crude zinc chloride vapor. In FIG. 2, the heating device 13 is described as a means for raising the temperature of the electric furnace dust, and the heating device 14 is indicated as a means for raising the temperature of the chloride gas.

  In this regard, since the technique disclosed in Patent Document 1 is based on a laboratory-level experiment result in which a small amount of reactant is performed in a furnace having a large heat capacity, the problem of this reaction stoppage is completely taken into consideration. There wasn't. Therefore, in the present invention, industrial utilization of the selective chlorination method can be realized by preliminarily heating the chlorinated gas and the water-washed electric furnace dust to a predetermined temperature (for example, a reaction temperature at which a selective chlorination reaction described later occurs). Is.

  However, if a flame such as a gas burner is used as the heating method, the combustion gas is reducible, so the positions of the “field” and “+” in FIG. Since it enters the chloride stable region and becomes iron chloride and evaporates together with zinc chloride, there arises a problem that it becomes difficult to separate zinc and iron. Furthermore, when such a reducing gas reacts with chlorine, there also arises a problem that dioxins are likely to be generated.

  Therefore, in the present invention, for example, a method of directly heating the chloride gas by installing an electric heater 15 as shown in FIG. Can do. In general, silicon carbide or metal electric heaters used in the air have no corrosion resistance against high-temperature chlorination gas, but it is possible to use such electric heaters by coating with quartz 16 having corrosion resistance against high-temperature chlorination gas. it can. This quartz-coated heater has been proven to withstand 1000 ° C. use. By using an electric heater in which a silicon carbide or metal heater is protected by quartz or the like as a heating device, the temperature can be raised while suppressing a decrease in oxygen partial pressure in the reaction, and generation of dioxins is also prevented.

<Selective Chlorination Step (S2)> In the selective chlorination step (S2), the chloride gas heated to a predetermined temperature is reacted with the waste, thereby allowing a selective chlorination reaction (oxidation) of zinc oxide, which is an endothermic reaction. A reaction of selectively chlorinating zinc) is carried out continuously to produce crude zinc chloride vapor. In FIG. 2, a chlorination furnace 17 is schematically shown as means for realizing this selective chlorination step. By reacting electric furnace dust with chloride gas at a high temperature, the zinc content in zinc oxide or zinc ferrite is selectively chlorinated and evaporated to take out in gaseous form as zinc chloride (chloride vapor), while iron is Fe ₂ O. The oxide such as ₃ remains as a residue as a solid. This oxide such as Fe ₂ O ₃ can be used as an iron source.

  FIG. 6 is a schematic view showing an example of a laboratory selective chlorination apparatus having a heating function. The apparatus 60 has a quartz reaction vessel 62 in which electric furnace dust 61 is accommodated, and a heating device 63 is disposed around the reaction vessel 62. According to the experiment of the present inventor using the apparatus 60 shown in FIG. 6, the powdered electric furnace dust 61 is deposited on the bottom of the porous plate 64 of the reaction vessel 62 by about 5 cm, and the reaction at about 1000 ° C. is performed from the bottom. When gas (the chlorine concentration is about 20 to 50%, the rest is air) is passed, the chlorine concentration in the gas that escapes to the top is sufficiently low even when the lower half of the electric furnace dust 61 reacts. Was confirmed. In this experiment, since there is a clear reaction interface, it can be said that the chemical reaction is not at the rate-determining stage, and selective chlorination can be performed at a practically sufficient rate.

  In this way, when the reaction is carried out with a small amount of unreacted electric furnace dust left, the chlorine concentration in the product gas is sufficiently low, so the exhaust gas after completion of the reaction, crude zinc chloride vapor, solid residue Sensible heat can be sufficiently used for heating the electric furnace dust and chloride gas before the reaction, which is preferable for energy saving.

  In the selective chlorination step (S2), it is possible to use a mixture of chlorine and oxygen or a mixture of chlorine and air as the chlorination gas, and considering that the allowable concentration range is wide, the internal pressure of the chlorination reaction vessel for safety. Can be made slightly lower than the atmospheric pressure outside so that the chloride gas does not leak outside. This is because even if the outside air slightly enters the reaction vessel and the chlorine concentration slightly changes, the influence on the reaction of formula (1) is very small.

<Molten salt production step (S3)> In this step (S3), the crude zinc chloride vapor produced in the selective chlorination step is condensed to obtain crude zinc chloride, and then the crude zinc chloride is purified to obtain an electrolytic bath. To obtain a molten salt (refined molten salt). Condensation for separating the crude zinc chloride and the exhaust gas is performed in the condensing tank 18, and purification is performed in the liquid purification tank 19. The crude zinc chloride vapor generated by the selective chlorination step (S2) contains a small amount of iron, lead, cadmium, and other metals that are more precious than zinc, mainly in the form of chlorides. In the step (S4), it is necessary to remove thoroughly before electrolysis in order to reduce and precipitate preferentially over zinc and lower the purity of zinc. Therefore, in the liquid purification tank 19 in this step (S3), a reducing agent is added to the molten salt obtained by mixing a supporting salt such as sodium chloride with respect to the crude zinc chloride to reduce and remove a metal that is more precious than zinc. To refine the molten salt.

  In this regard, Patent Document 1 discloses a reduction purification process and a distillation purification process as a method for removing iron, lead, cadmium, and the like in crude zinc chloride obtained by a selective chlorination method from zinc chloride. However, according to an additional study by the present inventor, the latter distillation purification process does not increase the purity of zinc chloride so that it can be directly introduced into the electrolysis process, and is applied when impurities are contained in large quantities. Since it is a rough separation, it can be omitted.

  As a reducing agent used for reduction and removal, the use of zinc powder is fundamental, but it is more desirable to use a mixture of zinc and magnesium powder as appropriate. The first reason is that when only zinc powder is used as a reducing agent, it is convenient in terms of reducing only noble metal chlorides than zinc, but the reduction temperature is set so that the zinc powder does not melt or aggregate. The melting point must be set to 419 ° C. or lower, and a temperature of approximately 350 ° C. or higher is necessary to maintain the melting of the salt. This is because in this temperature range, iron is solid, but lead and cadmium become liquid, which tends to cause clogging when the reduced product is filtered and removed, which is disadvantageous.

  The second reason is that lead or cadmium only needs to be dissolved in the solid zinc powder. However, if lead or cadmium covers the surface of the zinc powder as a liquid, contact with zinc is hindered, and the reduction reaction occurs. This is because there is a risk of not progressing. In particular, lead is hardly dissolved in zinc and is expected to cover the surface as a liquid in which a small amount of zinc is dissolved. There is a possibility that lead cannot be removed sufficiently in an electrolytic bath containing a large amount of lead.

  Therefore, when focusing on active magnesium as another reducing agent, it shows a phase diagram of eutectic type with lead and has a wide solid solution range. Therefore, as a reducing agent, powder of zinc and magnesium should be used as appropriate. Is desirable. In addition, even if magnesium dissolves in the molten salt as chloride, it does not affect electrolysis because it is lower than zinc. Thus, by using magnesium powder together as a reducing agent that removes the noble impurity metal from the molten salt mixed with the crude zinc chloride obtained in the selective chlorination step, the purification efficiency is improved and the impurity concentration is further reduced. Speeding up of the process is realized, and the purity of zinc obtained by electrolysis is also improved.

  When purifying using a reducing agent, the nature of zinc chloride becomes a problem. In other words, zinc chloride exhibits a unique property called a network structure at the atomic level in the molten state like fused quartz, and at low temperatures, it has a high viscosity and stickiness like water tank, and its fluidity is also conductive. It is very different from other chlorides, such as being easy to evaporate on the other hand. On the other hand, if the temperature is increased, the fluidity and conductivity are considerably improved. However, even when the melting point of metallic zinc is 419 ° C., the viscosity is about 200 times that of water. As described above, the reduction for removing impurities is performed at a temperature lower than the melting point of zinc. Therefore, due to the high viscosity, the reducing agent is not well mixed with the metal powder.

  A method for solving the problem of high viscosity of zinc chloride is mixing with a supporting salt (generally alkali chloride or alkaline earth chloride). By mixing, the network structure is divided and the viscosity is greatly reduced.

  By the way, in patent document 1, although the case of only zinc chloride and the case of the mixed salt which added the support salt of alkali chloride or alkaline-earth chloride as an electrolytic bath is made into a candidate (refer patent document 1 [0061]), Since the composition of the mixed salt is not shown, it is necessary to search for the optimum condition when the supporting salt is added.

  In this regard, according to the experiment of the present inventor, if sodium chloride is mixed at a mass ratio of about 15%, the viscosity is reduced to about 1/30 of the viscosity of zinc chloride at 380 ° C. It was confirmed that the reaction can be promoted. If sodium chloride is added, the content of sodium chloride is the mass ratio based on the melting temperature apparent from the phase diagram of the zinc chloride-sodium chloride system, the viscosity of the mixed molten salt according to literature values, the conductivity, the melting point of metallic zinc, etc. The composition range of 3 to 46% is preferable, and the electrolysis temperature is preferably in the range of 420 to 550 ° C.

  The effect of mixing the supporting salt with the molten salt (zinc chloride) as the electrolytic bath extends to the electrical conductivity in the molten salt electrolysis step (S4) described later. If sodium chloride as a supporting salt is mixed in the electrolytic bath by about 15% by mass, the conductivity is several tens of times that of plain zinc chloride at 450 ° C., which is assumed to be the electrolysis temperature. Furthermore, the viscosity can be lowered to a slightly higher degree than water. As a result, the electrolytic bath resistance during electrolysis is greatly reduced, so that a large current can flow and productivity is greatly improved. Naturally, the energy saving effect is great, and further, there is an effect such as improvement of the bubble breakage of chlorine gas generated from the anode, so that efficient electrolysis is possible.

<Molten salt electrolysis step (S4)> Using the purified molten salt as an electrolytic bath, zinc is deposited by electrolysis using an electrolytic bath 20 having a bipolar electrode to obtain zinc in a liquid state.

  FIG. 4 is a schematic diagram showing an example of an electrolytic cell 20 that realizes a molten salt electrolysis process. The electrolytic cell 20 has an insulating electrolytic cell main body 22 provided with a liquid zinc pumping port 21 and an upper lid 24 provided with a chlorine discharge port 23 and assembled to the electrolytic cell main body 22. A vent pipe 25 is attached to the discharge port 23. In the electrolytic cell main body 22, an insulating (ceramics or the like) partition plate 26 is incorporated in a substantially horizontal direction at a predetermined height position, and the upper surface of the partition plate 26 has a plurality of states inclined at a predetermined angle. Electrode plates 28 are arranged at a predetermined interval. The electrolytic cell 20 is filled with a molten salt as an electrolytic bath up to a predetermined height position. A liquid zinc metal pool 29 is formed below the partition plate 26.

  The partition plate 26 is provided with a plurality of through holes 27 having a small diameter, and the molten salt is electrolyzed on the upper side of the partition plate 26, and the chlorine 33 (chlorine generated at the positive electrode) generated thereby rises. The liquid zinc 32 (liquid deposited zinc at the negative electrode) is collected in the metal pool 29 through the through hole 27 formed in the partition plate 26 and is liquidated from the pumping port 21 at any time. It is configured to be pumped in the state. Although not shown, the electrolytic cell 20 is provided with a supply port for a molten salt electrolyte, and the molten salt that has been subjected to the processing of the molten salt generation step (S3) is supplied as needed.

  The electrode plate 28 is a graphite bipolar electrode. A bipolar electrode is a conductive plate that is sandwiched between at least one pair of positive and negative electrodes of an electrolytic cell usually composed of two positive and negative electrodes, and faces the original positive electrode. Becomes the negative electrode, and the surface facing the original negative electrode becomes the positive electrode, and functions as an electrode. For this reason, especially when the deposited metal is a liquid, a great effect is exhibited and a significant improvement in productivity can be realized.

  In order to dispose these electrode plates 28 on the partition plate 26 at a predetermined angle and a predetermined interval, concave mounting grooves (not shown) are formed on the inner side of the side wall of the electrolytic cell body 22 at a predetermined angle and a predetermined interval. Is formed. By inserting the plate-like electrode plates 28 into these mounting grooves, the respective electrode plates 28 are arranged at a predetermined angle and a predetermined interval. Conductive wires 30a and 30b are connected to the electrode plates 28a and 28b arranged at the end. In the illustrated case, the left electrode plate 28a is a graphite positive electrode, and the right electrode plate 28b is a graphite negative electrode. It has become.

  Any number of these electrode plates 28 may be arranged, but it is desirable to arrange them at a certain distance from each other. If the mutual distance is too close, the liquid zinc 32 deposited on the negative electrode side of each electrode plate 28 and the chlorine 33 generated on the positive electrode side of each electrode plate 28 cause a re-reaction. In order to efficiently recover the liquid zinc 32 deposited on the negative electrode side, a through hole 27 having a small diameter is formed below the negative electrode surface side of each electrode plate 28. The partition plate 26 is provided with the through hole 27. A recess is formed at the center, and the liquid zinc 32 deposited on the negative electrode surface is gathered toward the through hole 27 and flows down from the through hole 27 to the metal pool 29 below the electrolytic cell 20.

  By the way, in the bipolar electrode, in order to cause an electrode reaction on both sides of the electrode plate, a voltage corresponding to an electrolysis voltage (for example, about 1.6 V at 450 ° C. for zinc chloride electrolysis) is required between the adjacent electrode plates. Become. Here, if a metal pool of liquid metal is formed at the bottom of the electrolytic cell, the resistance of the metal is orders of magnitude smaller than that of the molten salt, so that a leakage electrode that passes through the metal pool is generated. This leakage current increases as the conductivity of the electrolytic bath increases and the resistance decreases.

  However, it is difficult to use the electrolytic cell disclosed in Patent Document 2 in the zinc recovery method disclosed in Patent Document 1. If the distance between the electrode plates is too close as in Patent Document 2, there is a problem that the liquid zinc deposited on the negative electrode side of the electrode plate and the chlorine generated on the positive electrode side of the electrode plate cause a re-reaction. It is because it is easy to occur. Furthermore, a thick graphite electrode where an electrode reaction should occur is avoided, and a leakage current flowing in the electrolytic bath is increased, resulting in a decrease in current efficiency. Such a problem becomes more apparent when the resistance of the electrolytic bath is reduced by adding a supporting salt to improve productivity as in the present invention.

  Therefore, in the present embodiment, the upper end of the electrode plate 28 is not submerged in the electrolytic bath, or the depth at which it is submerged is minimized, and a partition plate 26 made of insulating ceramics is installed at the lower end of the electrode so that current flows. By narrowing the path of the molten salt as much as possible, the generation of leakage current that tries to pass through the electrode plate 28 is minimized. By adopting an insulating partition plate 26 in the electrolytic cell 20 having the bipolar electrode plate 28 used to reduce the size and cost of the equipment, the leakage current passing under the electrode plate 28 is reduced. Decrease significantly. The partition plate 26 is provided with a through hole 27, which has a small diameter sufficient to allow a small droplet of zinc to pass therethrough, so that the leakage current passing through the metal pool 29 formed at the bottom of the electrolytic cell 20 is also minimal. It becomes. For this reason, in order to increase the number of electrodes per electrolytic cell without lowering the current efficiency, the equipment can be downsized and high productivity can be obtained.

  Chlorine 33 generated in the positive electrode by electrolysis is collected through the vent pipe 25 provided in the upper lid 24 of the electrolytic cell 20, and can be reused in the selective chlorination step (S2). At this time, it is desirable to keep the pressure inside the electrolytic cell 20 slightly lower than the atmospheric pressure for safety. Even if the outside air slightly enters the electrolytic cell 20 and the chlorine concentration is slightly changed, as described above, the allowable concentration range of chlorine in the selective chlorination step is wide, so the influence on the reaction of the formula (1) is extremely high. And has little effect on the selective chlorination step.

  The electrolytic bath (electrolytic tail solution) whose zinc chloride concentration has been reduced through the molten salt electrolysis step (S4) is transferred to the absorption tank 35 to replenish zinc recovered by electrolysis, and used for dissolving crude zinc chloride. Then, it refine | purifies in the liquid tank 19 and is supplied to the electrolytic cell 20 again. In this way, the molten salt circulates between the electrolytic bath 20 and the cleaning bath 19, but since the molten salt is at a high temperature, use of a normal pump such as that used in an aqueous solution electrolytic bath of a conventional zinc smelter is used. Is extremely difficult. Note that the electrolytic cell of Patent Document 2 is intended for high-purity zinc chloride, and zinc chloride consumed by electrolysis is merely replenished, and it is considered that purification is not considered.

  Therefore, a gas lift-up method as shown in FIG. 5 has been devised as an example of a low-cost method for circulating a high-temperature molten salt. FIG. 5 is a schematic view of an apparatus for transferring molten salt as an electrolytic bath accommodated in a low-position side bath to the high-position side bath. The molten salt transfer device 40 is a device for transferring a liquid molten salt from a lower tub 45 to a higher tub 46, and a transfer pipe 43 is disposed between the tubs 45, 46. . The molten salt is accommodated in the bathtub 45 in the lower position, and the lower end of the transfer pipe 43 is disposed so as to be immersed in the molten salt. The bathtub 45 is provided with a gas blowing pipe 41 for blowing gas into the lower end of the transfer pipe 43. The gas (bubbles) 47 discharged from the gas blowing pipe 41 floats through the transfer pipe 43. The upper end of the transfer pipe 43 is arranged in the bathtub 46 at a high position so that the molten salt can be poured into the bathtub 46.

  The molten salt transfer device 40 utilizes a phenomenon in which gas is blown from the lower end of the transfer pipe 43 to push up the liquid when the gas (bubble) 47 rises. It is the same principle used as a water circulation device. Transfer from the top to the bottom is by a natural flow due to gravity, and transfer from the bottom to the top is by a simple and low-cost method such as lift-up by gas. In the case where the electrolytic bath in which the molten salt electrolysis step (S4) is performed and the bathtub used in the molten salt generation step (S3) are arranged with a height difference, there is a difference between the high-position side bathtub and the low-position side bathtub. It is possible to circulate the molten salt between them. Needless to say, in the application of the present invention, the molten salt circulation method is not limited to the method shown in FIG.

  Here, experimental examples from the selective chlorination step (S2) to the molten salt electrolysis step (S4) will be described. When about 100 g of water-washed electric furnace dust 61 is placed on the quartz porous plate 64 of the quartz device 62 shown in FIG. 6 and the temperature is raised while flowing air, even if the temperature reaches 1000 ° C. However, when about 50% of chlorine gas was mixed with the flowing air, a remarkable change appeared immediately after. That is, it is confirmed that smoke such as smoke rises from the electric furnace dust 61, and this smoky thing becomes liquid at the low temperature portion at the top of the reaction vessel 62 and adheres as droplets to the inside of the reaction vessel 62. When the amount of droplets reaches a certain level, it flows into the cooling aggregation tube 65 made of quartz and solidifies there. In the state where there is enough unreacted electric furnace dust, almost no chlorine is detected in the exhaust. What is collected in the cooling aggregation tube 65 is crude zinc chloride colored yellowish brown. The cause of coloring is considered to be the color of iron chloride, which is an impurity. On the other hand, the residue remaining on the porous plate 64 changed from a brown color, which is the original color of the electric furnace dust, to a color close to black.

  By the way, although patent document 1 is based only on the 800 degreeC experiment using a zinc ferrite, since this temperature is not necessarily optimal, it is necessary to search for suitable reaction temperature. Therefore, when the temperature was changed and the chlorination experiment was conducted, the reaction rate was insufficient at less than 700 ° C., and unreacted substances remained. The reaction rate increases as the temperature increases, but reaches a peak when the temperature exceeds 1000 ° C., and if it exceeds 1100 ° C., it becomes difficult to keep the reaction tube warm, and the durability of the refractory decreases. Since this chlorination reaction is endothermic, a temperature exceeding the optimum temperature may be required in part. Therefore, when the present invention is industrially used, the reaction temperature is preferably between 700 and 1100 ° C. According to experimental examination, if it is the range of 700-1100 degreeC, it exists in the range of those balances, and can be selected according to the conditions of an apparatus or operation.

  When the obtained crude zinc chloride was analyzed, it contained about 0.5% of lead and cadmium in addition to about 0.5% of iron. On the other hand, most of the residue is iron oxide. About 0.5% zinc was contained, but lead and cadmium were not detected. The remaining components are soil constituent elements such as silicon and aluminum, and are oxides containing 50% or more of iron. Therefore, although they are slightly lower in quality than iron ore, they can be used as iron sources for electric furnaces if they are free of charge.

  By adding and stirring zinc powder at a temperature of 400 ° C. or less to a molten salt obtained by mixing and melting sodium chloride in a tan brown crude zinc chloride obtained by an experiment with a mass ratio of about 10%, a black precipitate is produced. The molten salt became colorless and transparent. The black precipitate contained lead and cadmium. Using this colorless and transparent molten salt as an electrolytic bath, electrolysis was performed at about 450 ° C. using a graphite electrode in a quartz electrolytic device to obtain metallic zinc having a purity exceeding 99.9%. In addition, it has been confirmed that zinc zinc having a purity of 99.99% or more can be obtained if the electrolytic bath is sufficiently purified.

  The present invention is not limited to electric furnace dust, but (a) waste containing zinc as a chloride, and (b) waste containing zinc chloride discharged in a dechlorination step from a chlorine-containing organic compound in the chemical industry. This can also be applied to wastes containing (c) zinc as an oxide. In particular, (c) can also be applied from the selective chlorination step.

  For the molten salt electrolysis step (S3), instead of the chloride obtained in the selective chlorination step (S2), waste containing zinc discharged mainly from other industrial processes in the form of chloride may be used. . By applying the chloride-based waste to the molten salt electrolysis method, the effect of supplementing the chlorine deficient in the selective chlorination step can be obtained.

  INDUSTRIAL APPLICABILITY The present invention can be applied to waste containing zinc such as actual electric furnace dust and zinc chloride waste that is a complex system containing many impurities, and recovers high-purity metallic zinc from the waste. be able to.

11 Flushing device 12 Exhaust port 13 Heating device 14 Heating device (electric heating)
DESCRIPTION OF SYMBOLS 15 Electric heater 16 Quartz 17 Chlorination furnace 18 Condensation tank 19 Purifying tank 20 Electrolysis tank 21 Pumping out port 22 Electrolytic tank main body 23 Outlet 24 Upper cover 25 Vent pipe 26 Partition plate 27 Through-hole 28 Electrode plate 28a Electrode plate arrange | positioned at the left end 28b Electrode plate 29 arranged at right end Metal pool 30a Conductive wire 30b to graphite positive electrode Conductive wire 31 to graphite negative electrode 31 Molten salt electrolytic bath 32 containing zinc chloride Liquid zinc 33 Chlorine 35 Absorption tank 40 Molten salt transfer device 41 Gas injection Pipe 42 Bubble 43 Transfer pipe 44 Discharged molten salt 45 Bath 46 in low position Bath 47 in high position Gas (bubble)
60 Apparatus 61 Electric furnace dust 62 Quartz reaction vessel 63 Heating apparatus 64 Porous quartz plate 65 Quartz cooling aggregation tube

Claims (9)

A method for recovering zinc from waste containing zinc,
A temperature raising step of preheating each of the chloride gas and the waste to a predetermined temperature;
A selective chlorination step of reacting the chlorinated gas heated to a predetermined temperature with the waste to cause a selective chlorination reaction of zinc as an endothermic reaction to generate crude zinc chloride vapor; ,
After the crude zinc chloride vapor is condensed to obtain crude zinc chloride, a molten salt production step for obtaining the molten salt by refining the crude zinc chloride;
A method for recovering zinc from zinc-containing waste, comprising a molten salt electrolysis step of depositing zinc by electrolyzing the molten salt as an electrolytic bath.   Prior to the temperature raising step, the alkali metal and other water-soluble compounds mainly contained as chlorides in the waste are washed away with water. Zinc recovery method.   The method for recovering zinc from zinc-containing waste according to claim 1 or 2, wherein the temperature raising step is performed by heating the chloride gas with electric heat while suppressing a decrease in oxygen partial pressure. .   The molten salt electrolysis step includes an electrolytic bath main body that stores molten salt as an electrolytic bath, and an insulating partition that is provided with a plurality of small-diameter through-holes and is incorporated substantially horizontally at a predetermined height position of the electrolytic bath main body. An electrolytic bath having a plate and a plurality of bipolar-type electrode plates disposed on the upper surface of the partition plate, and the zinc deposited by electrolysis is allowed to pass through the through-hole and the partition plate The method for recovering zinc from zinc-containing waste according to any one of claims 1 to 3, wherein the zinc is stored at a lower side.   5. The temperature raising step raises the temperature of each of the chlorinated gas and the waste until reaching a temperature of 700 ° C. or higher and 1100 ° C. or lower at which a selective chlorination reaction occurs. A method for recovering zinc from zinc-containing waste as described in 1.   The purification of the crude zinc chloride in the molten salt production step is performed by adding a supporting salt to the crude zinc chloride to increase the conductivity while reducing the viscosity of the crude zinc chloride, and then adding a reducing agent to the zinc. The method for recovering zinc from zinc-containing waste according to any one of claims 1 to 5, wherein a more noble metal is reduced and removed.   The method for recovering zinc from a zinc-containing waste according to claim 6, wherein zinc powder is used as the reducing agent, or zinc powder and magnesium powder are appropriately mixed as the reducing agent.   Use of an alkali chloride or alkaline earth chloride such as sodium chloride as the supporting salt, particularly when sodium chloride is used as the supporting salt, the content of sodium chloride is a composition range of 3 to 46% by mass ratio. The method for recovering zinc from zinc-containing waste according to claim 6 or 7, wherein the electrolysis temperature in the molten salt electrolysis step is in the range of 420 to 550 ° C.   In the case where the electrolytic bath in which the molten salt electrolysis step is performed and the bathtub used in the molten salt generation step are arranged with a height difference, a transfer pipe connecting the high-position side bathtub and the low-position side bathtub The zinc-containing material according to any one of claims 1 to 8, wherein the molten salt contained in the low-position side bathtub is transferred to the high-position side bathtub by blowing gas from the lower end side. How to recover zinc from waste.

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