Extraction of potassium from K-feldspar via the CaCl2 calcination route☆

Bo Yuan ,Chun Li ,,Bin Liang ,,Li Lü ,*,Hairong Yue ,Haoyi Sheng ,Longpo Ye ,Heping Xie

1 Multi-phases Mass Transfer and Reaction Engineering Laboratory,College of Chemical Engineering,Sichuan University,Chengdu 610065,China

2 Center of CCUS and CO2 Mineralization and Utilization,Chengdu 610065,China

Keywords:K-feldspar Calcium chloride Potassium extraction Calcination

A B S T R A C T The extraction of potassium from K-feldspar via a calcium chloride calcination route was studied with a focus on the effects of the calcination atmosphere,calcination temperature and time,mass ratio of CaCl2 to K-feldspar ore and particle size of the K-feldspar ore.The results demonstrated that a competing high-temperature hydrolysis reaction of calcium chloride with moisture in a damp atmosphere occurred concurrently with the conversion reaction of K-feldspar with CaCl2,thus reducing the amount of potassium extracted.The conversion reaction started at approximately 600°C and accelerated with increasing temperature.When the temperature rose above 900°C,the extraction of potassium gradually decreased due to the volatilization of the product,KCl.As much as approximately 41%of the potassium was volatilized in 40 min at 1100°C.The mass ratio of CaCl2/K-feldspar ore significantly affected the extraction.At a mass ratio of 1.15 and 900°C,the potassium extraction reached 91%in 40 min,while the extraction was reduced to only 22%at the theoretical mass ratio of 0.2.Optimal process conditions are as follows:ore particle size of 50–75 μm,tablet forming pressure of 3 MPa,dry nitrogen atmosphere,mass ratio of CaCl2/ore 1.15:1,calcination temperature of 900°C,and calcination time of 40 min.The XRD analysis revealed that a complex phase transition of the product SiO2 was also accompanied by the conversion reaction of K-feldspar/CaCl2.The SiO2 product formed at the initial stage was in the quartz phase at 900°C and was gradually transformed into cristobalite after 30 min.

1.Introduction

Potassium is one of the three most important nutritional elements for crop growth.The crop yield has been reported to be increased by 20%to 30%when potash fertilizers are applied to the potassiumdeficient soils[1].The water-soluble potassium resources in China,however,are very scarce and account for only 2.20%(approximately 2.1×108t K2O)of the total potash resources all over the world[2–4].At present,approximately 50%of the total potassium fertilizer demand for agriculture in China depends on imports[5–7].However,the waterinsoluble K-feldspar resources in China are extremely abundant,exceeding 200×108t K2O[8].A similar situation also exists in many other countries and regions[9–11].Therefore,the conversion of K-feldspar into water-soluble potassium fertilizers at a low cost and high efficiency is very important[12,13].

The existing methods for extracting potassium from K-feldspar can be divided into two categories:the wet and dry processes.The wet processes include the hydrofluoric acid method[14,15],the hydrofluoric acid–sulfuric acid method[16–20],the hydrofluoric acid–hydrochloric acid method[21],and the fluosilicate method[22,23].Although these wet processes have the advantage of a high potassium extraction ratio at low reaction temperatures,great quantities of the acidic waste gases,waste liquids and waste residues are discharged,resulting in serious environmental problems.

The dry processes refer mainly to the calcination of K-feldspar with various sodium-and calcium-containing additives,including sodium carbonate,sodium sulfate,sodium chloride,calcium carbonate,gypsum and calcium chloride,at higher temperatures to convert K-feldspar to water-soluble potassium salts,followed by water dissolution for the extraction of potassium.With mixed gypsum and calcium carbonate as additives,Bakr et al.[24]obtained a maximum potassium extraction of 80%at 1000°C,while Wang et al.[26,27]achieved an 84%–90%K-feldspar decomposition at 950–1050°C.Upon the addition of a third additive,sodium sulfate,the conversion temperature could be further reduced to 900°C.Using mixed calcium chloride and calcium carbonate as additives,a potassium extraction of 85%could be reached at 800–850°C[25].Zhao et al.[28]employed a first calcination with calcium carbonate and sodium carbonate as additives at 1280–1330°C,and then the NaOH dissolution route and the extraction of potassium reached 70%.Ma et al.[29]examined the sodium carbonate decomposition of K-feldspar with a maximum potassium extraction of 97.1%at 880°C.Han et al.[30,31]investigated the CaCl2and NaCl decomposition of K-feldspar,and the potassium extraction achieved 90%–99%at 960°C.

Clearly,among the various calcination additives mentioned above,the addition of chloride salts could considerably reduce the calcination temperature and hence is a more promising approach.Because of the difficulty of separating potassium chloride from sodium chloride,using only calcium chloride may be a better alternative.However,there have been few reports of the K-feldspar/calcium chloride calcination reaction.Peng et al.[32]considered the reaction to be a reversible liquid–solid reaction and experimentally determined the conversion ratio of K-feldspar under different reaction conditions.Han et al.[33]regarded the high temperature reaction as a cation exchange reaction between potassium ions in the silica-alumina-oxygen network structure of K-feldspar and calcium ions in the calcium chloride melt.Recently,Xie et al.[34]and Ye et al.[35]proposed a novel approach for the simultaneous extraction of potassium via the calcination of K-feldspar with CaCl2and aqueous carbonation of the potassium-extracted residue for CO2sequestration with the aim of reducing costs of both the potassium extraction and the CO2mineralization.However,until now,the process and mechanism of extracting potassium have not been well understood.

In the present study,the calcination reaction of K-feldspar with calcium chloride was systematically investigated by using various characterization techniques focusing on the effects of the calcination atmosphere,the calcination temperature and time,the particle size of the K-feldspar ore,and the mass ratio of CaCl2/K-feldspar ore for the potassium extraction and the phase transition.The mechanism was discussed.

2.Experimental

2.1.Materials

The K-feldspar ore used in the experiments was mined in Baoxing,Sichuan Province,China.The ore was crushed,ground and dried at 120°C for 2 h.The main minerals analyzed by X-ray diffraction(XRD)were microcline and quartz.To analyze the trace phase,the raw K-feldspar ore was identified with PLM(Polarized Light Microscopy)by observing the different interference colors under perpendicularly polarized light.As shown in Fig.1,K-feldspar was observed to be gray with a granular texture under the polarizing microscope.The major component of the K-feldspar was colorless under plane polarized light.The minor component that was white under the polarizing microscope and transparent under plane polarized light was unevenly distributed quartz,and the trace components were mica,calcite,sphene and hematite,which were colorful,brown,green and tawny under the polarizing microscope,respectively.

The chemical composition of K-feldspar is listed in Table 1.The chemical composition,except K2O,was measured by X-ray fluorescence spectrometry(XRF).

2.2.Calcination experiments

The K-feldspar ore powder sample with certain particle sizes was mixed uniformly with anhydrous calcium chloride powder according to the specific mass ratios in an agate mortar.The mixture was shaped into tablets with a diameter and thickness,respectively,of 10 mm and 5–7 mm using a tablet machine(HW-01,Tianjin,China)under 3 MPa pressure and was then dried in a muffle furnace at 300°C for 1 h to remove the water adsorbed during the mixing.Three tablets(approximately 4 g)placed in a porcelain boat were weighed accurately and calcined in a tube furnace at preset temperatures in a dry or damp nitrogen atmosphere at gas flow of 3×10?4m3·min?1for certain periods of time.The calcined slag thus obtained was withdrawn,cooled naturally to room temperature in a desiccator,pulverized and then leached.The dissolution was conducted in deionized water with the mass ratio of liquid to solid of 50:1 at 70°C for 30 min to extract potassium,then the slurry was filtered(the process is shown in Fig.2).

The contents of potassium and chlorine in the resulting filtrates were measured.The extraction ratio of potassium and the thermal hydrolysis ratio of calcium chloride were calculated.The calcined slag and leach residue were characterized by XRD and SEM.

2.3.Analysis and characterization

The content of potassium in the filtrate was measured by the potassium tetraphenylborate gravimetric method.To measure the content of potassium in the K-feldspar ore or leach residue,the solid samples were first digested.Approximately 0.5 g of K-feldspar ore or leach residue was mixed with 4 g of NaOH and 1 g of Na2O2in a nickel crucible,was smelted in a muffle furnace at 750°C for 10 min,and was then dissolved in a mixed solution of 30 ml of dilute sulfuric acid with a volume ratio of 98 wt.%sulfuric acid to deionized water 1:1 and 50 ml of dilute hydrochloric acid with a volume ratio of 31 wt%hydrochloric acid to deionized water 1:1.The content of potassium in the leach liquor was also measured by the potassium tetraphenylborate gravimetric method.

The content of chloride in the filtrate was determined by the silver chloride gravimetric method.

The XRD analyses of the calcined slag and leach residue were performed using a Philips X'pert PRO diffractometer employing graphitefiltered Cu Kαradiation(λ=0.15406 nm)with an accelerating voltage of 40 kV and tube current of 30 mA.Data points were acquired by step scanning with a ratio of 12(°)·min?1from 2θ=10°to 2θ=80°.

The surface morphology of the raw K-feldspar ore and the calcined slag was observed using a Hitachi S-4800 scanning electron microscope(SEM)at an accelerating voltage of 5 kV.The relative elemental abundance of the K-feldspar ore and the calcined slag was analyzed with an energy-dispersive X-ray spectrometer(EDS,Oxford IE 250).

3.Results and Discussion

The extraction of potassium from K-feldspar by calcium chloride calcination is a heterogeneous reaction.Generally,the factors affecting the extraction involved mainly the calcination temperature,reaction time,mass ratio of the CaCl2/K-feldspar ore and particle size of the K-feldspar ore.However,a preliminary experiment indicated that the existence of moisture in the calcination atmosphere also remarkably influenced the extraction.Therefore,the effect of water vapor was first examined.

3.1.Effect of moisture in the calcination atmosphere

A comparison test was conducted in dry and damp nitrogen atmospheres at 900°C,with a mass ratio of CaCl2/K-feldspar ore of 0.4 and a particle size of the K-feldspar ore of 75 to 150 μm.The damp nitrogen atmosphere,which simulated damp air with a relative humidity of 75%at 25°C,was prepared by the addition of a certain amount of deionized water to dry N2gas in a custom-designed damp nitrogen atmosphere device,which was heated for evaporation to obtain a mixture of gas with 18.5 g of moisture vapor per m3dry nitrogen and was introduced into the tube furnace at gas flow of 3×10?4m3·min?1.The extraction of potassium at different reaction times was measured and is shown in Fig.3.

Fig.1.PLM images of K-feldspar:(a)perpendicularly polarized light(+)and(b)plane polarized light(?).

Table 1 Chemical composition of K-feldspar ore

Fig.2.Schematic diagram of the experimental operations.

Fig.3.Effect of calcination atmosphere on the extraction of potassium.

As Fig.3 shows,the extraction of potassium in the damp N2atmosphere was approximately 5%lower than the extraction in a dry N2atmosphere when the calcination time exceeded 40 min.To explain this phenomenon,the chloride content remaining in the calcined slag was measured.The residual chloride content was found to be almost unchanged with the increasing reaction time when the calcination was in a dry N2atmosphere,while the chloride content under the damp N2atmosphere gradually decreased.The effluent gas from the tube furnace in the damp atmosphere was absorbed in deionized water,and the absorption solution was qualitatively treated using an acidic AgNO3solution with the formation of a large quantity of white precipitate,which was AgCl.These results indicated that the thermal hydrolysis of calcium chloride by the moisture in the damp N2atmosphere occurred.A similar reaction was also observed by Kondo et al.[36]and Jiang et al.[37]when they investigated the high-temperature stability of pure calcium chloride under a moisture-containing airstream,and they identified the reaction at ≥700°C as follows:

In the present study,the hydrolysis ratio of CaCl2was calculated based on the residual chloride content in the calcined slag and is also plotted in Fig.3.The hydrolysis increased monotonically with increasing calcination time and reached 50%at 40 min.These results revealed that an adverse hydrolysis of calcium chloride competed with its conversion reaction with K-feldspar,thus reducing the extraction of potassium.Therefore,minimizing the steam input from the atmosphere during the calcinations is very important.

The XRD patterns of the K-feldspar ore,slags calcined under different atmospheres and leach residues are shown in Fig.4.XRD analysis indicated that the main phases of the raw K-feldspar ore were microcline and quartz.As the mixture of K-feldspar ore and calcium chloride was calcined in a dry N2atmosphere,two new phases of anorthite(CaAl2Si2O8)and potassium chloride appeared in the calcined slag.Despite excessive amounts of calcium chloride,which was applied as a reactant,the diffraction peak of calcium chloride was not observed in the calcined slag due to the low crystallinity of calcium chloride that arose from the rapid cooling of the calcium chloride melt(CaCl2b.p.,782°C).The slag was dissolved to remove water-soluble product potassium chloride and the excessive calcium chloride and a hidden new phase of quartz(PDF 70-2516)was exposed.Accordingly,the calcination reaction of potassium feldspar and calcium chloride can be expressed as:

Fig.4.XRD patterns of the K-feldspar ore,calcined slag and leach residues.Calcination conditions:900°C,40 min.(a)K-feldspar ore;the calcined slags:(b)in a dry N2 atmosphere and(c)in a damp N2 atmosphere;and the leach residues after calcining:(d)in dry N2 atmosphere and(e)in a damp N2 atmosphere.

Under the damp N2atmosphere,another two new phases appeared,i.e.,pseudowollastonite(Ca3Si3O9)and wollastonite(CaSiO3),in addition to the anorthite and potassium chloride.The quartz(PDF 70-2516)that appeared in the dry N2atmosphere disappeared.According to the phase compositions in the calcined slag,the following two new reactions are likely to take place subsequent to the reactions(1)and(2)under the damp N2atmosphere:

The SEM images of the K-feldspar ore and the leach residues of the calcined slags in different atmospheres are shown in Fig.5.

The K-feldspar ore had a compact structure.After the ore was calcined with calcium chloride in a dry N2atmosphere and leached,a large quantity of minute particles(100–500 nm)was present in the leach residue.The combined SEM-EDS analysis indicated that the compact minute particles with an average diameter of ～500 nm were quartz,while the loose,relatively smaller particles were anorthite.For the residue obtained by leaching the slag calcined in the damp N2atmosphere,the compact quartz particles were seldom observed,while a dense strip particle,which was identified as pseudowollastonite and wollastonite,appeared.

Other studies reported more complicated product phases,including Ca12Al14O33and Ca12Al14O32Cl2[34,38],appearing in the CaCl2/K-feldspar system.In their investigations,air was directly introduced into the calcination system without drying pretreatment.A variation in the moisture content in air might be responsible for the difference in the product phases.To understand the intrinsic reaction of K-feldspar/CaCl2system,all of the experiments below were carried out under a dry nitrogen atmosphere.

3.2.Effect of calcination temperature

The effect of the calcination temperature on the extraction of potassium was investigated under a dry nitrogen atmosphere at a 75–150 μm particle size of the K-feldspar ore,a mass ratio of CaCl2/K-feldspar ore of 0.4 and a calcination time of 40 min.The results are shown in Fig.6.

The calcination reaction started at 600°C.With the increasing calcination temperature,the extraction of potassium increased monotonically until 900°C.Beyond that temperature,the extraction of potassium began to decrease.The maximum extraction at 900°C was 42%,while the value dropped to approximately 35%at 1100°C.

To understand the reason behind the decline of the potassium extraction in the high-temperature region,the potassium content in the leach residues of the slags that calcined at 900°C,1000°C and 1100°C was also measured,together with the potassium content in the leachates.The two items were added to obtain the total potassium in the calcined slags.Compared with the potassium in the K-feldspar reactant,a material balance for the potassium extraction was developed.The ratios of the potassium in the leach residues,leachates and calcined slags to the potassium in the K-feldspar reactant were calculated and are listed in Table 2.

The amount of potassium in the calcined slag was almost equal to the amount of potassium in the K-feldspar reactant at 900°C,but the difference increased with the increasing calcination temperature and reached 41%at 1100°C.According to the literature[39],the vapor pressure of pure potassium chloride increases from approximately 1.33 kPa at 1000°C rapidly to 13.3 kPa at 1100°C.In addition,we inferred that the existence of the excessive calcium chloride(melting point 782°C)in the slag might enhance the volatilization due to the possible formation of a eutectic point mixing with potassium chloride.The loss of potassium in the calcined slags at high temperatures was definitely due to the volatilization of the potassium chloride product.Although the potassium chloride in the effluent gas could be recovered,the process would become complicated.Therefore,the optimum calcination temperature should be below 1000°C.

Fig.5.SEM images of the K-feldspar and leach residues of the calcined slags.(a)K-feldspar ore;the leach residues after calcining:(b)in a dry N2 atmosphere and(c)in a damp N2 atmosphere.

Fig.6.Effect of calcination temperature on the extraction of potassium.

Table 2 Material balance for the potassium extraction at different calcination temperatures

Peng et al.[32]also reported a decrease in the extraction of potassium at temperatures higher than 1000°C.Unfortunately,they attributed the reason for this decrease to the sintering of the reactants,leading to potassium not being able to be extracted from the slag.With NaCl in place of CaCl2,Han et al.[33,40]found that the extraction of potassium from K-feldspar decreased at temperatures beyond 960°C.They thought this decrease was due to the coverage of the partially melting K-feldspar with the soluble product KCl.

The XRD patterns of the leach residues of the slags calcined at different temperatures are shown in Fig.7.

Fig.7.XRD patterns of the leach residues of slags calcined at different temperatures:(a)600°C,(b)700°C,(c)800°C,(d)900°C,(e)1000°C,and(f)1100°C.

The diffraction peaks of K-feldspar and anorthite gradually became weak and strong,respectively,with the increasing calcination temperature,indicating that the potassium extraction increased monotonically with temperature within the temperature range employed in the study,and the decrease in the potassium extraction beyond 900°C shown in Fig.6 was due to the high-temperature volatilization of the potassium chloride product.

The product of SiO2phase generated at 900°C and over 1000°C was,respectively,quartz(PDF 70-2516)and cristobalite(PDF 71-0785).A possible transition process could be:

3.3.Effect of the particle sizes of K-feldspar

The effect of the particle sizes of K-feldspar ore(150–300 μm,75–150 μm,50–75 μm,＜50 μm)on the extraction of potassium was investigated under a dry nitrogen atmosphere at 900°C,with a mass ratio of CaCl2/K-feldspar ore of 0.4 and a calcination time of 40 min.

The particle size distribution of K-feldspar ore is shown in Fig.8.The average diameters of K-feldspar ore 150–300 μm,75–150 μm,50–75 μm and ＜50 μm were 188.205 μm,148.833 μm,74.473 μm and 19.794 μm,respectively(RISE-2002,Henan,China).

The effect of the particle size of the K-feldspar ore on the extraction of potassium is shown in Fig.9.

Fig.9.Effect of the particle size of the K-feldspar ore on the extraction of potassium.

Fig.8.The distribution of particle size of the K-feldspar ore(a):150–300 μm;(b)75–150 μm;(c)50–75 μm;(d)～50 μm.

The extraction of potassium increased with the decreasing particle size of K-feldspar ore.Because of the low melting points of calcium chloride(782°C)and potassium chloride(770°C),the calcination reaction at 900°C could be regarded as a liquid–solid multiphase reaction with the formation of solid products(CaAl2Si2O8and SiO2).In theory,for a multiphase reaction controlled by either diffusion across the liquid boundary layer on the surface of solid particles or the surface reaction,small particle sizes may increase the contact area between the two phases,while for a multiphase reaction controlled by the internal diffusion across the solid product layer,small particle sizes may reduce the diffusion distance.Therefore,small K-feldspar particle sizes may always be beneficial to the extraction.

3.4.Effect of the mass ratio of CaCl2/K-feldspar ore

The effect of the mass ratio of CaCl2/K-feldspar ore on the extraction of potassium was investigated under a dry nitrogen atmosphere at 900°C,with a particle size of K-feldspar ore of 50–75 μm and a calcination time of 40 min.According to the potassium content in the Kfeldspar ore and Eq.(2),the theoretical mass ratio of CaCl2/K-feldspar ore was 0.2.Thus,the mass ratios employed in this series of tests were 0.2,0.4,0.6,0.8,1.0,1.1,1.15 and 1.2.The results are shown in Fig.10.

Fig.10.Effect of the mass ratio of CaCl2/K-feldspar ore on the extraction of potassium.

The extraction of potassium increased rapidly with the increasing mass ratio of CaCl2/K-feldspar ore.The extraction was only 22%at the theoretical mass ratio and reached 92%at six times the theoretical mass ratio(1.2).The equilibrium constant of Eq.(2)at 900°C was calculated to be only 0.167 using HSC Chemistry 5.0(commercial software from Outotec,Finland).Therefore,an excess of CaCl2was required to obtain a high potassium extraction.As the mass ratio of CaCl2/K-feldspar ore reached 1.2,adhesion occurred between the reactant and the porcelain boat,indicating that the optimum calcination mass ratio of CaCl2/K-feldspar ore should be below 1.2.And the ratio of potassium extracted reached 91%as the mass ratio was 1.15,so that the optimum calcination mass ratio of CaCl2/K-feldspar ore can be chosen 1.15.

The XRD patterns of the leach residues of the slags calcined at different mass ratios are shown in Fig.11.

With the increasing mass ratio of CaCl2/K-feldspar,the diffraction peaks of K-feldspar and anorthite gradually became weak and strong,respectively.At a mass ratio in excess of 1.0,the K-feldspar almost disappeared.The results were in accordance with the change in the potassium extraction shown in Fig.10.

Similarly,the product SiO2generated at 900°C and the low mass ratio was quartz(PDF 70-2516).However,when the mass ratio was above 0.8,the quartz transformed into cristobalite(PDF 71-0785).The excess of CaCl2might accelerate the crystal transition of quartz to cristobalite.

Fig.11.XRD patterns of the leach residues at different mass ratios of CaCl2/K-feldspar ore:(a)Raw K-feldspar ore,(b)0.2:1,(c)0.4:1,(d)0.6:1,(e)0.8:1,(f)1:1,(g)1.1:1,(h)1.15:1,(i)1.2:1.

3.5.Effect of the calcination time

The effect of the calcination time on the extraction of potassium was investigated under a dry nitrogen atmosphere at 900°C,with a mass ratio of CaCl2/K-feldspar ore of 1.15 and a particle size of K-feldspar ore of 50–75 μm.The results are shown in Fig.12.The extraction of potassium increased rapidly with the increasing calcination time from approximately 55%at 10 min to 91%at 40 min.Beyond 40 min,the extraction of potassium remained almost unchanged.

Fig.12.Effect of the calcination time on the extraction of potassium.

The XRD patterns of the leach residues of the slags calcined for different times are shown in Fig.13.

With the increasing calcination time,the diffraction peaks of Kfeldspar and anorthite,respectively,decreased and increased gradually.At a calcination time of more than 30 min,the diffraction peaks of Kfeldspar almost disappeared.

Fig.13.XRD patterns of the leach residues of the slags calcined for different times:(a)10 min,(b)20 min,(c)30 min,(d)40 min,(e)50 min,(f)60 min.

The product SiO2formed at the initial stage was quartz(PDF 70-2516).However,when the calcination time exceeded 30 min,the diffraction peaks of quartz weakened remarkably,while the diffraction peaks of cristobalite(PDF 71-0785)appeared and gradually became strong.Obviously,the calcination reaction was accompanied by the transition of the product SiO2phase from quartz to cristobalite.

4.Conclusions

The process of extracting potassium from K-feldspar via the calcium chloride calcination pathway was studied systematically.The factors affecting the extraction involved the calcination atmosphere,temperature,time,mass ratio of the CaCl2/K-feldspar ore and particle size of the K-feldspar ore.The results demonstrated that the high-temperature hydrolysis reaction of calcium chloride with moisture in a damp atmosphere occurred simultaneously during the conversion reaction of K-feldspar with calcium chloride,thus decreasing the potassium extraction.The potassium extraction increased with the increasing calcination temperature.At temperatures in excess of 900°C,the volatilization of the product KCl reduced the potassium extraction.The mass ratio of the CaCl2/K-feldspar ore affected the extraction significantly.At the ratio of 1.15 and at 900°C,the potassium extraction reached 91%in 40 min while the extraction was only 22%at a ratio of 0.2.Optimal process conditions are as follows:ore particle size of 50–75 μm,tablet forming pressure of 3 MPa,dry nitrogen atmosphere,mass ratio of CaCl2/ore 1.15:1,calcination temperature of 900°C,and calcination time of 40 min.

In addition,the XRD analysis demonstrated that the product SiO2was a quartz phase in the initial stage of reaction at 900°C and was gradually transformed into the cristobalite phase after 30 min.