| Mid-heavy rare earths elements are widely applied in new generation information technology,national defense and military industries owing to their excellent physical and chemical properties of magnetic,light,and electricity.The reserves of mid-heavy rare earth resources in China account for about 80%of the world’s available mining.The rare earth elements are mainly adsorbed on clay minerals in ion-exchangeable state.Currently,the primary industrial mining process for the ion-adsorption rare earth ore is in-situ leaching with ammonium sulfate,which has resulted in issues of ammonia nitrogen pollution,high consumption of leaching agent and low concentration of rare earths in leach solution.In our previous researches,a series of leaching technologies for the ion-adsorption rare earth ore using magnesium salt and its composite systems have been developed to eliminate the ammonia nitrogen pollution from the source.However,in the industrial practice ofin-situ leaching process using magnesium salt,the leaching results are different when leaching with various particle size or weathering extent of ion-adsorption rare earth ore by uniform leaching parameters.The consumption of leaching agent is high,and the concentration of rare earth in leach solution is low.Thus,in order to enhance the leaching of rare earths,reduce the leaching of impurity and shorten the leaching period,it is necessary to regulate the leaching parameters for leaching of various particle size of ion-adsorption rare earth ore.However,few studies have been reported on the selective enhanced leaching of rare earths in various particle size of ion-adsorption rare earth ore.In this work,column leaching using magnesium sulfate was employed for the leaching of the ion-adsorption rare earth ore with various particle sizes.The effects of MgSO4 concentration and pH of the leaching agent solution on the leaching behaviors of rare earth and impurity of aluminum were studied.And then,the occurrence transformation and leaching mechanism of aluminum were investigated through sequential extraction method during the magnesium sulfate leaching process.Finally,a regulating strategy was proposed for enhance leaching of rare earths and inhibit leaching of impurities in various particle size of ion-adsorption rare earth ore by magnesium salt.The main results are as follows:1.The effects of MgSO4 concentration and pH of the leaching agent solution on the leaching behavior of rare earths were studied in leaching with various particle sizes of rare earth ores.When the pH of the leaching agent solution was 2.0,increasing the MgSO4 concentration from 0.5 wt.%to 4.0 wt.%resulted in a gradual increase in the peak of rare earths concentration,while also causing a gradual decrease in the liquid-to-ore ratio required for maximum rare earth leaching rate.When the MgSO4 concentration was 4.0 wt.%,and the pH of the leaching agent solution increased from 1.5 to 5.0,the peak of rare earths concentration in the leach solution gradually decreased,however,only had a minor effect on the leaching rate of rare earths,which remained at around 100%.As the particle size of the rare earth ore decreases,the peak of rare earths concentration in the leach solution increases from 2.3 g/L to 21.5 g/L,and the optimal concentration of magnesium sulfate for achieving maximum leaching rate of rare earths increases from 0.5 wt.%to 3.0 wt.%.2.The effects of MgSO4 concentration and pH of the leaching agent solution on the leaching behavior of impurity aluminum were investigated during leaching with various particle sizes of rare earth ores.When the pH of the leaching agent solution was 2.0,and increasing the MgSO4 concentration from 0.5 wt.%to 4.0 wt.%resulted in varying concentration of aluminum in the leach solution depending on the particle size of rare earth ore,while finer particles led to an more higher leaching rate of impurity aluminum.When the pH of the leaching agent solution exceeded 3.0,the leaching rates of aluminum in the rare earth ores remained below 20%.Conversely,when the pH of the leaching agent solution falls below 3.0,the leaching rates of aluminum in the rare earth ores increase significantly(>86%).As the particle size of the rare earth ore gradually decreases,the peak of aluminum concentration in the leach solution decreased from 1.0 g/L to 0.6 g/L,leading to a gradual decline in the leaching rate of impurity aluminum.3.The occurrence transformation and leaching mechanism of impurity aluminum in magnesium sulfate leaching process were studied.When the pH of the leaching agent solution was high(～5.0),the main transitions were ion-exchangeable aluminum to watersoluble aluminum and water-soluble aluminum to hydroxyl-adsorbed aluminum,and increasing the MgSO4 concentration could accelerate the occurrence transformation and migration of aluminum.Meanwhile,the aluminum in the leach solution is mainly leached from the ion-exchangeable state.When the pH of the leaching agent solution was low(～2.0),the main transitions were hydroxyl-adsorbed aluminum to water-soluble aluminum and water-soluble aluminum to ion-exchangeable aluminum,and increasing the MgSO4 concentration could inhibit the back adsorption of water-soluble aluminum.The aluminum in the leach solution is mainly leached from the hydroxyl-adsorbed state.4.A regulating strategy for the selective enhanced leaching of rare earths from the rare earth ore was proposed.To achieve selective enhanced leaching of rare earths from coarsegrained rare earth ore,the MgSO4 concentration should be controlled within 1.0 wt.%to 2.0 wt.%and the pH of the leaching agent solution should be maintained at 3.0-4.0.Moreover,the optimum parameters for the leaching of medium-grained and fine-grained rare earth ore were MgSO4 concentration of 2.0-3.0 wt.%,and pH of the leaching agent solution of 3.04.0.In the actual magnesium sulfate leaching process,during the injection stage of highconcentration leaching agent solution,the pH of the leaching agent solution should be appropriately increased to minimize the leaching of hydroxyl-adsorbed aluminum in the rare earth ore,but during the injection stage of low-concentration leaching agent solution,the pH of the leaching agent solution should be appropriately lowered to accelerate the desorption of ion-exchangeable rare earths,shorten the leaching period.Thus,the purpose of reducing the leaching of impurity aluminum and enhancing the leaching of rare earths could be finally achieved. |
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