A method for recovering indium metal from waste liquid crystal displays

Abstract

The present invention discloses a method for recovering indium metal from waste liquid crystal displays, belonging to the technical field of resource recovery of waste electronic products. The indium tin oxide glass layer in the waste liquid crystal display is peeled off and pulverized to obtain glass powder; the glass powder is mixed with a sulfurizing agent and calcined at a temperature of 655-750°C under a reducing atmosphere, and the volatiles generated during the calcination process are collected to obtain indium materials. This method can achieve high-selectivity volatilization recovery of indium in waste liquid crystal displays through one-step low-temperature calcination, so as to efficiently separate from impurity elements such as Si, Al, and Mg, in order to obtain high-quality and easily utilizable indium smelting raw materials, and this method has a simple technical route, low cost, and safety, meeting the requirements of industrialization.

Description

Technical Field

[0001] The present invention relates to a method for treating waste liquid crystal displays, and particularly to a method for recovering indium metal from waste liquid crystal displays, belonging to the technical field of resource recovery of waste electronic products. Background Art

[0002] Waste liquid crystal displays are a typical type of solid waste generated after the scrapping of electronic and electrical products, which originate from the elimination and replacement of liquid crystal display modules in electronic devices such as televisions, computers, and mobile phones. Since waste liquid crystal displays contain heavy metal pollutants such as lead and mercury, alkyl cyclohexane compounds in the liquid crystal layer, and toxic and harmful substances such as halogen flame retardants in the backlight module, improper disposal will cause environmental risks such as soil heavy metal pollution and underground water body poisoning. At the same time, waste liquid crystal displays contain resources such as indium and high-purity silicon. Among them, indium, as the core material of transparent conductive films, has a grade far exceeding the industrial mining standard of primary ores and has significant economic value.

[0003] In liquid crystal displays, transparent conductive electrodes are usually formed by sputtering indium tin oxide (ITO) on the surface of a glass substrate. This structure has both excellent electrical conductivity and optical transparency characteristics. Its core components are a composite oxide of indium oxide (In2O3) and tin oxide (SnO2), and among them, the indium-tin ratio is usually above 10. The content of In2O3 in ITO glass in waste liquid crystal displays generally reaches more than 300 ppm, which can completely be used as a potential high-quality raw material for the indium industry.

[0004] Currently, some methods for recovering indium from waste liquid crystal displays have been disclosed in the prior art. For example, Chinese Patent (CN109112311B) discloses a method for recycling waste flat panel displays. The specific operation is to crush and acid-leach waste liquid crystal displays, and then perform extraction, zinc replacement, and electrolysis on the acid-leached solution to separate and recover indium metal. Chinese Patent (CN113234927B) discloses a device and method for recovering indium from waste liquid crystal displays. The specific operation is to spray-leach the glass substrate in waste liquid crystal displays, and use an indium resin adsorption device to adsorb and recover indium in the indium-containing leachate. The above two methods both recover indium from waste liquid crystal displays by wet methods. However, the existing wet recovery processes are lengthy, consume a large amount of acid, and the generated waste acid solution also needs to be disposed of additionally. Summary of the Invention

[0005] Aiming at the technical problems existing in the methods for recycling waste liquid crystal displays in the prior art, the purpose of the present invention is to provide a method for recovering indium metal from waste liquid crystal displays. This method can achieve high-selectivity volatilization recovery of indium in waste liquid crystal displays through one-step low-temperature roasting, so as to efficiently separate it from impurity elements such as Si, Al, and Mg, in order to obtain high-quality and easily utilizable indium smelting raw materials. Moreover, this method has a simple technical route, low cost, and is safe, meeting the requirements of industrialization.

[0006] The present invention provides a method for recovering indium metal from waste liquid crystal displays. The method involves peeling and pulverizing the indium tin oxide glass layer in the waste liquid crystal displays to obtain glass powder. The glass powder is mixed with a sulfiding agent and roasted at a temperature of 655 - 750 °C under a reducing atmosphere, and the volatiles generated during the roasting process are collected to obtain indium materials. The sulfiding agent is composed of iron sulfide and calcium sulfide in a mass percentage of 10% - 35%: 65 - 90%. The ratio of the sulfiding agent to the glass powder satisfies that the molar ratio of S / In elements is 1.55 - 2.55. The reducing atmosphere is an atmosphere containing H2, and the volume ratio of H2 / H2O in the reducing atmosphere is 1.85 - 2.55.

[0007] The key to the technical solution of the present invention lies in: on the one hand, taking advantage of the high saturated vapor pressure characteristics of low - price indium sulfide at low temperatures and the characteristics that impurity elements such as Si, Al, and Mg are difficult to volatilize at low temperatures, so that indium in waste liquid crystal displays can be converted into the form of low - price indium sulfide and recovered by high - selective volatilization through sulfiding means; on the other hand, fully utilizing the reducing atmosphere to reduce the Gibbs free energy of the indium sulfide oxidation reaction, to strengthen the selective sulfidation and volatilization of indium in waste liquid crystal displays at low temperatures, and to ensure that the glass phase is not melted at low temperatures, thereby improving the indium sulfidation volatilization efficiency. In summary, the present invention realizes the high - selective sulfidation and volatilization of indium in waste liquid crystal displays at low temperatures by using a sulfiding agent and a reducing atmosphere to obtain high - quality and easily utilizable indium smelting raw materials.

[0008] Another important innovation in the technical solution of the present invention is: using a sulfiding agent mainly composed of iron sulfide and calcium sulfide, mainly because these two sulfides do not introduce impurity elements that are easily volatile at low temperatures, nor do they introduce substances that lower the melting point of the system. Moreover, the introduced iron and calcium are easy to form high - melting - point phases, enabling the sulfiding agent to provide a framework support effect for the system while exerting its sulfiding effect and not affecting the sulfidation reaction. If there are impurities in the sulfiding agent that lower the melting point of the system, it will cause the system to melt and block the volatilization of indium sulfide. If volatile substances are introduced into the sulfiding agent, it will affect the purity of the recovered indium materials.

[0009] As a preferred solution, the particle size of the glass powder satisfies that the mass ratio of the particle size less than 0.074 mm reaches 30% - 60%. When the particle size of the glass powder is too coarse, the contact between the glass powder and the sulfiding agent is insufficient, making it difficult to occur the sulfidation reaction of indium oxide, resulting in poor indium recovery rate. When the particle size of the glass powder is too fine, it is unstable in the air flow, and a large amount of energy is wasted in the fine grinding stage.

[0010] The method for stripping and pulverizing the indium tin oxide glass layer in waste liquid crystal displays according to the present invention uses mature methods such as low-temperature air heating, mechanical stripping, centrifugal crushing, and ball milling in the prior art. However, the requirement for the pulverization particle size is that 30% to 60% should be less than 0.074 mm.

[0011] The vulcanizing agent of the present invention is composed of iron sulfide and calcium sulfide in a mass percentage of 10% to 35%: 65 to 90%. The proper proportioning of iron sulfide and calcium sulfide is mainly based on the comprehensive consideration of the sulfur release efficiency, reaction controllability, and cost of the two. When the two are proportioned properly, the best indium sulfide effect can be achieved. When the proportion of iron sulfide is too high or too low, it is not conducive to the formation of a high melting point skeleton with calcium, resulting in a decrease in the volatilization effect.

[0012] The ratio of the vulcanizing agent to the glass powder in the present invention is such that the molar ratio of S / In elements is 1.55 to 2.55. If the proportion of the vulcanizing agent is too low, the vulcanization reaction will be incomplete, and part of the indium will volatilize in the form of indium oxide. Its volatilization effect is inferior to that of indium sulfide, resulting in a low indium recovery rate. If the proportion of the vulcanizing agent is too high, the introduction of excessive iron will inevitably consume additional H2 gas, which hinders the indium sulfide volatilization behavior, resulting in a decrease in the vulcanization reaction rate and volatilization efficiency.

[0013] The reducing atmosphere of the present invention is an H2-containing atmosphere, and the volume ratio of H2 / H2O in the reducing atmosphere is 1.85 to 2.55. If the partial pressure of H2 is too low, the direct sulfide reaction becomes the main reaction, and its Gibbs free energy is higher, resulting in a sharp decrease in the indium volatilization rate. If the partial pressure of H2 is too high, the system is prone to over-reduction, and indium oxide is directly reduced to metallic indium. Metallic indium volatilization requires a higher temperature, and the glass phase is easily melted at high temperatures.

[0014] As a preferred solution, the roasting time is 2 to 3 hours. Based on the action of the reducing gas, the Gibbs free energy of the indium sulfide reaction of indium oxide is reduced, so the formation and volatilization of indium sulfide can occur at a lower temperature of 655 to 750 °C. Therefore, the temperature can be controlled below the melting temperature of the glass for the reaction. However, too low a temperature makes the sulfide reaction difficult to occur, and too high a temperature will cause the glass phase to melt, deteriorating the indium volatilization recovery. A roasting time of more than 2 hours can achieve a good indium recovery effect, and too long a time has no positive impact on the result.

[0015] Compared with the prior art, the beneficial technical effects brought by the technical solution of the present invention:

[0016] The present invention can realize the selective sulfurization volatilization recovery of indium in waste liquid crystal displays through one-step low-temperature roasting, and the separation from other impurity elements, so as to obtain high-quality and easy-to-use indium smelting raw materials, providing a new way for the large-scale disposal and utilization of the resources of waste liquid crystal displays, and solving the problems of long process flow and high acid consumption in traditional wet leaching processes, and the easy melting of glass in traditional pyrometallurgical processes affecting the recovery effect.

[0017] The present invention makes full use of the characteristics of the high saturated vapor pressure of low-valent indium sulfide at relatively low temperatures, adds an appropriate amount of sulfurizing agent to selectively convert indium in waste liquid crystal displays into low-valent indium sulfide for volatilization. At the same time, with the help of a reducing atmosphere, the Gibbs free energy of indium for sulfidation reaction is reduced to strengthen the low-temperature and high-efficiency sulfidation of indium, and ensure that the glass phase is not melted, improving the volatilization efficiency of low-valent indium sulfide. Impurity elements such as Si, Al, and Mg in the system are difficult to react and volatilize at low temperatures, thus realizing the selective recovery of metallic indium and obtaining high-quality and easy-to-use indium smelting raw materials at the same time.

[0018] The present invention can be implemented using mature large-scale industrial equipment such as rotary kilns and reduction shaft furnaces. The technical route is simple and the cost is low, greatly improving the recovery efficiency and resource recycling rate of waste liquid crystal displays. Detailed implementation manners

[0019] The following examples are intended to further illustrate the content of the present invention rather than limit the protection scope of the claims of the present invention.

[0020] Comparative example 1

[0021] This comparative example is mainly used to illustrate that if the proportion of iron sulfide in the sulfurizing agent is improper, it is not conducive to the volatilization and separation of indium in waste liquid crystal displays.

[0022] Compared with Example 1, the only difference is that the proportion of iron sulfide in the sulfurizing agent is 60%.

[0023] The collected products were tested and analyzed: the indium content was 48.4%, and the indium recovery rate was only 46.8%.

[0024] Comparative example 2

[0025] This comparative example is mainly used to illustrate that if the molar ratio of S / In elements is too low, it is not conducive to the selective volatilization of indium.

[0026] Compared with Example 1, the only difference is that the molar ratio of S / In elements is controlled to be 1.20.

[0027] The collected products were tested and analyzed: the indium content was 38.3%, and the indium recovery rate was only 36.8%.

[0028] Comparative example 3

[0029] This comparative example is mainly used to illustrate that if the volume ratio of H2 / H2O is too low, it is not conducive to reduction sulfidation and volatilization.

[0030] Compared with Example 1, the only difference is that the volume ratio of H2 / H2O is 1.55.

[0031] The collected products were tested and analyzed: the indium content was 48.7%, and the indium recovery rate was only 46.1%.

[0032] Comparative Example 4

[0033] This comparative example is mainly used to illustrate that if the roasting temperature is too high, it will cause the glass phase to melt, which is not conducive to volatilization and separation.

[0034] Compared with Example 1, the only difference is that the roasting temperature is 950 °C.

[0035] The collected products were tested and analyzed: the indium content was 47.1%, and the indium recovery rate was only 32.9%.

[0036] Example 1

[0037] Using a certain waste liquid crystal screen (the In2O3 content in ITO glass is 530 ppm), iron sulfide, calcium sulfide and hydrogen as raw materials. First, the indium tin oxide glass layer in the waste liquid crystal screen was peeled off by heating with hot air and crushed to a particle size less than 0.074 mm with a proportion of 30%. Then, a sulfurizing agent was added, with iron sulfide accounting for 10%, and the S / In element molar ratio was controlled to be 2.55, and the mixture was evenly mixed; the mixed raw materials were subjected to hydrogen reduction roasting, with the H2 / H2O volume ratio being 2.55, the roasting temperature being 750 °C, and holding for 2 hours. After the roasting was completed, the volatiles collected were the indium raw materials.

[0038] The collected products were tested and analyzed. The indium content was 49.8%, and the indium recovery rate was 96.4%.

[0039] Example 2

[0040] Using a certain waste liquid crystal screen (the In2O3 content in ITO glass is 530 ppm), iron sulfide, calcium sulfide and hydrogen as raw materials. First, the indium tin oxide glass layer in the waste liquid crystal screen was peeled off by heating with hot air and crushed to a particle size less than 0.074 mm with a proportion of 60%. Then, a sulfurizing agent was added, with iron sulfide accounting for 30%, and the S / In element molar ratio was controlled to be 1.55, and the mixture was evenly mixed; the mixed raw materials were subjected to hydrogen reduction roasting, with the H2 / H2O volume ratio being 2.55, the roasting temperature being 675 °C, and holding for 2 hours. After the roasting was completed, the volatiles collected were the indium raw materials.

[0041] The collected products were tested and analyzed. The indium content was 49.6%, and the indium recovery rate was 97.7%.

[0042] Example 3

[0043] Using a certain waste liquid crystal screen (the In2O3 content in the ITO glass is 530 ppm), iron sulfide, calcium sulfide, and hydrogen as raw materials. First, the indium tin oxide glass layer in the waste liquid crystal screen is peeled off by heating with hot air and crushed to a proportion of less than 0.074 mm accounting for 50%. Then, a sulfurizing agent is added, with iron sulfide accounting for 35%. The S / In element molar ratio is controlled at 1.95, and the mixture is evenly mixed. The mixed raw materials are subjected to hydrogen reduction roasting, with the H2 / H2O volume ratio being 1.85, the roasting temperature being 750 °C, and holding for 2 hours. After the roasting is completed, the volatiles collected are the indium raw materials.

[0044] The collected product is tested and analyzed. The indium content is 49.4%, and the indium recovery rate is 96.9%.

[0045] Example 4

[0046] Using a certain waste liquid crystal screen (the In2O3 content in the ITO glass is 530 ppm), iron sulfide, calcium sulfide, and hydrogen as raw materials. First, the indium tin oxide glass layer in the waste liquid crystal screen is peeled off by heating with hot air and crushed to a proportion of less than 0.074 mm accounting for 30%. Then, a sulfurizing agent is added, with iron sulfide accounting for 20%. The S / In element molar ratio is controlled at 1.95, and the mixture is evenly mixed. The mixed raw materials are subjected to hydrogen reduction roasting, with the H2 / H2O volume ratio being 1.85, the roasting temperature being 655 °C, and holding for 3 hours. After the roasting is completed, the volatiles collected are the indium raw materials.

[0047] The collected product is tested and analyzed. The indium content is 49.4%, and the indium recovery rate is 96.5%.

Claims

1. A method for recovering indium metal from waste liquid crystal displays, characterized in that: Peel and crush the indium tin oxide glass layer in the waste liquid crystal screen to obtain glass powder; mix the glass powder with a sulfurizing agent, place it in a reducing atmosphere and roast it at a temperature of 655-750 °C, and collect the volatiles generated during the roasting process to obtain indium material; the sulfurizing agent is composed of iron sulfide and calcium sulfide according to the mass percentage of 10%-35%: 65%-90%; the ratio of the sulfurizing agent to the glass powder satisfies that the molar ratio of S / In elements is 1.55-2.55; the reducing atmosphere is an atmosphere containing H2; the volume ratio of H2 / H2O in the reducing atmosphere is 1.85-2.

55.

2. The method for recovering indium metal from waste liquid crystal displays according to claim 1, characterized in that: The particle size of the glass powder satisfies that the mass ratio of the particle size less than 0.074 mm reaches 30%-60%.

3. The method for recovering indium metal from waste liquid crystal displays according to claim 1, wherein: The roasting time is 2-3 hours.