Low-Temperature Acid Leaching Of Phosphorus-Potassium Associated Ore: Kinetics And Process Intensification

The growth of agricultural crops needs potash fertilizer.At present,the global production of potash fertilizer is primarily based on soluble potassium resources.Unfortunately,China is poor in soluble potassium resources but rich in insoluble potash resources.Recently,the phosphorus-potassium associated ore,which contains 6-12%P₂O₅ and 5-10%K₂O,was found with a reserve of 800 million tons in Yichang city,China.Considering the natural symbiotic association of phosphorus and potassium in the ore,the simultaneous extraction of phosphorus and potassium from the ore to produce PK compound fertilizer,which is urgently needed in China,will alleviate the shortage of PK compound fertilizer.The efficient extraction of soluble potassium from the ore is the key because potassium in the ore mainly exists in K-feldspar?KAl Si₃O₈?,which has a very stable Al-Si-O tetrahedron structure and cannot be decomposed by acid or alkali at normal temperature and pressure.In this work,low-temperature acid leaching process was developed to leach K from the phosphorus-potassium associated ore and the leaching kinetics and process intensification strategies were investigated,and the following main conclusions were obtained.?1?The effect of fluoride on the acid leaching process of phosphorus-potassium associated ore was studied.The results showed that the leaching promotion effect was in the following order:Ca F₂>NH₄F>Na F>KF.After 6 h of leaching at 90?,the dissolution fractions of potassium could reach 89.5%,82.2%,86.8%and 78.1%,respectively,when Ca F₂,NH₄F,Na F and KF were added as additive.Considering the leaching promotion effect and cost,Ca F₂ was the most suitable additive.?2?The leaching behaviors of potassium,aluminum and phosphorus from phosphorus-potassium associated ore in HCl-Ca F₂ media were studied.Effects of HCl concentration,Ca F₂ addition and temperature on the dissolution fractions of potassium,aluminum and phosphorus were further investigated.It was found that compared with aluminum,potassium was preferentially leached out though they were incorporated in the same mineralogical phase of potash feldspar in the ore.Moreover,the dissolution fraction of potassium was always higher than that of aluminum in all the investigated conditions due to the existence of amorphous alumina-silicate secondary precipitation and/or the formation of insoluble Al F complex in the leaching process.?3?To intensify the leaching process,a series of additives affecting the dissolution fraction of potassium were investigated.The experiment results showed that traditional phosphoric acid extraction agents such as monohydric alcohols and organophosphates would hinder the leaching of potassium,while surfactant,small-molecular-weight compounds containing hydroxyl groups,and hydroxyl-containing polymers could enhance the dissolution fraction of potassium.Moreover,the dissolution fraction of potassium could increase with the increasing of alcohol chain length,and lauryl alcohol had the best effect.The dissolution fraction of potassium increased from 89.50%to 97.60%under the condition of temperature 90?,hydrochloric acid concentration 22.5%?m/m?,hydrochloric acid volume 80 m L,liquid/solid ratio 4:1,mass ratio of ore to Ca F₂ 5:1,reaction time 6 h and the addition of lauryl alcohol 1 m L.In addition,the enhanced leaching observed in the presence of lauryl alcohol was analyzed by IR,SEM,XRD and XRF,and could be attributed to the hydrophobicity mechanism.The kinetic analysis showed that there were two distinct stages in the leaching process and the kinetics of both stages followed the classical shrinking core model with chemical reaction being the rate-controlling step.?4?The concentrations of P and K in the leaching solution are relatively low,and the acid consumption is relatively large,which is due to the low content of P and K in the ore and the relatively large liquid-to-solid ratio in the low-temperature acid leaching process.In addition,Ca in fluorapatite and dolomite can react with the generated phosphoric acid to generate Ca-containing phosphate sediments and hinder the leaching of potassium.Considering the above-mentioned issues,a sequential leaching process with leachate recycling was developed,in which P and K were sequentially leached out from P-K associated ore and the obtained P-and K-containing leachates were recycled into the next leaching process.The sequential leaching process with two times leachate circulation obtained a potassium dissolution fraction of 81.6%and a phosphorus dissolution fraction of95.6%,with were notably higher than the potassium dissolution fraction of71.6%and the phosphorus dissolution fraction of 74.6%obtained on simultaneous leaching process with two times leachate circulation.Moreover,the pilot-scale leaching of potassium from the phosphorus-potassium associated ore by using the sequential leaching process was studied,the effects of heating form and stirring type on potassium leaching efficiency were further investigated,and some suggestions were proposed.This work can provide basic date and theoretical support for the leaching of soluble potassium from insoluble potassium ores.