Study On Removing Chromium From Waste Residue By Hydrothermal Leaching Method

Chromium salt is one of the important inorganic compounds.It is a type of chemical raw material that is developed in my country.It is widely used in many fields such as electroplating,advanced alloy materials,printing and dyeing,leather,catalysis,spices,ceramics,anticorrosion,medicine,etc.,involving the national economy 10%of the product variety.my country’s annual demand for it is more than 200,000 tons.At present,more than 30 companies,large and small in China,directly produce dichromate,with an annual production capacity of more than 300,000 tons,and the total output ranks first in the world.But at the same time,every ton of chromium salt produced will produce 2.5-3 tons of chromium slag,and the chromium slag produced is one of the most important sources of heavy metal industrial pollution.In the past 50 years,about 22,000 tons of chromium slag was discharged into the environment,most of which entered the soil.In the early stage of pollution,it is often difficult for people to notice the accumulation of heavy metals such as Cr in the soil.However,once the soil is contaminated by heavy metals,It is difficult to completely eliminate.Heavy metals are generally not easy to leaches with water,and they are also difficult to be decomposed by soil microorganisms.They are mainly adsorbed in soil colloids,accumulate,and then migrate through the transfer of plants themselves.Eventually,it is transformed into more toxic substances through the food chain or other means,which is a serious harm to human health.The current methods of processing chromium include electrolytic reduction,chelation extraction and plant absorption,but all have their advantages and disadvantages.This article attempts to improve the leaching efficiency of chromium in the soil through the combination of acidification and heat treatment.So as to achieve the purpose of reducing pollution and protecting the environment.The main research results are as follows:(1)In the single-factor experiment,the chromium ions in the waste slag were leached in large amounts with the increase of time,temperature and acid concentration,but it had no relationship with the amount of acid used,that is,the liquid-solid ratio.It can be seen from the experimental results that temperature has a greater influence on the leaching efficiency of chromium ions,and the liquid-solid ratio has the least influence on the leaching efficiency of chromium ions.(2)The orthogonal test of hydrothermal leaching is carried out on four factors of time,temperature,acid concentration and liquid-solid ratio.The leaching efficiency of chromium ions is observed through the combination of factor levels,and the influence of each factor on the test is different.Analyze whether there is an interaction between the factors.It is concluded that the effects of time,temperature,and concentration are highly significant,and the interaction between liquid-solid ratio and various factors is not significant.The optimal leaching conditions are 12 h,180℃,10 mol/L and a liquid-tosolid ratio of 12.5:1.Under these conditions,the leaching rate of chromium ions is 85.1%.(3)A four-factor compared experiment was arranged to study the influence of the four factors on chromium leaching efficiency under normal pressure conditions.The results obtained are as follows:temperature is highly significant,time has a certain effect,and the concentration and liquid-solid ratio are not significant.The optimal leaching conditions are 12 h,90℃,7 mol/L and a liquid-to-solid ratio of 5:1.Under these conditions,the leaching rate of chromium ions is 18.4%.(4)Under the optimal conditions,the waste residue after hydrothermal leaching was leached twice.The total secondary leaching rate of chromium ions is 96.5%.Compared with the one-time leaching rate of 85.1%,an increase of 11.4%.(5)Purify and recycle the waste liquid after hydrothermal leaching.The process of waste liquid treatment and recovery is given to realize the recovery and reuse of chromium slag.