Effect et al. [17] reported that dissolution of

Effect of reaction
temperature on colemanite
waste dissolution

 

            The
experiments were performed at different temperatures between 30 oC
and 70 oC.

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The influence of temperature on the dissolution
efficiency of CC was studied at a sulfuric acid concentration of 1.0 mol/L with
a solid/liquid ratio of 50 g/L and a stirring speed of 500 rpm.

            The
dissolution efficiency was 79.75% at 30oC and 97.84% at 70oC
for 25 minutes.

The dissolution efficiency increased with
increasing temperature.

This is expected from the exponential
dependence of the rate constant in the Arrhenius equation.

 

Abali et al. 17 reported
that dissolution of ulexite in oxalic acid solutions increased with increasing
temperature.

Fig. 8. Effect of reaction temperature on colemanite waste dissolution.

(acid concentration of 1.0 mol/L, solid to liquid ratio of 50 g/L,
stirring speed of 500 rpm).

 

            As
seen in Figure 8, the maximum dissolution efficiency was obtained at 70 oC.
It was observed that colemanite was completely dissolved at 70 °C in this
study.

            Boric
acid crystallization starts below 70 oC. Thus when the dissolution solution from a reactor is
transported in pipes, the boric acid solution temperature will decrease in the
pipes and filters and the boric acid solution will crystallize, so that the
pipes and filters will be clogged by these crystals. For this reason,
industrial dissolution of concentrated colemanite with sulfuric acid is
performed at 85 ± 3 °C.

Characterization
of leaching and crystallization products

 

            The
leaching process of CC in sulfuric acid solution takes place via the following
set of reactions. The dissolution of sulfuric acid is obtained in an aqueous
medium as follows:

            The
overall reaction is as follows:

            The
reaction of colemanite with sulfuric acid results in the products gypsum and
boric acid. The dissolution of calcite is obtained as follows:

            The
reaction between calcite and sulfuric acid results in gypsum.

            The
optimum conditions were found to be 1.0 mol/L sulfuric acid concentrations due
to high acid concentration requiring for colemanite dissolution and gypsum
formation, a 500 rpm stirring speed, a 50 g/L solid to liquid ratio and a
solution temperature of 70 °C.  50 g/L
solid CC was added to the solution prepared for optimum conditions and was leached for 30 minutes. The leach solution
was filtered with filter paper. The liquid phase was
crystallized in a water bath at a temperature of 35 oC to obtain
boric acid crystals. The liquid phase was not dried at ambient temperature due
to the hygroscopic nature of gypsum. Instead, it was dried in an oven at 110 oC
for XRD analysis. As seen in Figure 9, anhydrite (gypsum) and sassolite (boric
acid) were observed in the XRD analysis.

 

 

Fig. 9. X-ray diffraction analysis of crystallized
liquid phase.

(acid concentration of 1.0 mol/L, solid to liquid ratio of 50 g/L,
speed of 500 rpm, for 30 minutes).

 

            As the
solid phase was dried at 110 oC, the crystal water evaporated from
the gypsum, which became anhydrous because of its low dehydration temperature. The CC was leached using a sulfuric acid
solution at the aforementioned optimum conditions for 120 minutes. The leachant
solutions were filtered, and the liquid phase was crystallized at 35 oC.

The obtained crystals were
dried at room temperature due to the low dehydration temperature of boric acid,
and the crystallized liquid phase was analyzed by XRD (Rigaku D/Max-2200/PC).

 

Fig. 10. X-ray diffraction analysis of crystallized
liquid phase for 120 min.

(acid concentration of 1.0 mol/L, solid to liquid ratio of 50 g/L,
speed of 500 rpm, for 120 minutes).

 

            Figure 10
shows the XRD patterns of the crystals. Only boric acid (H3BO3)
peaks were observed by XRD analysis. The result of XRD is in agreement with
equation (3). The colemanite and sulfuric
acid reaction equation (3) resulted in boric acid.

            An SEM (JSM–6060 JEOL)
image of crystalline boric acid is given in Figure 11. The boric acid
crystals, which have a triclinic structure, can be observed in the SEM images. The XRD and SEM analyses show that pure boric acid crystals can be
produced using CC. As a result of chemical analysis, the solid residue
was found to contain 30.2% CaO and % 35.11 % SO3. The coarse
particle size of gypsum did not allow it to pass the filtration media with the leaching
solution.

 

 

Fig. 11. SEM images of crystal boric acid a) x1000 and
b) x2500.

(acid concentration of 1.0 mol/L, solid to liquid ratio of 50 g/L,
stirring speed of 500 rpm, for 120 minutes).

 

To obtain pure boric acid crystals, the gypsum
present in the solution must be completely separated.