For pretreatment in stainless steel strip pickling lines an electrolytic pickling process using appr. 20 % sodium sulfate solution is often used. The electrolyte itself is not spent during this process. Metals like iron(III), nickel(II) and chromium(III) are precipitated in the neutral solution as metal hydroxides. The chromium(VI) compounds formed by oxidation have not directly be separated as solids but removed in order to achieve a longer service life of the electrolyte. This is performed by the Steuler Electrolyt Purification (SEP) process through reduction of chromium(VI).
The neolith reduction system is based on the reduction of chromium (VI) using reducing agents like hydrogen peroxide or sodium bisulphite at low pH value. In the first step sulphuric acid is metered pH controlled for the reduction of the pH value. At the same time the reducing agent is metered rH controlled in order to reduce chromium (VI) to chromium (III). In the second step the liquid is neutralized with sodium hydroxide and therefore the dissolved metals are precipitated as metal hydroxides. Following the formed particles are flocculated, settled and dewatered in a chamber filter press. The added sodium hydroxide and sulphuric acid form new neolith solution to replace the neolith loss in the filter cake water and drag out of the neolith solution on the strip surface. |
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