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When treating various steel grades and forms with chemical pickling in the standard pickling acids / acid mixtures, the coating of metal oxides (mill or roll scale) resulting from annealing or milling is removed and charged or enriched in the form of solids or dissolved metals in the pickling acids. For an optimum pickling plant operation, with regard to quality and throughput, maximum concentration ranges should be maintained. With increased solid and metal concentrations, the achievable plant throughput sinks, due to decreasing pickling efficiency. Slurry and precipitation lead to additional plant standstill and reduces the quality of the steel surfaces generated. In order to prevent this effect, Steuler Anlagenbau has utilised the following processes in the surface treatment plants, partly also modular and extendable (retro-fitable) :
Solids separation utilising micro-filtration The solids, homogenized through continuous pickling bath blending, are led to the micro-filtration plant with the pickling solution in a partial stream. The core of this separation phase is one or more cross-flow microfiltration modules, overflown at high speed, in which the pickling solution is pressed through the membrane by operating pressure. The solids are retained completely on the surface of the membrane, strongly enriched in the circulation and centrifuged through a small partial stream. The combination of hydraulic design, periodical back rinse and chemical treatment, when necessary, can achieve a very long service life of the utilized acid resistant micro-filtration membranes. The Steuler Acid Microfiltration (SAM) is mainly used for the filtration of mixed acids (HF / HNO3), HF / H2SO4 and HCl in approx. 20 reference plants for approx. 20 years. Acid recovery Based on the experience with over 40 reference plants for more than 25 years, the following processes are selected for separating dissolved metals, depending on acid composition, plant capacity and customer-tailored requirements: Steuler Acid Cooling Crystallization (SACC) In this process, the separation of metals from sulphuric acid pickling solutions takes place through a multi-stage cooling of the solution. Through this, the solubility of the ferrous (II) sulphate is reduced and this crystallizes as FeSO4x7H2O. The metal depleted sulphuric acid is directly returned to the pickling process. Particular benefits are achieved when the ferrous sulphate yielded is utilised as raw material or precipitant. This process is only applicable to sulphuric acids. Steuler Acid Purification Unit (SAPU) The principle of this process is the separation and retention of the free acids of metals in a resin bed (retardation). The metal salt containing, acid depleted solution is disposed of in the waste water treatment plant. The acid absorbed in the resin bed is re-eluted with water in a second stage and returned to the pickling. Basically, a complete solids removal for example through Steuler Acid Microfiltration SAM, as well as a cooling in the supply is necessary, depending on temperature, acid type and concentration. The retardation process can be utilised for practically all pickling solutions from HCl, H2SO4 und HF/HNO3. Due to the low investment costs, this process has a quick payback, alone through the acid saving for uses with mixed acids (HF / HNO3), even by small throughput capacities. Steuler Total Acid Regeneration (STAR) With the above mentioned process, the free acids are each returned to the pickling process, the metal salts (bonding of dissolved metal cations with the corresponding acid anions) are further precipitated as by-product or waste water stream. In the STAR process, the entire acids for the pickling process are recovered and the metals are used as metal oxides. This is based on a process combination of membrane separation process for separating free acids in the first stage and upgrading the metal salts to a concentration at which these crystallizes due to solubility. The resulting crystal slurry is dewatered and oxidized to metal oxides in a thermal roasting process. With this well proved plant technology, the metal salts achieve first-time recovery rates for HFtotal and HNO3 total of distinctly more than 90%. This process can be utilized for processes with mixed acids (HF/HNO3) and capacities of more than 100 kg/h metal removal. In summary, for all the mentioned regeneration processes from Steuler, the following benefits are achieved:
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Processes:
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