Technologies for the resource utilization of sulfuric acid waste from steel industry pickling processes

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This technology is: a method for the resource utilization of acid waste generated during steel pickling. The process involves: waste sulfuric acid – precipitation – acid preparation – cooling and crystallization – solid-liquid separation (with the sulfuric acid being reused and ferrous sulfate being sold externally). In cold-worked steel pipes, acid pickling of the surface is required due to the demands of the production process; dilute sulfuric acid with an initial concentration of less than 25% is generally used for this purpose. As the pickling process progresses, the concentration of sulfuric acid decreases, and waste acid with a concentration below 4% is no longer usable when it contains more than 280 g/L of ferrous sulfate. For unusable waste acid, **the environmental protection authorities have strict regulations. Currently, most companies handle waste acid by entrusting it to qualified third-party environmental protection firms for treatment; this involves paying a certain fee to such firms. Another method is acid-base neutralization, where slaked lime is added to the waste acid until the pH reaches 7, after which filtration and sedimentation take place. The water obtained after filtration can be used to rinse pipes after acid cleaning, while the waste residue is sent to qualified environmental protection firms for disposal. However, either approach will result in waste of resources and indirect environmental pollution; moreover, leaks during transportation can also cause serious harm to people and the environment. At present, China has strict regulations regarding the transportation of such products. Taking the above circumstances into account, this process solves all the aforementioned problems by using physical methods to treat waste acid, achieving zero emissions and zero pollution. Moreover, the original waste acid can be recycled as a resource; 95% of the resulting ferrous sulfate heptahydrate can be sold as a commodity. This not only addresses the pollution issue but also generates significant economic value for enterprises. Ferrous sulfate itself is a flocculating substance; this invention takes advantage of the different solubilities of ferrous sulfate at various temperatures by lowering the temperature of the waste acid solution, thereby causing ferrous sulfate to precipitate in crystalline form, followed by solid-liquid separation using a solid-liquid separator. The separated waste acid liquid can be re-prepared and used as new liquid, while the ferrous sulfate heptahydrate (FeSO4 7H2O) of chemical grade with a purity of 95% appears as pale lavender-green crystals; its molecular weight is 278.03 and its specific gravity ranges from 1.895 to 1.898. In dry air, it can undergo weathering and dehydration to form white ferrous sulfate powder. Ferrous sulfate is a widely used chemical product that can be employed in the production of iron oxide pigments (such as iron red and iron yellow), as well as in the manufacture of other chemical substances like malachite green, indigo blue, and yellow prussiate. It is also widely used in pharmaceuticals or feed additives. Specific operations and techniques of the plan: The main equipment required for this process is arranged in a three-dimensional layout, primarily to facilitate handling, reduce production costs, and make efficient use of resources. 1. The waste acid generated in the pickling workshop, with a concentration of less than 4% and an iron sulfate content of over 280 g/L, is discharged through pipes into a waste acid storage tank for sedimentation. A heating device is installed at the bottom of the sedimentation tank, just like in the workshop’s pickling tanks, in order to prevent insufficient recovery of iron sulfate due to its crystallization during winter. 2. Use a diaphragm pump to transfer the settled supernatant acid waste from the waste acid storage tank into the cooling crystallization reactor. The reactor usually has a capacity of 3–6 cubic meters, and it is continuously stirred by an agitator at a speed of 60 revolutions per minute. The temperature of the waste acid in the reaction vessel is reduced to a certain level through compression refrigeration. 3. The cooled and crystallized waste acid liquid is pumped to the solid-liquid separation system; the separated ferrous sulfate is sold as a product, while the waste acid liquid, after being mixed with concentrated sulfuric acid, is directly pumped to the pickling tanks in the workshop for use. 4. At this point, the iron sulfate content in the waste acid solution is around 40 g/L. Since the separated ferrous sulfate exists in the form of crystals of ferrous sulfate heptahydrate, some water is removed during the crystallization process; as a result, the concentration of the original 5% waste acid increases to around 7% after ferrous sulfate is separated out. 5. The crystals of ferrous sulfate heptahydrate separated by the separator can be packaged and sold after a short period of time. The waste acid content in the crystals of ferrous sulfate heptahydrate is less than about 1%; after a short period of time, a small amount of waste acid containing saturated iron sulfate will flow out. This waste acid is collected and sent through pipes to a waste acid storage tank for recycling. 6. In the acid preparation tank, an appropriate amount of concentrated sulfuric acid or a small quantity of water is added according to production needs to prepare an acid solution with a concentration of 20–25%, which is then used in a cyclic manner in the production tanks of the pickling workshop. 7. In actual production, in the pickling tanks located in the workshop, a certain amount of ferrous sulfate and black sludge precipitates at the bottom after waste acid is discharged; these residues are then removed and placed in a sediment storage tank. Furthermore, the distance between the production tanks in ordinary workshops and the waste acid storage tanks is usually relatively large. Due to the high iron content in the waste acid, some of it crystallizes and deposits within the discharge pipes, which not only blocks the flow in these pipes but also hinders the recovery of ferrous sulfate. Therefore, it is necessary to clean these pipes regularly based on production requirements. The sediment that is removed is also placed in a sediment storage tank. A heating device is installed at the bottom of the sediment storage tank, as solubility is low in winter. Feature: Through this treatment process, zero discharge of waste acid liquid is achieved, which helps to reduce environmental pollution during the original transportation and treatment processes. Ferrous sulfate heptahydrate with a purity of 95% can be produced and sold as a commodity. It eliminates the financial costs associated with previous third-party treatment and neutralization processes, while also generating significant economic value for the company. Since this process is carried out using physical methods, no chemical substances need to be added, and no other substances are produced.