Research On Chromium Resourcilization Technique Of Mixed Electroplating Sludge

On the basis of the review on the electroplating sludge treatment, the recovery of Cr aswell as the preparation of Cr(OH)3, the recovery of Cr from the phosphate precipitates isstudied in this dissertation, which is produced from sulphuric acid leaching solutions of mixedelectroplating sludge by selective phosphate precipitation. The pressure alkali-decompositiontechnique is proposed for the production of Cr(OH)3from the phosphate precipitates. Thisresearch provides a new high-efficiency approach for the comprehensive recovery of Cr frommixed electroplating sludge. The pressure alkali-decomposition technique is an essentialsupplement for the selective phosphate precipitation method. These two combined techniquesare meaningful to the harmlessness and resourcilization of Cr from mixed electroplatingsludge. The main research contents are as follows:The feasibility of P/Cr separation in alkali system is tested firstly on the basis of thethermodynamic analysis and exploring experiments. The results show that P can be separatedfrom Cr effectively and Cr(OH)3can be produced from phosphate precipitates by pressurealkali-decomposition.Based on the L16(45) orthogonal array, the effects of initial alkali concentration([NaOH]ini.), reaction temperature, agitation speed and liquid to solid ratio are discussed onthe extraction of P and Cr in pressure alkali-decomposition. The range analysis shows that[NaOH]ini., reaction temperature, agitation speed and liquid to solid ratio are in the decreasingorder in influencing P/Cr separation. Furthermore, the variance analysis shows that [NaOH]ini.has a significant influence on the extraction of Cr and a very significant influence on theextraction of P, while other factor, such as reaction temperature, agitation speed and liquid tosolid ratio have little influence on the extraction of Cr or P. F test results are the same to thatof P test.The pressure alkali-decomposition process is further studied. The initial concentration ofNaOH is increased to a proper value for effective removing of P. The optimum technique ofpressure alkali-decomposition of phosphate precipitates is determined as: T=90℃,[NaOH]ini.=150g·L-1, L/S=5(mL/g), rotation time of1.0h and agitation speed of600r·min-1.Under the conditions stated above, the extraction of P is achieved as higher as97%, by whichP is successfully separated from Cr. The leaching solutions can be reused after the appropriatesupplement of alkali. The extraction of P can be maintained above90%and P/Cr separation is still effective even after10recycles. The dissolving loss of Cr doesn’t exist and thus therecovery of Cr is ensured because of the recycling of the leaching solutions.The pressure alkali-decomposition product is analyzed by mineralogical methods, suchas scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-rayanalysis (EDX) and microscope. The results show that amorphous Cr(OH)3is produced fromphosphate precipitates by pressure alkali-decomposition. Cr content in Cr(OH)3products ashigh as32%, while the impurities, such as Ni, Zn and Fe as low as0.33%,0.21%and0.26%respectively. The quality of Cr(OH)3satisfies the product standard of basic chrome sulphate.