Research On Harmless Treatment And Utilization Technology Of Chromium-containing Leather Offcuts

The main components of chromium-containing leather offcuts are collagen and a small amount of trivalent chromium introduced by chromium tanning agents.In this paper,the leather offcuts are hydrolyzed into small soluble molecules by chemical methods,and the chromium in them is separated from the hydrolyzed protein solution by precipitation.The separated hydrolyzed protein is modified to prepare gypsum retarder and the hydrolyzed filtrate is further treated to prepare alkaline chromium sulfate chrome tanning agent,which is recycled to the leather production chain.The subject can be harmless treatment and comprehensive utilization of chromium-containing leather offcuts,turning waste into treasure and resource utilization and reducing environmental pollution.This project firstly adopts Ca O and Na OH compound alkali solution to carry out alkaline hydrolysis of leather offcuts,after the completion of hydrolysis,the hydrolysis solution is filtered,the filtrate is adjusted p H and dried,and the filter residue is washed.By investigating the experimental conditions of Ca O to Na OH ratio,hydrolysis temperature,hydrolysis time,alkali dosage and solid-liquid ratio,the best hydrolysis process conditions were determined as follows:Ca O to Na OH ratio of 10:5,compound alkali dosage of 18%,hydrolysis temperature of 100℃,hydrolysis time of 2.5 h,and solid-liquid ratio of 1:4.5during hydrolysis,and the chromium-containing leather offcuts were treated under these conditions.The yield of hydrolysis filtrate was 77%,free amino acid content was 15.7×10^-3 mol/g and chromium content was 26.57 ppm.The results showed that the chromium content of hydrolysis solution was low,the yield of hydrolysis solution was high,and almost all chromium went into the filtrate,and the effect of chromium removal was good.Next,the effect of hydrolyzed protein and its modification by different carboxylic acids on the setting time of gypsum was investigated.It was found that retarder M,prepared under the conditions of modification temperature of 5℃,modification time of 6 h,modifier maleic anhydride ratio of 15%and solid-liquid ratio of 1:260,could make the initial setting time of gypsum reach 65 min（dosing 0.035%）,flexural strength of 8.8 MPa and compressive strength of The results show that the prepared gypsum retarder M has low cost,good retarding effect and little effect on the strength of gypsum.The amino acid type and content of the prepared gypsum retarder M were analyzed,and its effect on the hydration temperature and calcium ion concentration during gypsum coagulation was analyzed,and the mechanism of its retarding effect was studied by means of scanning electron microscopy and XRD.The results show that retarder M contains a large number of amino acids,and these exposed amino and carboxyl groups play a chelating effect on calcium ions,which can limit their saturation,and then delay the exothermic time of hydration of gypsum and prolong the setting time of gypsum,but have a certain effect on the shape of gypsum crystals,resulting in the loss of gypsum mechanical properties.The chromium tanning agent-alkaline chromium sulfate can be prepared by reacting the hydrolyzed filter residue with sulfuric acid after water washing.Through the investigation of sulfuric acid concentration,reaction temperature,reaction time,solid-liquid ratio and other factors,the optimal process conditions were determined as follows:sulfuric acid concentration of 3.7%,reaction temperature of 50℃,reaction time of 20 min and solid-liquid ratio of 1:8.The chromium content in the alkali chromium sulfate prepared under the conditions of 25.43%and alkalinity of 23.80%met the industrial product standard,while the recovery of chromium could reach 70.4%.