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Optimization of Dissolution of Ulexite in Phosphate Acid Solutions.

Byline: Tuba Hatice Dogan and Ahmet Yartasi

Keywords: Ulexite ore Taguchi optimization Phosphate acid.

Introduction

Boron consists of complex chemical compounds. Its compounds are used in many fields such as detergents disinfectants cosmetics in the production of medicines and in the industries of glass dye and plating polymer refractory materials steel etc. In addition they are used in some production industries as catalysts in rockets as fuel and in nuclear technology as a radiation trapper [1 2]. Ulexite is found large deposits in Turkey and is a sodium calcium borate hydrate mineral (Na2O.2CaO.5B2O3.16H2O). Boron compounds as raw materials have a very important place in industry. Because they are used in many technical applications [3].

Some studies on the dissolution kinetic of boron minerals in various solutions have been made. The dissolution kinetics of ulexite in acetic acid solutions [4] were examined and found that the dissolution rate of ulexite increased with increasing solution concentration and temperature and decreasing particle size and solid-to-liquid ratio. The activation energy for the process was 55.8 kj.mol-1. Dissolution kinetics of ulexite in ammonia solutions saturated with CO2 [5] and tincal in phosphoric acid solutions [6] were investigated and the dissolution rates were found to be based on the first order pseudo homogeneous reaction model. The dissolution kinetics of ulexite in perchloric acid solutions was investigated and the activation energy was found to be 19.12 kj mol-1 [7]. Leaching kinetics of ulexite in phosphoric acid [8] and colemanite in potassium hydrogen sulphate [9] were studied and the leaching processes were determined to be diffusion controlled.

In a study [10] the dissolution of colemanite in (NH4)2SO4 solutions was studied and the activation energy was found as 40.46 kj mol-1. The dissolution kinetics of ulexite in borax pentahydrate solutions saturated with carbon dioxide [11] and colemanite in phosphoric acid solutions [12] were studied. It was determined that the dissolution rates were controlled by surface chemical reaction. In another study [13] the dissolution of ulexite in H2SO4 H3PO4 HNO3 and HCl solutions was studied and this process was found to be controlled by diffusion.

For industrial processes the optimization of dissolution conditions of the ores has an important place. Therefore many researchers have been made studies on these subjects using various methods such as Taguchi the factorial experiment design the orthogonal central composite design and response surface methodology. In a study dissolution of magnesite in citric acid solutions was optimized using Taguchi technique and the optimum conditions were found to be 2M acid concentration 120 min. reaction period 75oC reaction temperature 0.125 g/mL solid-liquid ratio and -319 m particle size [14]. The optimization of the boric acid extraction from colemanite ore was examined by the fractional factorial design and central composite design techniques and found that the effectiveness of this process at the optimum conditions was about 99.9% [15]. In another study [3] the optimization of production of H3BO3 from ulexite was investigated by using the 2n factorial design method.

It was found that the highest dissolution approximately 100% was reached at the conditions of 80oC 140 minute 500rpm and the solid/liquid ratio of 1 /4. One of the optimization techniques is Taguchi Orthogonal Array (OA) analysis. It is used to find the optimum parameters of process by making the minimum number of experiments. Taguchi technique has many advantages. One of the main advantages of this technique relative to other statistical techniques the parameters affecting an experiment can be studied as controlling and not controlling. Another advantage the technique can be applied to experimental design containing a great number of design factors [16]. In this study ulexite ore was dissolved in phosphate acid solutions using the experimental parameters of particle size solid-to-liquid ratio reaction temperature phosphate acid concentration stirring speed and reaction time.

The optimum dissolution conditions were determined by Taguchi method. Reaction products were found to be boric acid sodium ihydrogen phosphate and calcium dihydrogen phosphate. These reaction products have wide application areas in industry [8].

Results and Discussion

Dissolution Reactions

During the experiments the pH was measured and found in the range of 1.5-2.5. Dihydrogen phosphates occur at these pH values [17]. Therefore the dissolution reactions are follows:Equation

Statistical Analysis

Taguchi method is used for optimizing a process with one or multiple performance characteristics. The application of this method involves eight steps which make up a Robust Design cycle view of planning and carrying out the experiments and analyzing and verifying the experimental results [18]

Three different performance characteristics are used as the optimization criteria. They are the larger-the-better the smaller the better and the nominal-the-best. In this study larger-the-better was used. Performance characteristic (S/N) was calculated using Eq. (4) [18]:

Table-1: Experimental parameters and their values

###Levels

###Parameters

###1###2###3###4###5

###A###Reaction temperature (oC)###50###60###70###80###90

###B###Particle size (m)###-1000+850###-850+452###-300+212###-212+150###-50+125

###C###Stirring speed (rpm)###200###300###400###500###600

###D###Solid-to-liquid ratio (g.mL-1)###0.05###0.10###0.15###0.20###0.25

###E###Acid concentration (M)###0.25###0.30###0.70###1.0###1.5

###F###Reaction time (min)###5###15###30###45###60

Minitab-13 computer software package was used to analyze the results of experiments. A statistical analysis of variance (ANOVA) was performed to determine the effective parameters and their confidence levels on the dissolution process. The ANOVA accounts values known as sums of squares degrees of freedom etc. and presents them in a standard table form (Table 3).

Table-2: L25 (56) experimental plan table and results of experiments.

Experiment###Parameters and their levels###Conversion fraction of B2O3

###S/N ratio

###no###A###B###C###D###E###F###Experiment ( I) B2O3 %###Experiment (II)B2O3 %###Average B2O3 %

###1###1###1###1###1###1###1###5198###5514###5356###345655

###2###1###2###2###2###2###2###4578###4203###4391###328265

###3###1###3###3###3###3###3###7685###7836###7761###377966

###4###1###4###4###4###4###4###6669###6339###6504###362552

###5###1###5###5###5###5###5###4898###4853###4876###337601

###6###2###1###2###3###4###5###7522###7569###7546###375536

###7###2###2###3###4###5###1###7855###7997###7926###379800

###8###2###3###4###5###1###2###3888###3796###3842###316893

###9###2###4###5###1###2###3###8444###8055###8250###383213

###10###2###5###1###2###3###4###8972###8654###8813###388982

###11###3###1###3###5###2###4###2059###1742###1901###254866

###12###3###2###4###1###3###5###9999###9999###9999###399991

###13###3###3###5###2###4###1###8461###8503###8482###385699

###14###3###4###1###3###5###2###8302###8302###8302###383837

###15###3###5###2###4###1###3###5481###5414###5448###347235

###16###4###1###4###2###5###3###8379###8742###8561###386441

###17###4###2###5###3###1###4###6129###6229###6179###358175

###18###4###3###1###4###2###5###3779###3980###3880###317668

###19###4###4###2###5###3###1###7609###7355###7482###374766

###20###4###5###3###1###4###2###9667###9292###9480###395306

###21###5###1###5###4###3###2###7651###7255###7453###374374

###22###5###2###1###5###4###3###8189###8646###8418###384941

###23###5###3###2###1###5###4###9999###9803###9901###399123

###24###5###4###3###2###1###5###6711###6191###6451###361714

###25###5###5###4###3###2###1###7570###7767###7669###376921

Table-3: The ANOVA table

###Sum of squares###Degrees of freedom###Mean of squares###F###Percentage of the contribution (%)

A###Reaction temperature (oC)###257111###4###64278###17234###1246

B###Particle size (m)###112221###4###28055###7522###544

C###Stirring speed (rpm)###19348###4###4837###1297###094

D###Solid-to-liquid ratio(g.mL-1)###635941###4###158985###42627###3081

E###Acid concentration (M)###926956###4###231739###62134###4491

F###Reaction time (min)###103087###4###25772###6910###499

###Error###9324###25###373

###Total###2063988###49

Figure 1 shows the degree of the effects of parameters on the performance characteristics. The highest S/N value is the optimal level of a process parameter. Fig. 1C shows the change of performance characteristics with stirring speed. Level 1 for C parameter (stirring speed) is 200 rpm. The experiments for level 1 of C parameter are experiments numbered as 1 10 14 18 and 22. Therefore S/N value for level 1 of C parameter is the average of those obtained from experiments numbers 1 10 14 18 and 22. All values in Figure 1 were calculated in the same way. The highest value in each graph is the optimum value for that parameter [2]. When the Figure 1 is examined it can be seen that A5 (90oC) B4 (-212+150m) C4 (500rpm) D1 (0.05gmL-1) E3 (0.70M) and F3 (30 minutes) are the optimum conditions. However when the design parameters are analyzed in detail and paid attention to industrial applications

it was thought that there might be other options in which one can get more economical results as well as 100 % percent dissolution. Selecting a larger particular size can reduce crushing and grinding costs. Because of ineffectiveness of the stirring speed 200 rpm can be used instead of 500 rpm in order to save energy. To increase the efficiency of industrial-scale applications the high proportion of the solid-liquid is promoted. Considering all these conditions new experiments were done in the laboratory. Then new conditions were determined. The dissolution percentage of ulexite in phosphate acid solutions was 100 % under the following conditions: A2 (60oC) B2 (-850+452m) C1 (200rpm) D3 (0.15gmL-1) E3 (0.70M) and F3 (30minutes). Therefore for this process conditions of A2 B2 C1 D3 E3 and F3 were taken as optimum dissolution conditions. According to these results the acid concentration and solid-to-liquid ratio were the most effective parameters for this process.

As shown in Table-2 the experiment in optimum dissolution conditions (A2 B2 C1 D3 E3 and F3) was not in experimental plan table. Therefore it was performed later.

Table-4: Optimum dissolution conditions observed and predicted dissolved quantities of the ulexite.

###Parameters###Value###Level

###Reaction temperature (oC)###60###2

###Particle size (m)###-850+452###2

###Stirring speed (rpm)###200###1

###Solid-to-liquid ratio (g.mL-1)###015###3

###Acid concentration (M)###070###3

###Reaction time (min)###30###3

###Observed dissolved quantity for B2O3 (%)###100

###Predicted dissolved quantity for B2O3 (%)###100

###Confidence limits of prediction for B2O3 (%)###9527-100

ExperimentalThe ulexite ore used in the study was provided from a region of Bigadic Balikesir Turkey. The ore was cleaned manually from visible impurities it was ground and sieved by ASTM standard sieves to obtain the nominal particle size fractions of -1000+850 -850+425 -300+212 - 212+150 and -150+125m in diameter. The chemical composition of the original sample was found as follows: 41.27%B2O3 13.34%CaO 6.83%Na2O 34.09%H2O 3.22%MgO 0.034%SiO2 and 1.22% others. X-ray diffractogram of ulexite ore was given in Fig. 2.

The Taguchi method was used to determine optimum conditions for the dissolution of ulexite in phosphate acid solutions. The results of the dissolution experiments were analyzed using the MINITAB-13 statistical package.

Conclusions

The significant results obtained for this study are as follows:

1. The effective parameters on the dissolution of ulexite ore in phosphate acid solutions were acid concentration solid-to-liquid ratio reaction temperature particle size reaction time and stirring speed respectively.

2. The optimum conditions were 60oC for reaction temperature 0.15g.mL-1 for solid-to-liquid ratio 30 minutes for reaction time 0.70M for acid concentration -850+452m for particle size and 200rpm stirring speed. Under these conditions dissolution percentage of ulexite in phosphate acid solution was 100 (Table-4).

3. The predicted and observed dissolution values were very close to each other. This means that the Taguchi method can be applied to this dissolution process successfully.

4. In the Taguchi method the optimum conditions obtained in a laboratory environment can be applied in real production environments. Therefore the results of this study will be useful in industrial scale.

It was thought that boric acid sodium dihydrogen phosphate and calcium dihydrogen phosphate could be produced by this process. Dissolution of ulexite ore in H3PO4 solutions will be useful in solving problems encountered in the production of boric acid such as decreasing reaction yield and filtration etc.

Nomenclature

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Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Aug 31, 2014
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