Separation of Magnesium Hydroxide and Barium Sulphate from a Barium Sulphate – Magnesium Hydroxide Mixed Sludge by Carbonation: The Effect of Temperature

The solids that result from mine wastewater treatment usually contain elevated levels of contaminants that were originally contained in the wastewater. These must be carefully disposed or treated to avoid shifting of the original pollutants in the waste stream to the final disposal site where they may again become free to contaminate the environment. A more reasonable approach to ultimate solids disposal is to view the sludge as a resource that can be recycled or reused. In South Africa, reverse osmosis is already being used for desalination of mine water and huge sludge volumes are also produced. The Tshwane University of Technology-Magnesium-Barium-Oxide (TUT-MBO) process and its variations is an alternative technology that offers the benefit of lower cost as magnesium hydroxide, barium hydroxide and coal are the main process raw materials. In the first stage Mg(OH)2 is dosed to raise the pH of the acid mine drainage to 9 for removal of free acid, iron(II) oxidized to iron(III) and all other metals precipitated as metal hydroxides. In the second stage Ba(OH)2 is dosed for magnesium and sulphate removal as Mg(OH)2 and BaSO4 respectively. The resultant, mixed BaSO4/Mg(OH)2 sludge is treated in a thermal stage to produce BaS and MgO. The aim of this study was to separate magnesium hydroxide from barium sulphate, produced in the second stage of the TUT-MBO Process. Magnesium hydroxide is separated from barium sulphate through the dissolution of Mg(OH)2 with CO2 to Mg(HCO3)2. The results showed that: (a) By adding CO2 to a BaSO4/Mg(OH)2 sludge, selective dissolution of Mg(OH)2 occurred due to the relatively high solubility of Mg(HCO3)2 and the low solubility of BaSO4 and, (b) the solubility of Mg(HCO3)2 increased with decreasing temperatures and increasing pressures. *Corresponding author: Timothy T Rukuni, Department of Environment, Water & Earth Sciences, Tshwane University of Technology, Private Bag X680, 0001, Pretoria, South Africa, Tel: +2771 554 3236; E-mail: chivovot@tut.ac.za Received May 16, 2012; Accepted June 16, 2012; Published June 19, 2012 Citation: Rukuni TT, Maree JP, Zvinowanda CM (2012) Separation of Magnesium Hydroxide and Barium Sulphate from a Barium Sulphate – Magnesium Hydroxide Mixed Sludge by Carbonation: The Effect of Temperature. J Civil Environ Eng 2:116. doi:10.4172/2165-784X.1000116 Copyright: © 2012 Rukuni TT, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
South Africa currently faces both a shortage of water as well as a deterioration of the quality of the available water due to industrial activities such as mining and power generation. The gold mines in Gauteng are expected to decant 345 ML/day of acid mine drainage (AMD) when underground voids have filled up. This water will pollute surface water resources and cause serious environmental impacts [1]. South Africa was one of the first countries to implement commercial scale drinking water reclamation from mine water using reverse osmosis. The cost of reverse osmosis amounts to R10/m 3 and produces gypsum, ferric hydroxide rich sludge and brine that have to be disposed at a cost on sludge disposal dumps and brine ponds.
Tshwane University of Technology (TUT) has developed innovative technologies for the neutralization and desalination of acid mine drainage while avoiding gypsum crystallization. These technologies are the Magnesium-Barium-Oxide (TUT-MBO) process and its variations namely; Magnesium-Barium-Bicarbonate (TUT-MBB) process, where magnesium bicarbonate is used instead of magnesium hydroxide, and the Ammonium-Barium (TUT-NB) process [2], where ammonium hydroxide is used instead of magnesium hydroxide to avoid the formation of mixed sludges. Laboratory and pilot studies have demonstrated that magnesium hydroxide or magnesium bicarbonate can be used for removal of metals through precipitation as metal hydroxides. In the processes, magnesium and sulphate removal is achieved by treatment with Ba(OH) 2 .
The TUT-MBO process and its variations offer the benefit of lower cost as magnesium hydroxide, barium hydroxide and coal are the main process raw materials [3,4]. It produces minimum amounts of sludge as process raw materials (Mg(OH) 2 or Mg(HCO 3 ) 2 and Ba(OH) 2 ) are recovered from the sludge. Sulphur, a valuable industrial raw material, can also be recovered from the sulphate removed from mine water.
The TUT-MBO process involves two stages. In the first stage, Mg(OH) 2 or Mg(HCO 3 ) 2 is dosed to raise the pH of the AMD to 9 for removal of free acid, iron(II) as iron(III), and all other metals as metal hydroxides. In the second stage Ba(OH) 2 is dosed for magnesium and sulphate removal as Mg(OH) 2 and BaSO 4 , respectively. The resultant, mixed BaSO 4 /Mg(OH) 2 sludge is treated in a thermal stage to produce a mixture of BaS and MgO. To avoid this mixture it would be beneficial if BaSO 4 and Mg(OH) 2 could be separated prior to thermal treatment. This would allow pure BaSO 4 to go to the thermal stage where it will be reduced to BaS. The Magnesium bicarbonate solution (TUT-MBB variation) can be used again in the first stage of TUT-MBO process instead of Mg(OH) 2 . Figure 1 shows the schematic diagram of the TUT-MBB process.
The aim of this study was to explore the separation of magnesium hydroxide from barium sulphate, produced as a mixed sludge in the second stage of the TUT-MBO process, through dissolution of Mg(OH) 2 by converting it to Mg(HCO 3 ) 2 with CO 2 . By adding CO 2 to a BaSO 4 /Mg(OH) 2 sludge, selective dissolution of Mg(OH) 2 occurs according to the following reaction:  (2) Where K sp (T, S, p) is the solubility product (T is temperature, S is solubility and p is pressure) [5] with the pressure dependency adjustments by Millero et al. [6].
Note: All the square-bracketed species are stoichiometric concentrations (molality or mol /kg) and disregard any complex formation or ion pairs. It is thus necessary to specify not only the pH scale used in the evaluation of the dissociation constants [7], but also their dependency on ionic strength, temperature, and pressure.

Materials and Methods
Commercial grade Mg(OH) 2 (60 g) and analytical grade (14 g) BaSO 4 were mixed with deionized water and made up to volume (2 L). Bottled CO 2 was used for dissolving Mg(OH) 2 . Batch studies were carried out using a completely-mixed pressurized reactor ( Figure 2). It consisted of a 3 L reaction vessel equipped with a BirCraft stirrer, temperature sensor, pressure gauge, pH and conductivity control sensors. This unit was designed to handle pressures up to 10 bar and a maximum temperature of 150°C. The main body of the reactor was constructed from a Class 12 uPVC pipe with a wall thickness of 3.2 mm and an internal diameter of 560 mm. The height of the main body was 520 mm with a uPVC base and top plates that were each 65 mm thick. The dosage points were 300 mm above the effluent off-take point that was fitted at the bottom of the reactor. Calcium carbonate and barium sulphate were first fed into the reactor from the top and deionized water was pumped into the reaction vessel with a Watson-Marlow pump with continuous stirring. Carbon dioxide was dissolved in water, under pressure in the reactor, to lower the pH to around 6 and increase the pressure to desired levels. A pH control unit was used to control CO 2 dosage by monitoring the pH. At the end of the reaction the solution was filtered under pressure and the filtrate released from the pressure vessel through a valve.
The total dissolved carbonate species were measured by titrating the sample with standard hydrochloric acid to pH 4.5. The volume of acid was used to calculate the total alkalinity of the sample. From this calculation, the carbonate and hydroxide species concentrations were calculated, and the total carbonate species concentration (as mg/L CaCO 3 ).
Magnesium hardness was determined by titration with EDTA (0.02 M) using P & R indicator and NaOH buffer. Total hardness was determined with EDTA (0.02 M) using EriochromeBlack T indicator and ammonia buffer.
Barium concentrations were determined by titrating the sample with standard 0.02 M EDTA using methylthymol blue indicator, potassium nitrate indicator and NaOH buffer (for pH 12).
The pH, conductivity and pressure in the reactor were measured directly.  [8,9].

Separation of Mg(OH) 2 and BaSO 4
The mixture of Mg(OH) 2 and BaSO 4 sludge produced by the TUT-MBO process cannot be separated by solubility differences due to the low solubility of both Mg(OH) 2   Ba). As CO 2 is produced as a waste product in the TUT-MBO process, it was decided to investigate whether Mg(OH) 2 could be separated from the almost insoluble BaSO 4 by dissolving it as Mg(HCO 3 ) 2 (Equation1), by contacting the sludge mixture with CO 2 . Figure 3 shows that 60 g/L Mg(OH) 2 dissolved partially when contacted with CO 2 at 1 atm. . It also showed the ionic balance between the measured calcium concentration and the total alkalinity of the system.
Over the same period the pH dropped from 9.6 to 7.6. The dissolution of Mg(OH) 2 increased with decreased pH due to the increased formation of soluble Mg(HCO 3 ) 2 . Therefore, CO 2 dosing lowers the pH, and magnesium hydroxide is converted to Mg(HCO 3 ) 2 .
Alkalinity was used to monitor the formation of Mg(HCO 3 ) 2 and includes the parameters listed in Equation (4). As the system became enriched in CO 2 , the extent of dissolution decreased as a function of changes in the Mg(OH) 2 saturation state to yield both magnesium ions and Alk (Equation 1). (4) Figure 4 shows the contrast between the solubilities of Mg(OH) 2 (21100 mg/L as CaCO 3 ) and BaSO 4 (50 mg/L as CaCO 3 ). The results showed that, as expected, BaSO 4 does not dissolve when contacted with CO 2 and consequently there were negligible losses of BaSO 4 due to dissolution.

Alk = 2[CO
The effect of BaSO 4 on the rate of formation and solubility of Mg(HCO 3 ) 2 was studied. Figure 5 showed that BaSO 4 had no effect on

Effect of temperature on solubility
In the previous section it was shown that Mg(OH) 2 can be dissolved by the formation of Mg(HCO 3 ) 2 , through CO 2 addition without affecting the low solubility of BaSO 4 . The sludge separation process will be more effective at higher solubility values for Mg(HCO 3 ) 2 . Therefore, it was decided to determine the effect of temperature on the solubilisation of Mg(OH) 2 as Mg(HCO 3 ) 2 . According to Henry's law (Equation 5), it was expected that the solubility should increase with decreasing temperatures. Figures 6 and 7 confirmed the validity of Henry's law. In the case of Figure 6, only Mg(OH) 2 was present in the slurry used, whilst in Figure 7, BaSO 4 was also present. As in the previous case, it was found that BaSO 4 had no effect on the solubilisation of Mg(OH) 2 when contacted with CO 2 . These figures also showed that the concentration of Mg 2+ in solution increased to the maximum value in the first 20 minutes and stabilized for another 20 minutes and then fell to a final stable level. This implied that at a high concentration of 60 g/L, not all of the magnesium hydroxide will be carbonated to form Mg(HCO 3 ) 2 . P gas = kC (at constant T) Where, P = gas partial pressure (Pa) k = Henry's law constant (Pa m 3 mol) C = concentration of the gas (mol/L) Alaee et al. [10] showed that the air/water Henry's Law constant (K) is defined as the ratio of the concentration of a chemical in the gas phase to its concentration in the aqueous phase.
Where, K is in Pa.m 3 mol; P, is partial pressure (Pa) and C, is aqueous concentration (mol/L).
The temperature effect on the Henry constant K can be expressed as: Where T is temperature in K, A and B are constants of the Van't Hoff equation.
Ten-Hulscher et al. [11] showed that B is the ratio of the enthalpy of volatilization to the gas constant, ΔH o /R in K -1 , and A is the ratio of the entropy of volatilization to the gas constant ΔS o /R, resulting in a dimensionless value. Table 1 shows a comparison of the predicted and determined values for the effect of various parameters on the solubilisation of Mg(OH) 2 and BaSO 4 in a CO 2 -rich solution. The predictions were done using the Visual MINTEQ [8,9] model. The model was designed to simulate equilibrium and speciation of inorganic solutes in natural waters.

Measured versus predicted solubility values
Temperature and sulphate: Figures 6 (experiment 1) and 7 (experiment 2) compare the solubility of Mg(HCO 3 ) 2 at 1 atm CO 2 when no BaSO 4 and 14 g/L BaSO 4 , respectively, were present over the temperature range 0-45°C. Both sets of results showed that the final "solubility" of Mg(OH) 2 increases with decreasing temperature. The measured "solubility" values for Mg(OH) 2 when contacted with CO 2 were lower than predicted by Visual Minteq. This can be ascribed to the high concentrations in solution which exceeded the model's operation range.
The results of experiment 2 in Table 1 show the predicted values for Ba 2+ at 0°C in the treated water. The Ba 2+ concentration increased from 6.4 to 24.3 mg/L for a temperature change of 45°C. It is preferred to have Ba 2+ concentrations of less than 2 mg/L in treated water as this is within the expected range in natural waters. This can be achieved by allowing a low residual sulphate concentration of at least 10 mg/L in the treated water. The residual sulphate ion will act as a common ion which decreases barium solubility. • Mg(OH) 2 had a high solubility of about 22 700 mg/l when in contact with CO 2 at 1 atm, while BaSO 4 is almost completely insoluble.
• The solubility of Mg(OH) 2 increases with decreasing temperature and increasing pressure.
• The Visual Minteq model was a powerful tool to predict the "solubility" of Mg(OH) 2 and BaSO 4 when contacted with CO 2 .
Thus, the TUT-MBO process offers a sustainable method for neutralization, metal removal and desalination of AMD and recovery of saleable and reusable products from the mixed sludge produced. Because Mg(HCO 3 ) 2 has high solubility at low temperatures and high pressure, the practical optimal operation conditions for the dissolution reaction are temperatures close to 0°C and atmospheric pressure. This is because high pressures need sophisticated reactor designs and the systems are more complicated to run that those at atmospheric pressure conditions. Pressure: Rukuni at al. [12] showed that the solubility of Ca(HCO 3 ) 2 is influenced by pressure. It was also planned to investigate the effect of pressure on Mg(HCO 3 ) 2 solubility. Due to the high solubility of Mg(HCO 3 ) 2 at ambient pressure and temperature this was seen as less practical in the context of the process being developed. The high concentration also does not allow the use of the Visual Minteq [8,9] model to predict the solubility at various pressures.

Cost
The process cost for this process is low because all the process raw materials (CO 2 and the Mg(OH) 2 -BaSO 4 sludge) are waste products of the TUT-MBO process (Figure 1). It is also foreseen that the cost can also be kept low in other applications by producing CO 2 on-site by burning coal and scrubbing in water than the purchase of pure CO 2 .

Conclusions
It was found that: • Mg(OH) 2 can be separated from BaSO 4 and Mg(OH) 2 in a mixed sludge by carbonating it to Mg(HCO 3 ) 2 using CO 2 .
• The dissolution rate of Mg(OH) 2 in the presence of CO 2 is fast in the initial 20 minutes of the reaction.