Dissolution kinetics of borogypsum in
di-ammonium hydrogen phosphate solutions
Havva Mumcu S‚ims‚ek
a, R
€ovs‚en Guliyev
b, Ays‚e Vildan Bes‚e
c,*aDepartment of Chemical Engineering, Osmaniye Korkut Ata University, 80000, Osmaniye, Turkey bDepartment of Environmental Engineering, Ardahan University, 75000, Ardahan, Turkey c
Department of Chemical Engineering, Atatu¨rk University, 25240, Erzurum, Turkey
a r t i c l e i n f o
Article history:Received 27 February 2018 Received in revised form 22 May 2018
Accepted 12 July 2018
Available online 2 August 2018 Keywords:
Borogypsum
Di-ammonium hydrogen phosphate Dissolution
Kinetics
a b s t r a c t
This work examines the dissolution kinetics of borogypsum in di-ammonium hydrogen phosphate solutions ((NH4)2PO4) in a batch reactor. The parameters selected were the
re-action temperature (15e53C), di-ammonium hydrogen phosphate concentration (1e4 M),
stirring speed (50e800 rpm), and solid/liquid ratio (1/50e1/5). The dissolution rate increased by increasing the temperature (from 0.32 to 0.82), di-ammonium hydrogen phosphate concentration (from 0.35 to 0.821), and by decreasing solid-to-liquid ratio (from 0.77 to 0.24). The dissolution rate increased up to stirring speed of 600 rpm (from 0.135 to 0.56), and then decreased with increasing stirring speed (from 0.56 to 0.351). The dissolu-tion rate was described by first-order pseudo-homogeneous reacdissolu-tion model. The activadissolu-tion energy of this study was calculated as 42.103 kJ mol1. A kinetics model including the used parameters in this study was suggested as follows:
lnð1 XÞ ¼ 223:63C0:911ðS=LÞ0:7689W0:6212eð5064:2=TÞt
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Boron minerals are natural compounds containing boron oxide (B2O3) in different proportions in their structure. Despite
the fact that there are more than 230 boron minerals in nature, the boron minerals widely used by industry worldwide are colemanite, kernite, tincal and ulexite The total amount of the world's boron reserves is about 4.5 billion tons, Turkey has 72.2% of these reserves by 3.3 billion tons[1e3]. USA, China, Russia, Peru, Bolivia, Chile and Argentina are the other countries with boron reserves. The boron minerals commonly
found in Turkey are tincal, colemanite and ulexite in which the colemanite constitutes approximately 70% of the total reserve[4].
One of the most important boron compounds is boric acid. It is used to product a number of materials such as fiber glass, borosilicate glass, inorganic borate salts, boron alloy, borate esters and heat-resistant materials. Boric acid is produced by the reaction between the sulfuric acid at about 90C and the colemanite, crunched and/or concentrated by physical methods, in Turkey. The reaction of this process can be written as follows:
* Corresponding author.
E-mail address:vbese@atauni.edu.tr(A.V. Bes‚e).
Available online at
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ScienceDirect
journal home page: www.elsevier.com/loca te/he
https://doi.org/10.1016/j.ijhydene.2018.07.089
2CaO:3B2O3:5H2Oþ 2H2SO4þ 6H2O/6H3BO3þ 2CaSO4:2H2O
(1) As can be seen from the above reaction, the main product of the reaction is boric acid and by-product is gypsum. At the end of the reaction process, the gypsum is removed by filtra-tion of the reactor contents and the boric acid is crystallized by cooling about at 45C of the hot mother liquor[5]. Because the gypsum sludge contains about 3e6% B2O3, it is called
bor-ogypsum. Approximately 3.3 105tons per year of boric acid
and 6.5e7 105tons per year of borogypsum are produced in
the factories located in Bandırma and Emet of Etimaden Inc. in Turkey[6]. Borogypsum discarded from the plant are stored in open areas. Boron compounds in borogypsum can be dis-solved by rain which leads to pollution of the soil and water. Since high boron concentrations are harmful to the environ-ment, borogypsum should be considered in the category of hazardous waste[7].
Many studies have been focused to evaluate borogypsum in different fields. Some of these are about the possible use of borogypsum in the production of cement and asphalt. Bon-cukcuoglu et al. [8] have investigated the effects of adding borogypsum instead of natural gypsum to clinker in cements production. It has been reported that compressive strength of produced concrete by using borogypsum would be better than production concrete with natural gypsum. Demirbas‚ and Karslioglu [9] have studied the effects on the compressive strength of Portland cement directly mixed borogypsum and sludge. It was observed that the compressive strength of cement mixture decreased with increasing the percent of the sludge. In a study, it was reported that using borogypsum instead of natural gypsum retarded the setting period of cement[10]. Sevim and Tu¨men[11]have investigated effects of borogypsum on the strength and fresh properties of con-crete. They have recommended the use of borogypsum as a set retarder for Portland cement. Alp et al.[12]studied usage of arsenical borogypsum as a set retarder in cement industry. Abi[6]has tested the suitability of adding borogypsum to the brick over a range of brick compositions. It has been reported that the brick samples containing 10% borogypsum had the best compressive strength. Radiation transmission of con-crete including boron waste for 59.54 and 80.99 keV gamma rays have been measured by Demir and Keles‚[13]. The results have been shown that borogypsum is suitable for gamma ray shielding material to cement. Ku¨tu¨k-Sert and Ku¨tu¨k[14]have used borogypsum as a filler aggregate in asphalt concrete. It is recommended that borogypsum can be used for road and highway construction. Elbeyli et al.[15]have reported that the mortar prepared from the cement added borogypsum gives better mechanical strength than natural gypsum. Zaimoglu [16]has investigated the usage of borogypsum in modification of granular soils. The results shown that optimum water content and dry density of granular soils increased when added borogypsum.
Some researchers have removed the harmful pollutants in borogypsum by leaching to reduce the environmental impact. Delfini et al.[7]have investigated the leaching of the arsenic in borogypsum with Na2S. Arsenic content in waste was reduced
from a level up to 2000 ppm to a value less than 500 ppm after leaching. Demirbas‚ et al.[17]have studied recovery of boric acid from boronic wastes by using water and carbon dioxide-or sulfur dioxide-saturated water. They have repdioxide-orted that more than 90% of B2O3 recovery was found as boric acid.
€Ozmala and Erdogan[18]have recovered lithium from bor-ogypsum using lithium ion-sieves in aqueous solution. Dem-irbas‚[19]has recovered the lithium from borogypsum with water leaching. Borogypsum as a sorbent have been used to remove of Cd2þ [20] and Sr2þ [21]from aqueous solutions. Elbeyli and Pis‚kin[22]have studied the thermal behavior of the borogypsum and calculated kinetic parameters for dehy-dration reactions. Guliyev[23]has used the borogypsum to produce fertilizers containing boron. As can be seen from the literature survey, borogypsum generally has been used as an additive material for cement and asphalt.
To generate (NH4)2SO4fertilizer containing boron, the
re-action between borogypsum and di-ammonium hydrogen phosphate solutions was considered. In the present study, the dissolution kinetics of borogypsum in the di-ammonium hydrogen phosphate solutions were investigated and a mathematical expression representing the dissolution pro-cess for chosen parameters was derived.
Materials and methods
Materials
The borogypsum samples used in this study were obtained from The Boric Acid Plant in Emet, Turkey. The borogypsum samples were dried at laboratory conditions and then sieved by using ASTM standard sieves. The150 mm fraction was used in all experiments. Spectrophotometric and gravimetric method were used to determine the chemical composition of borogypsum. The chemical composition and X-ray diffracto-gram and SEM image of borogypsum used in the experiment are given inTable 1andFig. 1, respectively.
Experimental procedures
(NH4)2HPO4concentration, temperature, solid-to-liquid ratio,
and stirring speed were chosen as parameters to determine their effects on the dissolution of borogypsum. The parame-ters used in the study and their values are given inTable 2. The parameter levels were determined according to the pre-liminary results. While the effect of a parameter on dissolu-tion of borogypsum was examined, the values of other parameters shown with asterisks inTable 2were kept con-stant. In order to be able to see the effect of the examined
Table 1e Chemical analysis of borogypsum.
Component B2O3 SiO2 SO3 CaO MgO Fe2O3 Al2O3 Na2O SrO As2O3 H2O
% (w) 4.00 7.50 35.40 22.90 1.30 0.82 0.93 0.12 0.30 0.10 26.63
parameter more clearly, the values of the parameters kept constant were chosen from the intermediate values outside their maximum or minimum values.
Experiments were carried out in a spherical glass reactor of 500 mL volume at atmospheric pressure. It was equipped with a temperature controller, a mechanical stirrer and a back cooler. 200 mL of di-ammonium hydrogen phosphate solution
was put into the reactor in the determined concentration. After reaching the desired temperature, the borogypsum was added into solution by the determined amount, while the contents of the reactor were stirred at a determined speed. At certain intervals, the solution taken from the reactor was filtered and the amount of sulfate was determined.
Analysis of sulfate content
The amount of sulfate was determined by using turbidimetric method[24]. The turbidimetric method is based on the prin-ciple of light scattering by the particles in the aqueous solu-tion. If barium and sulfate react in aqueous solution, a turbid solution occurs (Eq.(2)) which allows the concentration of the sulfate which can be measured by using a spectrophotometer. SO24ðaqÞþ Ba2þ
aq/BaSO4ðsÞ (2)
The sample to be analyzed was diluted to 100 mL with distilled water with in 250 mL Erlenmeyer flask. 5 mL of the Fig. 1e X-ray diffractogram (a) and SEM image (b) of borogypsum.
Table 2e Parameters and their values used to experiments. Parameter Value Concentration (M) 1, 2a, 3, 4 Solid-to-liquid ratio (g mL1) 1/5, 1/10, 1/15, 1/20a, 1/25, 1/50 Stirring speed (rpm) 50, 200, 400, 600a, 800 Temperature (C) 15, 23, 33a, 43, 53
a While the effect of one parameter was investigated, the values of
conditioning reagent consisting of 50 mL glycerol, 30 mL concentrated HCl, 300 mL distilled water, 100 mL ethyl alcohol and 75 g NaCl was added in the flask and stirred gently. Then about 0.3 g of BaCl2was added to the flask and stirred the
contents of the flask for one minute. After the completion of the stirring time, the contents were poured into a spectro-photometer cell. The absorbance of the resulting solution was measured at 420 nm by using a UV spectrophotometer (Termo- Evolution 220). Results were expressed as milligrams of sulfate per 1000 mL solution (ppm). The conversion fraction of borogypsum was calculated as follows:
XCaSO4 ¼
amount of sulfate passing to the solution
amount of sulfate in the borogypsum (3)
Results and discussion
Dissolution reactions
It is estimated that the reactions between borogypsum and (NH4)2HPO4solution are as follows:
CaSO4:2H2OðsÞ#Caþ2ðaqÞþ SO24ðaqÞþ 2H2OðaqÞ (4)
ðNH4Þ2HPO4ðaqÞ#2NHþ4ðaqÞþ HPO24ðaqÞ (5)
and the total reaction is as follows:
CaSO4:2H2OðsÞþ ðNH4Þ2HPO4ðaqÞ/CaHPO4:2H2OðsÞþ ðNH4Þ2SO4ðaqÞ
(6) Fig. 2 shows the X-ray diffractogram and SEM image of solid product produced during the reaction. As seen inFig. 2 the main solid product was CaHPO4$2H2O as expected
ac-cording to Eq.(6).
Effect of parameters on dissolution of borogypsum
The effect of (NH4)2HPO4concentration on dissolution rate of
borogypsum was investigated in concentration of 1, 2, 3 and 4 M. During the experiments, solid/liquid ratio, temperature and stirring speed were kept constant as 1/20 g mL1, 33C and 600 rpm, respectively. Results were shown inFig. 3. As shown inFig. 3, the dissolution rate of borogypsum increased with increasing the (NH4)2HPO4concentration. This case can
be explained by the presence of sufficient (NH4)2HPO4to react
with borogypsum.
The effect of solid-to-liquid ratio was studied in the range of 1/5e1/50 g mL1. While the effect of solid/liquid ratio was
examined, (NH4)2HPO4concentration, temperature and
stir-ring speed were kept constant as 2 M, 33C and 600 rpm, respectively. As shown inFig. 4, an increase of solid-to-liquid ratio decreased dissolution rate of borogypsum. This behavior can be explained by the reduction in the amount of dia-mmonium hydrogen phosphate per unit of solid.
The values of 50, 200, 400, 600 and 800 rpm of stirring speed were used to determine the effect of stirring speed on disso-lution rate, while (NH4)2HPO4concentration of 2 M,
tempera-ture of 33C and solid/liquid ratio of 1/20 g mL1were kept constant. Results of this study are shown in Fig. 5. The
conversion values increased up to 600 rpm and decreased up to this value. This behavior can be evaluated by considering two situations separately. Firstly, up to 600 rpm increasing speed provides to be more homogeneously dispersed of the solid particles in the solution. This case provides a better solid-liquid contact and so conversion value increases. Sec-ondly, increasing stirring speed values cause vortex formation and the particles move together with the fluid which prevents effective contact of solid and fluid.
The effect of the temperature on the dissolution rate of borogypsum was investigated at 15, 23, 33, 43 and 53C. In these experiments, (NH4)2HPO4 concentration, solid/liquid
ratio, and stirring speed were fixed at 2 M, 1/20 g mL1and 600 rpm, respectively. Results are plotted in Fig. 6. The in-crease in reaction temperature inin-creased the dissolution rate. Since the reaction rate constant depends on temperature as exponential according to the Arrhenius equation, this case is an expected result.
Behaviors similar to the effects of parameters on bor-ogypsum dissolution have been reported by previous workers [25e31].
Kinetic analysis
Kinetics of non-catalytic liquidesolid reactions can be analyzed by using the heterogeneous and homogeneous re-action models. Chemical rere-action at the particle surface, diffusion through the fluid film or diffusion through the product layer can control the rate of reaction between a solid particle and a liquid reactant in the heterogeneous models. It is assumed that the liquid penetrates into the solid particle and always reacts throughout the particle for the homoge-neous reaction models. These models are known as first order homogeneous model and second order pseudo-homogeneous model[32].
The experimental data were applied both to the hetero-geneous and homohetero-geneous models to determine kinetics of reaction between borogypsum and (NH4)2HPO4. Applied
models and obtained regression coefficients for these models are summarized inTable 3. According to statistical analysis, first order pseudo-homogeneous model with the highest R2
value could be considered to be suitable for describing the kinetic of this study. So, the rate expression can be written as follows:
lnð1 XÞ ¼ kt (7)
As there is a linear relationship betweeneln (1-X) and time as seen in Eq.(7), the plot ofeln (1X) versus time for each of parameters must be straight line.Figs. 7e10represent varia-tion of [-ln (1- XCaSO4)] with reaction time for (NH4)2PO4
con-centration, solid-to-liquid ratio, stirring speed and reaction temperature, respectively. All of these figures were obtained straight lines passed through the origin. So, it can be said that reaction of occurring between borogypsum and di-ammonium hydrogen phosphate is represented by first order pseudo-homogeneous model.
It can be described a semi-empirical model which includes effects of parameters used for the dissolution experiments on the rate constant of reaction as follows:
Fig. 2e X-ray diffractogram (a) and SEM image (b) of solid product (temperature: 33C; solid/liquid ratio: 1/20 g mL¡1; (NH4)2HPO4concentration: 2 M; stirring speed: 600 rpm).
Fig. 3e Effect of (NH4)2HPO4concentration on dissolution of
borogypsum (temperature: 33C; solid/liquid ratio: 1/ 20 g mL¡1; stirring speed: 600 rpm).
Fig. 4e Effect of solid-to-liquid ratio on dissolution of borogypsum (temperature: 33C; (NH4)2HPO4
k¼ k0CaðS=LÞbWceðEa=RTÞ (8)
If Eqs.(7) and (8)is combined, Eq.(9)is obtained:
lnð1 XÞ ¼ k0CaðS=LÞbWceðEa=RTÞt (9)
where ko, C, S/L, W, T, Ea, t a, b and c represent pre-exponential
factor (min.), concentration of di-ammonium hydrogen phosphate (g mL1), solid-to-liquid ratio (g mL1), stirring speed (rpm), temperature (K), activation energy (kJ mol1), time (min.) and constants.
The constants a, b, and c in Eq.(9)calculated by using the apparent rate constants which were obtained from slopes of straights inFigs. 7e10and their values were 0.911,0.7689
and 0.6212, respectively. The reaction rate constant depends on the temperature and Arrhenius equation can be used to determine the relationship between k and T[33,34]:
k¼ koexp Ea RT (10) According to Arrhenius equation (Eq. (10)), a plot of the natural logarithm of k value versus 1/T gives a straight line with a slope Ea/R and intercept ln ko.Fig. 11shows the values
of lnk versus 1/T according to the linearized Arrhenius equa-tion for this process. The activaequa-tion energy value was deter-mined from the slope of line as 41.623 kJ mol1. The relationships between lnk and 1/T is shown in Eq.(11)with R2.
ln k¼ 5:332 5064T:2 R2¼ 0:9846 (11)
Fig. 5e Effect of stirring speed on dissolution of borogypsum (temperature: 33C; solid/liquid ratio: 1/ 20 g mL¡1; (NH4)2HPO4concentration: 2 M).
Fig. 6e Effect of temperature on dissolution of borogypsum (solid/liquid ratio: 1/20 g mL¡1; (NH4)2HPO4concentration:
2 M; stirring speed: 600 rpm).
Table 3e Integrated rate equations used in this process and regression coefficients.
Rate equation Rate-controlling step R2
t/t*¼1(1X)1/3 Surface chemical reaction 0.989
t/t*¼1(1X)2/3 Fluid film diffusion control model 0.987
t/t*¼1e3(1X)2/3þ2(1X) Diffusion control through the ash or product layer 0.986
k.t¼ln(1X) The first-order pseudo-homogeneous model 0.991 k.t¼1/(1X) The second-order pseudo-homogeneous model 0.978
Fig. 7e Variation of [ lnð1 XCaSO4Þ] with time for different
(NH4)2HPO4concentration.
Fig. 8e Variation of [ lnð1 XCaSO4Þ] with time for different
solid-to-liquid ratio.
Dissolution reaction rates of ulexite in ammonia solutions saturated with CO2[35], calcined colemanite in ammonium
carbonate solutions[36], smithsonite in boric acid solutions [37]and calcined ulexite in ammonium carbonate solutions [38]have been described by first order pseudo-homogeneous model and values of their activations energy reported as 55, 59.3, 62 and 35.3 kJ mol1, respectively.
And finally, the kinetics model including the used param-eters in this study can be written as follows:
lnð1 XÞ ¼ 223:63C0:911ðS=LÞ0:7689W0:6212eð5064:2=TÞt (12)
The theoretical conversion values were calculated using the semi-empirical model (Eq.(12)). To test fit between the experimental (Xexp) and theoretical (Xtheo) conversion values,
a plot of values of Xtheoversus Xexpis shown inFig. 12. As can
be seen fromFig. 12, the experimental and theoretical values are in good agreement.
Conclusion
In this study, the dissolution kinetics of borogypsum in di-ammonium hydrogen phosphate solutions was investigated. The effect of selected parameters on the rate dissolution of bor-ogypsum was examined. Increasing di-ammonium hydrogen phosphate concentration, reaction temperature, stirring speed (up to 600 rpm) and decreasing solid-to-liquid ratio were increased the dissolution rate of borogypsum. The experimental data was fitted the heterogeneous and homogeneous models and the first-order pseudo-homogeneous reaction model was found to be a better model for describing the dissolution kinetics of borogypsum. Arrhenius equation was applied to determine the activation energies of dissolution process and its value calculated as 42.103 kJ mol1. A mathematical expression containing the effect of the parameters used in the study was obtained.
(NH4)2SO4and CaHPO4 were produced with the reaction
between borogypsum and di-ammonium hydrogen phos-phate in this study. The main product of the reaction was (NH4)2SO4which can be used as a boron containing fertilizer.
CaHPO4insoluble in water was produced as by-products in the
study. It is used as a fertilizer, in the plastics, food, and glass industries, and in medicine[27]. For these reasons, it can be said that this process is more advantageous than from other processes of the evaluation of borogypsum.
Acknowledgements
The authors thanks to the Emet Boric Acid Plant Etimaden Inc. for providing the borogypsum. Thanks are also to Prof. Dr. Ahmet Bes‚e of Atatu¨rk University American Culture and Liter-ature Department for his proofreading of the paper in English. Fig. 9e Variation of [ lnð1 XCaSO4Þ] with time for different
stirring speed.
Fig. 10e Variation of [ lnð1 XCaSO4Þ] with time for
different reaction temperature.
Fig. 11e Arrhenius plot for dissolution of borogypsum.
Fig. 12e Agreement between of experimental conversion values and calculated conversion values from semi-empirical model.
Nomenclature
a, b, c model constants
C concentration of di-ammonium hydrogen phosphate (g mL1)
Ea energy of activation (kJ/mol)
k reaction rate constant (min.) ko pre-exponential factor (min.)
L amount of liquid (mL)
R the universal gas constant (8.314 J/mol K) R2 regression coefficient
S amount of solid (g)
t time (min.)
t* time for complete conversion (min.)
T temperature (K)
W stirring speed (rpm)
r e f e r e n c e s
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