Some thoughts on the comprehensive high-value reuse project for sulfuric acid slag

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Some Thoughts on the Comprehensive High-Value Reuse of Sulfuric Acid Slag – Introduction As is well known, sulfuric acid slag is one of the by-products produced during the production of sulfuric acid from pyrite. With the advancement of slag treatment technologies today, black sulfuric acid slag is a resource with a relatively high iron content; it also contains valuable metals such as gold, silver, copper, lead, and zinc, as well as substances like silicon dioxide. How to improve the utilization value of sulfuric acid slag and recover valuable elements. Avoiding excessive storage of sulfuric acid slag, which could cause environmental pollution, is of utmost importance for those working in this field. I. In a fluidized bed furnace, when semi-coke and pyrite are subjected to high-temperature reactions, the mineral powder undergoes desulfurization, oxidation, high-temperature melting, agglomeration, and solidification, resulting in the formation of sulfuric acid slag. Due to its special formation process, sulfuric acid slag has a complex composition; it is somewhat corrosive to equipment, has high hardness, and exhibits significant friction. The selection of equipment is also very important when handling it. By communicating with vertical mill manufacturers and wet grinding mill manufacturers. Sulfuric acid slag contains a certain amount of moisture, typically around 13-15%, and the leachate is acidic. A large amount of heat is generated in the lapping machine, leading to sticking to the walls. If the moisture content is high, water vapor will form at the outlet of the wet grinder, mix with the material, and block the discharge outlet. Dust removal pipes also get clogged very easily. With the use of vertical mills, there are currently no successful precedents in China. There are several issues associated with using vertical mills to process sulfuric acid slag. One of them is whether a material layer can be formed; sulfuric acid slag is already very fine, and after air separation and grading, only a small portion of it is able to form a layer on the mill drum. Second, after grading the materials, is it still necessary to mix them? At present, it seems that vertical mills are better suited for obtaining stable materials. The moisture content, particle size, and production capacity are all relatively stable, and transportation is also convenient, features that are not available with existing equipment. II. High-power mixers: If the particle size of sulfuric acid is one of the key factors in ball formation, then the uniformity of the mixture is another crucial aspect. Sulfuric acid slag, bentonite, and calcium chloride solution are mixed using a mixer, and the resulting mixture is then conveyed to a balling disc for ball formation. And uniform mixing of the materials, consistent moisture levels, and smooth transportation are key to ball formation. The current choice of mixer is not ideal enough. During the mixing process of sulfuric acid slag, bentonite, and calcium chloride solution, significant resistance is generated. The blades of the mixer, as well as the drive shaft, are subject to high stress, resulting in shaft breakage or bending in the mixer. It severely affects production. When selecting a mixer, it is also necessary to base the decision on practical considerations: choose a mixer with a large mixing capacity, the ability to achieve uniform mixing, corrosion resistance, and one that is suitable for this type of material. III. Green ball dryer – The selection of a green ball dryer is also very important. There are significant problems with the heat source gas circulation in the current system. The temperature is not uniform enough. There is a low-temperature zone, resulting in a limited effective drying area for the grate dryer. The pores of the chain plate are severely blocked. The moisture content of the green balls after drying is within 1%. The screening efficiency of the vibrating screen at the outlet side is not ideal; when the layer thickness is large and thorough drying is not possible, a large amount of material gets pulverized, and the drying capacity is limited, failing to meet the production requirements. Redesign is needed to address issues such as small pores in the chain plates, short stroke length, and uneven distribution of heat sources. IV. Heat exchange design: Heat exchange issues are also one of the key problems that need to be addressed. This system uses natural gas as a heat source, and careful design along with effective methods are required to make full use of the heat. The heat exchanger designed currently has limited capacity, and the heat exchange temperature does not meet the design requirements. The heat exchange methods and forms in vertical furnaces need to be improved. Inside the shaft furnace, the heat exchange path with the pellets is only 696 mm. Due to the operation of the exhaust fan, there are cases of short circuits and blocked gas pipes, which affect the heat exchange efficiency. The heat is diluted by cold air and enters subsequent systems, having a significant impact on smoke recovery. V. Selection of purification equipment: The biggest issue in purification and recycling is the choice and application of the material used for the equipment. Due to the presence of chlorine in the flue gas, the requirements for materials under high temperatures are almost stringent. Neither stainless steel nor Hastelloy possesses strong resistance to chlorine. Corrosion resistance should be given priority, along with considerations for equipment weight and foundation design. It is the link that currently severely restricts production. Investors must decide whether to solve the problem by reducing the flue gas temperature or by choosing appropriate materials for the equipment. VI. Separation of precious metals: The separation of precious metals is related to issues of profitability and environmental protection. By separating valuable metals such as gold, silver, copper, lead, and zinc, the heavy metal content in gypsum is reduced, thereby avoiding the classification as hazardous waste and eliminating environmental concerns. Separating precious metals holds economic value, increases corporate revenue, and also represents a way to achieve resource regeneration. It meets environmental protection requirements as well as the needs of social development. At present, only the gold sludge can be separated; the sequential separation of the remaining components requires careful design and operation to be achieved, and this represents a potential source of significant profits. VII. Consensus and Prospects Through the summary and reflection on the pilot production, practical production experience and data regarding the balling properties of sulfuric acid slag, production equipment, and operational control have been initially obtained. Based on the actual production conditions in each workshop, the following consensus was reached: 1. The process principle is reliable; pilot production has established the process route, and according to this principle, pellets have been successfully produced along with high-quality gold concentrate. All components are now connected properly. 2. The technical solution is feasible; through trial production, the technical approach and plan have been put into practice. Due to equipment issues, there are limited nodes, but the overall technical solution has proven to be feasible. 3. The prospects are very bright. Once the equipment and certain processes are resolved, the project can achieve industrial-scale production, offering broad prospects for application in the field of resource recycling as well as in the regional economy.