Research On Clean Metallurgical Production Technology For Recovery Of Cadmium From Copper Cadmium Slag

The recovery and utilization of solid waste is a major subject of today's environmental protection.A mass of copper-cadmium slag is produced in hydrometallurgy smelter plants in China every year,in which has rich-containing copper,cadmium and zinc etc elements,among them cadmium is third toxic metal element after mercury,lead which caused to pollute environment and threaten to the human health.If copper-cadmium slag does not treated effectively,lots of value metal resources will not only be wasted,but also led environmental pollution severe.Therefore,how to efficiently and cleanly treat copper-cadmium slag,recover the valuable metals,and at the same time cut down environmental pollution,which has became the technical challenge that the lead-zinc smelting enterprises in China faced with.Aimming at the technology present situation of copper-cadmium slag recovery during the process of zinc smelting in China and basing on analysis the existing technology at home and abroad,through technology integration and self-dependent innovation,the selective extraction technology of Cu-Cd slag was developed,and cadmium could be leached efficiently,the leaching rate of Cd and Cu were 99.56%and 37%,which really achieve effective separation of copper and cadmium in Cu-Cd slag;the refining cadmium production process of "selective leaching for Cu-Cd slag-zinc powder replacement-sponge cadmium melting-zinc powder replacement again-vacuum continuous distillation" was developed,and the traditional cadmium electro-deposition or intermittent distillation process were replaced by vacuum continuous distillation that solved the technical problems in vacuum continuous distillation production.So as to realize industrial production with total Cd recovery of 96.39%and refining-cadmium purity more than 99.99%In this paper,the analysis on basic theory of the mechanism for selective separation of copper-cadmium in cadmium copper slag,crude cadmium continuous vacuum distillation was carried out from two aspects of thermodynamics and kinetics.The main research content and results as follows:(1)Through the multi-element assay,XRD,microstructure and energy spectrum analysis on raw materials of cadmium-copper slag leaching test were carried out,the conclusions were made as follows:a)major valuable elements in Cu-Cd slag are Zn,Cd and Cu,the composition is Zn about 22%,Cd about 15%and Cu about 10%respectively;b)98%copper in Cu-Cd slag is existing at metal monoplasmatic form,and the main existing form of zinc is zinc sulfate,and most of cadmium is existing at metal cadmium form and associated with a few of cadmium sulfate.(2)Research on process condition for selectively leaching copper-cadmium slag by using the sulfuric acid,making sponge cadmium by zinc powder secondary displacement was conducted that found out the best leaching conditions for cadmium-copper slag:the liquid solid ratio = 6:1,the period of time 4 h,the temperature is 80?85 ?,initial acid concentration 10?15 g/L,leaching terminal pH value was 5.2;best conditions for the first replacement cadmium by zinc powder:temperature is 50?55 ?,time 1h,zinc powder dosage 80%that got sponge cadmium contained 78.42%cadmium and 3.14%zinc;the best process conditions for second replacement cadmium by zinc powder:temperature 50?55?,time 0.5?1 h,feeding time of zinc 10 min that can get sponge cadmium contained 85.12%copper,2.17%zinc and 0.26%cadmium.Cadmium content in sponge cadmium can raise above 85%,zinc content dropping to below 5%so as to solve the technical problem of high containing-Zn in crude cadmium during the process of vacuum continuous distillation.(3)Through testing research,the process conditions for high-grade sponge cadmium alkaline refining and making coarse cadmium were optimized that obtained more optimum:technological parameters:smelting temperature 450?500?,time 2?3h,the best proportion of sponge cadmium and alkali is 1:0.2.The testing results show that this technical process has better effects of removing impurities with removal rate of Zn,Pb,As and Cu of 99.7%,48.9%,97.5%and 45.1%respective.The cadmium content in coarse cadmium product is more than 99%with zinc content of about 0.004%so as to achieve requirements for production of refining cadmium by continuous vacuum distillation.(4)Testing research on the pure cadmium preparation process by using continuous vacuum distillation of crude cadmium were conducted that obtained the optimum technological conditions:distillation temperature 420?,vacuum degree 10?20 Pa that 99.995%containing-cadmium in refining cadmium product can be reached and its quality can meet the national standard.Due to under the condition of vacuum,smelting temperature only is 420?,compared with the atmospheric distillation process of current domestic advanced,the smelting temperature decreased by more than 400?(atmospheric distillation smelting temperature is 850-950?)so that refining energy consumption reduced by 50%,caustic sludge decreased 40%,and the direct recovery of cadmium can reach 98.5%.Through above mentioned research work,some technical problems of low leaching rate of Cu-Cd slag,high impurity sponge cadmium and high zinc-containing in crude cadmium that cannot meet requirements for continuous vacuum distillation process,were preferably worked out so as to realize continuous production of pure cadmium vacuum distillation.During the process of this project implementation,the "crude cadmium automatic feeding device"(authorized patent no.:ZL 2011 2 8528.8 021)and "the device for crude cadmium and caustic sludge removal from the bottom(2011 authorized patent no.:ZL 2011 2 8470.7)as well as adopting full sealed crude cadmium furnace fume hood were also successfully developed that optimized the working environment,reduced the cadmium pollution to environment and improved the process automation;In addition,by vacuum distillation and sinter board dust catch technology,the off-gas emission reaching the standard was achieved that really reached cleaner production.On this basis,refining cadmium demonstration project construction with 800t/a capacity has been completed and commercial production has been achieved successfully that got better economic and environmental benefits and finally verified the technical economy and feasibility of this technology.