Boron is an important element in the world from both strategic and industrial points of view. Althou — Organic Chemistry Chemistry Question
Colemanite mineral as boron source
Boron is an important element in the world from both strategic and industrial points of view. Although the element is not directly used, its compounds have a wide range of applications almost in all manufacturing areas, except food. Boron is oxophilic and, therefore, occurs primarily as oxide (borates) in nature. Borate minerals occur in a few locations in the world. The largest reserves of boron minerals are in the western part of Turkey. One of the most important borate minerals is colemanite with the formula 2 CaO⋅3 B2O3⋅5 H2O. Boric acid (H3BO3) is produced in Turkey and Europe mainly by the reaction of colemanite with sulfuric acid.
The reaction is carried out at temperatures above 80 °C. Calcium sulfate dihydrate (gypsum, CaSO4·2 H2O) crystallizes from the reaction solution and the crystals are filtered out from the hot solution. Subsequently, boric acid crystallizes from the solution when it is cooled down to room temperature. Filtration of gypsum crystals from the reaction solution is a crucial process in the boric acid production for achieving high purity and high efficiency, as the subsequent crystallization of boric acid from the supernatant solution is substantially affected by contaminations. The reaction of sulfuric acid with colemanite takes place in two steps: In the first step colemanite is dissolved in sulfuric acid forming the calcium(II) ion and boric acid. In the second step, calcium sulfate, formed from Ca2+ and SO4 2− ions, precipitates as gypsum crystals. In an experiment, 184.6 g colemanite containing 37.71 % by mass of B2O3 and 20.79 % by mass of CaO is dissolved in aqueous sulfuric acid yielding initially solution of boric acid at 80 °C (c = 1.554 mol dm-3). The reaction is carried out in a closed system so that the volume of the solution remains essentially constant. The concentration of calcium ions in this saturated solution at 80 °C is c(Ca2+) = 0.0310 mol dm-3.
Write a balanced equation for the dissolution of colemanite in sulfuric acid.
Model Answer
2 CaO·3B2O3·5 H2O(s) + 2 H2SO4(aq) + 6 H2O(l) → 2 CaSO4·2 H2O(s) + 6 H3BO3(aq)
Calculate the amount of gypsum obtained from the crystallization.
Model Answer
mass of B2O3 in 184.6 g colemanite = 184.6 × 37.71 / 100 = 69.61 g
n(B2O3) = 69.61g / 69.6 g mol-1 = 1.00 mol B2O3
Since the initial concentration of H3BO3 is 1.554 mol dm-3, the initial concentration of B2O3 is 0.777 mol dm-3.
Total volume of the solution is: V = 1.000 mol / 0.777 mol dm-3 = 1.287 dm3
n(Ca2+) in the saturated solution: c(Ca2+) = 0.0310 mol dm-3
n(Ca2+) = 0.0310 mol dm-3 × 1.287 dm3 = 0.0400 mol
n(CaO) in 184.6 g colemanite:
n(CaO) = 184.6 g × (20.9 g CaO / 100) × (1 mol CaO / 56.08 g CaO) = 0.6843 mol
n(Ca2+) precipitated as gypsum = 0.6843 – 0.040 = 0.644 mol
Mass of gypsum precipitated = 0.644 mol × 172.0 g · mol−1 = 111 g
Calculate the mass of calcium ions remained in the solution.
Model Answer
n(Ca2+) remained in the solution = 0.0310 mol dm-3 × 1.287 dm3 = 0.0400 mol
mass of Ca2+ in the solution: m(Ca2+) = 0.0400 mol × 40.0 g · mol−1 = 1.60 g
Calculate the theoretical amount of boric acid that can be obtained in this
Model Answer
The sample contains 37.71 % by mass of B2O3.
m(H3BO3) = 184.6 g × 0.3771 / 69.6 g B2O3 × 2 × 61.8 g H3BO3 = 124 g H3BO3
After hot filtration of gypsum crystals, boric acid is obtained by crystallization when the solution is cooled down to room temperature. The boric acid obtained is still contaminated by sulfate ions. The sulfur contamination is not desired in industrial use of boric acid, such as production of borosilicate glasses.
Can the sulfate contamination of the product be removed by recrystallization of boric acid in aqueous solution?
Model Answer
Yes.