Study On The Green And Efficient Leaching Technology For Ion-adsorption Type Rare Earths Ore With Magnesium Salt System

The ion-adsorption type rare earths ore is a new kind of exogenous rare earth minerals, which was first discovered in 1969 in Ganzhou city, Jiangxi province, China. There are many advantages of the ores, such as complete in rare earth partition, low radioactivity, and rich in the middle and heavy rare earth elements. It is valuable strategic mineral resources of China which located in Jiangxi, Guangdong, Guangxi and other provinces in South China. It is the main resources of middle and heavy rare earth in the world. Its development and utilization could solve the problem that monazite, bastnaesite and mixed rare earth mine almost only produce light rare earth products and lack of middle and heavy rare earth products. Nowadays, rare earth is recovered from the ion-adsorption type rare earths ore by in-situ leaching process with ammonium salt in the industry. In other words, rare earth is leached with (NH4)2SO4 and then removal of impurities from the leaching solution with ammonium bicarbonate, and enriched by precipitating with ammonium bicarbonate, and finally the ionic rare earth concentrate is obtained after roasting. Although the process has been widely used in industrial production, there are still some probllems as follows:(1) The consumption of ammonium sulfate leaching agent is high, leading to serious ammonia-nitrogen pollution. For the granite ion-adsorption type rare earths ore,7-9 tons ammonium sulfate would be consumed for every tons of rare earth concentrate. However, for the volcanic ion-adsorption type rare earths ore which widely deposits in guangxi region, above 12 tons ammonium sulfate would be consumed for every tons of rare earth concentrate because of its poor permeability. According to statistics, about 60000 tons of ionic rare earth concentrates are manufactured every year in china, Thus it can be seen that there would be a lot of ammonia-nitrogen going into the soil and groundwater in the leaching process of ion adsorption type rare earths ore, causing serious pollution to the environment. Meanwhile, leaching tail solution containing a large amount of ammonia-nitrogen, could be difficult to handle. (2) Calcium, magnesium and other nutrients in the soil would be exchanged into leaching solution and lost in the leaching process, resulting in imbalance for soil nutrient ratio, which would lead to growth difficulty for plant and have an adverse effect on rehabilitation of leaching tailings. (3) Leaching selectivity of ammonium sulfate is a little poor, leading to high aluminum content in leaching solution. Furthermore, ammonium sulfate could only leach most of rare earth in ion-exchangeable phase, but not the colloidal phase and mineral phases. Therefore, the volcanic ion-adsorption type rare earths ore from Liutang Rare Earth Mine area located in the Chongzuo City was employed as the research object in the present work. Based on the adsorption and migration mechanism research of rare earth by clay minerals and the leaching process research of ion-adsorption type rare earths ore, the effect of compound leaching agent with magnesium/calcium/ammonium salt on rare earth leaching and environmental influence was innovatively systematically studied. And then the green and efficient leaching technology for the ion-adsorption type rare earths ore with magnesium salt system was developed, and the efficient ecological friendly leaching of the ion-adsorption type rare earths ore was expected to achieve with this technology. The main research contents and results were as follows:1. Through the research on the adsorption and migration of rare earth ions by clay minerals, it was shown that the adsorption of lanthanum, neodymium, yttrium and magnesium by kaoline was in line with the Langmuir monolayer adsorption model. The Gibbs free energy of adsorption was -32.69 kJ/mol?-32.05 kJ/mol?-31.44 kJ/mol?-28.97 kJ/mol, respectively, which showed that there were minor differences in the adsorption of rare earth and non-rare earth element by clay minerals, thereby explained the metallogenic characteristics of the ion-adsorption type rare earths ore and the formation of fractionation effect. Furthermore, experiments revealed that adsorption of rare earth by clay minerals was divided into specific adsorption and nonspecific adsorption. The rare earth ions which were absorbed by nonspecific adsorption accounts for over 93% of the total rare earth adsorption, and it could be desorbed by neutral or weak acid salt solution. The rare earth ions which were absorbed by specific adsorption accounts for about 93% of the total rare earth adsorption, this section could only use the reinforcement measures, such as improving solution acidity for desorption.2. The leaching process research of ion-adsorption type rare earths ore was studied in this paper, a leaching electric double layer model which was suitable for the ion-adsorption type rare earths ore was established based on Goup-Chapman theroy, and the potential where the distance to be r from the center of clay molecules was obtained, shown as ?(r)=zje/?e-Ka(Ka+1)e-Kr/r=zjeeK(a-r)/?(Ka+1)r..It revcaled the ability to leaching ions-exchange phase rare earth by different inorganic salt solution.The higher the concentration of leaching agent was,the smaller the cationic hydrated radius was,and the higher the cationic valence state was,the greater the rare earth leaching ability was.3.The cation with reduction properties added in the leaching agent solution could generate a redox reaction with part of the high valence rare earth elements in colloidal phase (such as tetravalent cerium),forming soluble low valence rare earth ions(such as trivalent cerium ions)into leaching solution.Therefore,the rare earth efficiency was improved,and the cerium partition in the leaching solution was increased.Moreover,the greater the acidity of leaching agent was,the higher the reducing ion content was,the higher the rare earth efficiency was and the bigger the cerium partition was in the leaching solution.4.Kinetics study on leaching rare earth from the ion-adsorption type rare earths ore with magnesium sulfate was investigated in order to reveal the rare earths leaching mechanism. Leaching kinetics equation with inner diffusion controls was acquired under the condition of agitation leaching,which showed to be 1-2/3?-(1-?)2/3=0.011r0-1.217e9.48*103/8.314T. Furthermore,the column leaching was introduced to simulate the actual in-situ leaching. Through tthe study on the effect of technological factors and mineral properties,it was shown that the rare earth leaching efficiency was above 93%in 298 K,a flow rate of 0.60 mL/min, 0.20 mol/L magnesium sulfate solution,pH around 5.70 and a particle size of 1.0mm,and the aluminum leaching effciency was onlly 50%on this condition.The rare earth efficiency would be almost the same,and the leaching efficiency of aluminum could be reduced by 13? using MgS04 as leaching agent instead of(NH4)2S04.There was great significance in the purification and enrichment of rare earth in leaching solution.5.The differences in leaching characteristics and mass transfer between rare earth and non-rare earth impurities were investigated in the present study,it showed that there was a small separation coefficient between rare earth and non-rare earth impurities.The leaching kinetic of Ca was controlled by inner diffusion process,and its kinetic equation was 1-2/3?-(1-?)2/3=0.016r0-1.094e-8.314Tt.The leaching rate of calcium was bigger than rare earth because the absorption ability of clay to bivalent cations was lower than to trivalent cations.What is more,leaching reaction and hydrolysis reaction of aluminum existed in the system when the pH of MgS04 solution was around 5.7.The higher the temperature was,the bigger the particle size of ore samples was, the more significant influence was on hydrolysis reaction, so as to effectively reduce the aluminum ion concentration in the leaching solution. As a result, the aluminum leaching efficiency was lower than the rare earth leaching efficiency and the calcium leaching efficiency. Moreover, in the column leaching process with MgSO4, there was a peak concentration in every curve, and the order of the time-to-peak was Mg<Ca<RE<Al. Hence, it could be seen that there were a small separation coefficient between rare earth and non-rare earth impurities in leaching process.6. Magnesium salt compound leaching of the ion-adsorption type rare earths ore could make the ratio of the ion-exchangeable calcium and ion-exchangeable magnesium in the tailings meet the requirements of soil and plants, and may realize the ecological friendly leaching of the ion-adsorption type rare earths ore. When the ratio of magnesium sulfate, ammonium chloride and calcium chloride was 15:25:60 in leaching agent, the content of ion-exchangeable magnesium and ion-exchangeable calcium in the water washing tailings was 628.57 mg/kg and 77.77 mg/kg, respectively. The ratio of ion-exchangeable calcium and ion-exchangeable magnesium was 8.08, which was in accord with the requirements of the soil nutrient ratio. Moreover, the rare earth efficiency could be above 94% and the aluminum leaching efficiency was only 49%, which the leaching rate of aluminum could be reduced and rare earth efficiency would be almost the same by using magnesium-ammonium-calcium compound leaching agent instead of (NH4)SO4. Besides, planting experiments with soybeans on water washing tailings were taken. It could be seen that the average plant height of soybean which were planted on water washing tailings with magnesium-ammonium-calcium compound leaching process was the highest, up to 25.5 cm. The growth of soybeans on water washing tailings with ammonium sulfate leaching process or magnesium sulfate leaching process was limited because of the lack of nutrients, and the average plant height of the soybeans was only 16.1cm?21.5cm. Otherwise, the root and stem leaves of soybeans on water washing tailings with ammonium sulfate leaching process went yellow because of the lack of magnesium. The growing points of soybeans on water washing tailings with magnesium sulfate leaching process were necrosis because of the lack of calcium, and the lack of calcium may be adverse to the formation of the fruit.