Recovery of sulphur and calcium carbonate from waste gypsum

Gypsum is produced as a waste product by various industries, e.g. the fertiliser industry, the mining industry and power stations. Gypsum waste disposal sites are responsible for the leaching of saline water into surface and underground water. The aim of this investigation was to evaluate a process for converting waste gypsum into sulphur and calcium carbonate. The process evaluated consisted of the following stages: reduction of gypsum to calcium sulphide; H2S-stripping and sulphur production. Thermal reduction showed that gypsum could be reduced to CaS with activated carbon in a tube furnace operating at 1 100 °C. The CaS yield was 96%. The CaS formed was suspended in water to form a CaS slurry. The reaction of gaseous CO2 with the CaS slurry leads to the stripping off of H2S gas and the precipitation of CaCO3. During batch studies sulphide was stripped from 44 000 mg/l to less than 60 mg/l (as S). The H2S generated in the previous step was then reacted in the PIPco process to form elemental sulphur


Introduction
Brine and sludge disposal represents a major fraction of the cost during treatment of mining effluents.Gypsum-rich sludge is produced during the following water treatment activities: • Neutralisation of acidic effluents with limestone or lime to produce gypsum and metal hydroxides (Geldenhuys et al., 2001) • Desalination of industrial effluents from the brine when it is saturated with respect to gypsum (Van Zyl et al., 2000).
Most of the gypsum waste produced by industry is unsuitable for further use, e.g.production of plasterboard.These stockpiles create environmental concerns such as airborne dust as well as effluent problems as gypsum is slightly soluble (2 000 mg/ℓ) in water.Therefore, a need exists to develop methods to convert poor-quality gypsum into a useful product, namely sulphur.Sulphur is an essential raw material for many manufacturing industries such as fertilisers, acids, rayon, steel, petroleum, insecticides, titanium dioxide, explosives, etc. (Cork et al., 1986).Catalytic and thermal reduction processes (Rameshni and Santo, 2005) and biological processes for sulphur recovery (Maree et al., 2004) are expensive, difficult to operate, have high fuel consumption and limited ability to control temperature and side reactions.
As far as the supply and demand for sulphur is concerned, Africa is a major importer of sulphur (Maree et al., 2005).Countries like Zambia and the DRC are importing large tons of sulphur at high cost to manufacture sulphuric acid for the reduction of oxidised ores.These costs are inflated by the cost of transportation whilst sulphur is a cheap product.South Africa alone imports 1.5 m. t of sulphur per year from the Middle East and Canada (Ratlabala, 2003).The demand for sulphur is also expected to increase in line with increased fertiliser usage and exports (Agnello et al., 2003).
In view of serious shortages of foreign exchange, it is becoming increasingly difficult for these African countries to import sulphur.Consequently, industries depending on the use of sulphur are facing shut-down unless other sources are identified.Most African countries have large amounts of waste gypsum generated by industrial activity.Even the costly sulphuric acid produced from imported sulphur mostly ends up as gypsum once used.Gypsum is a good source for the recovery of sulphur (Wewerka et al., 1982).
The recovery of sulphur and CaCO 3 from gypsum is economically feasible.From 1 t of gypsum it is calculated that 0.18 t of sulphur and 0.58 t of CaCO 3 can be recovered.From 1 t of gypsum, sulphur with a value of R180 can be recovered and CaCO 3 with a value of R116.The prices of sulphur and CaCO 3 were taken at R1 000/t and R200/t respectively.This compares favourably with the cost of the main raw material, coal.At a coal to gypsum ratio of 0.3, and a coal cost of R200/t, the cost of the coal amounts to R60/t of gypsum.This is significantly less than the combined value of R296 of sulphur and CaCO 3 .This value would be even higher if chemically pure CaCO 3 were to be recovered.The price of chemically pure CaCO 3 amounts to R3 000/t compared to the R200/t for waste or mined CaCO 3 .
The basic steps of the sulphur-recovery process from gypsum are: • Reduction of gypsum to calcium sulphide using reducing agents (Matsuya and Yamane, 1981), for example, coal or activated carbon (Eq.( 1)), carbon disulphide (Eq.( 2)), carbon monoxide (Eq.( 3)) and hydrogen (Eq.( 4)) • Suspending the calcium sulphide obtained from Eqs. ( 1) to (4) in water to form a CaS slurry.From this slurry, the H 2 S is stripped off with CO 2 (Eq.( 5)) and converted to elemental sulphur via the PIPco process (Eq.( 6)) In the PIPco process elemental sulphur is produced from SO 2 and H 2 S gas (Ray et al., 1990).Comparable processes are the sodium phosphate process and the sodium citrate process (Bekassy-Molnar et al., 2005).These processes utilise a buffer (sodium phosphate and sodium citrate respectively) to absorb SO 2 which is then used as an oxidising agent in the conversion of H 2 S to elemental sulphur.Due to solubility limitations these buffers only allow low concentrations of SO 2 in solution.However, in contrast, the PIPco process uses potassium citrate buffer solution to absorb SO 2 (Eqs.( 7) and ( 8)).The potassium citrate buffer allows the solution to dissolve high concentrations of SO 2 resulting in increased sulphur recovery.The H 2 S produced in Eq. ( 5) is bubbled through the SO 2 -rich buffer solution, initially forming intermediates such as S 2 O 3 2-(Eqs.( 9) to ( 11)), then elemental sulphur as per Eq. ( 6) (Gryka, 1992).
The overall reaction given by Eq. ( 6) consists of various steps with intermediate components (Eqs.( 7) to ( 12)): The aim of this investigation was to evaluate the various stages of the sulphur-recovery process on laboratory scale prior to full-scale implementation.Figure 1 shows the process flow-diagram of the gypsum treatment process.The following individual stages were studied: • Production of calcium sulphide from gypsum (A) • H 2 S-stripping with CO 2 and production of CaCO 3 (B) • Sulphur production (C).
Carbon: Activated carbon (Merck) with a carbon content of 100% was used as reducing agent.CO 2 : CO 2 gas (Air Liquide) was used for H 2 S-stripping.Potassium citrate solution: SO 2 -rich potassium citrate buffer solution was used for the absorption of the stripped H 2 S-gas.
The results of X-ray fluorescence analyses (ARL9400XP spectrometer) of the gypsum compounds are summarised in Table 1.

Equipment
Thermal studies: Tube and muffle furnaces were used for thermal decomposition of gypsum.A silica tube was used for the reduction reaction and samples were contained in silica boats.
Nitrogen gas was passed through the reaction tube as an inert gas in the tube furnace.In the muffle furnace no N 2 was used, therefore some oxygen was present.
H 2 S-stripping at atmospheric pressure studies: Figure 2 shows the set-up used for H 2 S-stripping.It consisted of three reactors connected in series, all equipped with glass spargers.The first reactor (1 ℓ) contained calcium sulphide slurry from which sulphide was stripped.The remaining two reactors (1 ℓ) contained an SO 2 -rich potassium citrate buffer solution into which H 2 S gas was absorbed and sulphur formed.Pressurised system for H 2 S-stripping studies: Figure 3 shows the 5 ℓ pressurised reactor, containing a hollow shaft stirrer capable of a maximum pressure of 300 kPa and a maximum operating temperature of 300°C, used in CaS stripping experiments.

Experimental procedure
Thermal studies: Stoichiometric amounts of gypsum (5 g) and a reducing agent (activated carbon, 1.05 g) were mixed.The mixtures were placed in silica boats and heated at elevated temperature (900ºC to 1 100ºC) in the tube furnace and muffle furnace for various reaction times.Reaction products from the furnace were allowed to cool in a nitrogen atmosphere.
H 2 S stripping at atmospheric pressure: The calcium sulphide product (approximately 60 g) from the thermal studies was dissolved in 2 ℓ of water and placed in the first reactor.The potassium citrate buffer solution dosed with SO 2 was placed in the second reactor.The CO 2 used to strip the H 2 S gas was introduced into the sulphide solution via a pump.The stripped H 2 S-gas was trapped in the potassium citrate buffer solution and converted to sulphur.

Pressurised system for H 2 S-stripping studies:
The CO 2 was fed into the pressurised & stirred reactor from the cylinder.The gas was allowed to flow at pressure through the hollow shaft, finned, mechanical stirrer and mixed with the calcium sulphide slurry.
The reactor was then pressurised to the desired experimental pressure with CO 2 fed from the cylinder.The stirrer was started and the off-gas valve was opened to the flow rate specific to each experiment.At the experimental pressure and stirring rate the gas in the headspace above the slurry was also sucked back into the slurry for further reaction.

Analytical procedure
The calcium sulphide and other compounds formed during the process were analysed using an automated Siemens D501 XRD spectrometer.The titration procedure to determine the concentration of sulphite (SO 3 2- ) and thiosulphate (S 2 O 3 2- ) was developed by Pfizer and is accurate to ± 0.01mol/ℓ (Gryka, 2005).The purity of sulphur recovered was analysed using the LECO Combustion Techniques.

Thermal studies
Table 2 shows the effects of various reaction parameters on the CaS yield during the thermal conversion of gypsum to CaS using pure gypsum.Effect of time: Expt. 1 (Table 2) showed that good conversion yields (> 96%) were achieved after a reaction time of 20min.
After a reaction time of 5 min, the yield was only 45%.
Effect of temperature: Expt. 2 (Table 2) showed that for a carbon: gypsum mole ratio of 3:1, after a reaction time of 20 min, the conversion percentage increased from 15% at 900 °C to 96% at 1 100 °C.This could be due to the high activation energy required for the reduction of calcium sulphate to calcium sulphide.

Effect of carbon: gypsum mole ratio:
Expt. 3 (Table 2) showed that when no carbon was added, no CaS was formed.The addition of 2 and 3 moles carbon, respectively, showed high percentage conversion of gypsum to calcium sulphide (88% and 96% respectively).

Effect of particle size:
Expt. 4 (Table 2) showed that the formation of the reduced product calcium sulphide, is dependent upon the particle size of gypsum, the smaller the particle size, the higher is the conversion under specific conditions.This can be ascribed to higher reactant surface areas for smaller particle size.
Table 3 shows the effects of various reaction parameters on the CaS yield during the thermal conversion of gypsum to CaS using synthetic gypsum: From Table 3, the XRD results showed that the use of pellets results in a 92% conversion, while the use of powder yielded a 90% conversion.Therefore the reactant mixture in the form of pellets instead of powder only increased conversion by 2%.The 2% difference between the pellets and the powder cannot be seen as significant.Pellets may have a larger accessible bulk surface area for heat transfer than the powder, but the powder has a higher overall surface area.
The results in Table 3 further showed that the tube furnace (92%) is more efficient in converting gypsum to CaS than the muffle furnace (80%).The presence of oxygen in the muffle furnace resulted in the formation of several oxygen-containing compounds such as MgAl 2 O 4 and Ca 2 Al 2 SiO 7 .However, the tube furnace purged with nitrogen does not favour production of oxygen-containing compounds.

Effect of CO 2 flow rate on the formation of intermediate compounds
Figures 4 to 7 and Tables 4 and 5 show the results, as well as the experimental conditions, when sulphide was stripped with 100% CO 2 from a CaS slurry, followed by absorption of the stripped H 2 S gas in a SO 2 -rich potassium citrate solution.The effect of CO 2 flow rate was investigated by conducting an experiment at 520 mℓ/min and at 1 112 mℓ/min.
Figure 4 shows the relationship between the concentrations of the various species vs. time when CO 2 was added at a flow rate of 520 mℓ/min.The initial CaS concentration in the slurry was 2 167 mmole/ℓ and the pH of the slurry was 12,2.CaS has a low solubility as indicated when zero CO 2 was added.During CO 2 addition the pH dropped from 12.2 to 8.2.The sulphide concentration in the slurry reactor reached a maximum concentration of 1 375 mmole/ℓ (44 000 mg/ℓ S) due to the formation of Ca(HS) 2 in solution (Eq.( 14)).The difference of 792 mmole/ℓ Ca(HS) 2 (2 167 to 1 375) was present as a solid due to its solubility.With further CO 2 addition, the pH dropped down to 6.9 and sulphide was stripped completely (to less than 60 mg/ℓ S) (Eq.( 15)).The stripped H 2 S reacted with the SO 3 2-in the SO 2 /citric acid reactors.The SO 3 2--concentration in Reactor 1 dropped sharply, while that in Reactor 2 dropped slowly.The fast drop in Reactor 1 can be ascribed to the formation of sulphur (Eq.( 17)) and possibly due to some of the SO 2 being stripped with CO 2 .The following parameters were kept constant: temperature = 1100 °C, mole ratio (carbon: gypsum) = 3: 1, time = 20 min, activated carbon and a tube furnace unless otherwise stated.

745
The slow drop in the SO 3 2--concentration in Reactor 2 can be ascribed to SO 2 -stripping with CO 2 .
Figure 5, shows the relationship between load removed or formed of the various parameters as a function of time.It is noted that: • 2 167 mmole CaS was initially slurried • 1 375 mmole of the formed Ca(HS) 2 was in solution and the balance as a solid as the solubility of Ca(HS) 2 was exceeded • 2 180 mmole SO 3 2-was removed, which is more than the CaS that had been slurried.This shows that a portion of the SO 2 is stripped with CO 2 .This observation explains why the Pipco process needs to be operated under excess H 2 S-conditions.
This finding is of importance as it shows that Ca (HS) 2 can possibly be recovered as a valuable product.The experiment described above for 520 mℓ/min CO 2 was repeated for a CO 2 flow of 1 112 mℓ/min (Figs. 6 and 7 and Table 5).Similar conclusions were made except for the behaviour of SO 3 2-in the SO 2 /citrate reactor.The following similar observations were made: • 2 167 mmole CaS was initially slurried • 1 300 mmole remained in solution as Ca(HS) 2 , and the balance was in solid form as Ca(HS) 2 • 2 310 mmole SO 3 2-was removed which is also more than expected from the amount of CaS that was slurried.This shows that a portion of the SO 2 is stripped with CO 2 .
The following different observations were made for the different CO 2 flow rates: The increase in SO 3 2--concentration during the initial period (Fig. 6) can be ascribed to the formation of an intermediate compound when H 2 S is contacted with the SO 2 / citrate solution.The intermediate compound is oxidised to sulphate from a much lower valence (valence of S species) when contacted with iodine, compared to SO 3 2-, which has a valence of +4.The intermediate could be S 3 O 4 2-, with a valence of +2.Reaction 18 shows the reaction of S 3 O 4 2-with iodine.This was determined by way of elimination of the reactions with iodine of the various sulphur species.• S 2 O 3 2-is oxidised to S 4 O 6 2-(Eq.(13)) and the latter will not be further oxidised with iodine • SO 3 2-is oxidised to SO 4 2-(Eq.( 12))  flow rates (Fig. 8).This was expected and is also confirmed by the results reported in the previous section.• The rate of sulphide stripping increased with increased temperature (Fig. 9).The results indicate that at higher temperature, more CaS dissolves into solution as Ca(SH) 2 and more sulphide is stripped off with CO 2 gas.At lower temperature less CaS dissolved therefore and low sulphide was stripped.
• The rate of sulphide stripping increased with increasing stirring rate.The finding could be due to the higher rate of CaS dissolution at faster speed (Fig. 10).• The rate of the sulphide stripping increased with a decrease in pressure (Fig. 11).This can be attributed to the solubility of CO 2 gas and H 2 S gas that increased at an increased pressure.

Conclusions
The reduction of gypsum to CaS takes place at a temperature of 1 100°C.A good conversion was obtained when a reducing agent was used.Controlling the amount of carbon added, relative to the amount of gypsum, higher reduction was achieved when the molar ratio gypsum to carbon was 1:3.The smaller particle size of gypsum yielded higher reduction percentages due to the higher reactant surface areas for smaller particles.The reaction time between gypsum and carbon was also found to be shorter.The optimum time found was 20 min.The use of pellets resulted in better conversion of gypsum to CaS than the use of powder mixture.Depending on the presence of oxygen in the muffle furnace, the reaction mixture obtained after heating at 1 100°C consisted of oxygen-containing compounds.The tube furnace which had been purged with nitrogen yielded no oxygen-containing compounds.The thermal decomposition of gypsum to CaS should, therefore be carried out in an oxygen-deficient environment.
The H 2 S stripping studies led to the following conclusions:  H 2 S gas can be stripped with CO 2 from a CaS slurry with the simultaneous production of CaCO 3  Sulphur can be produced from the stripped H 2 S. Sulphur with a purity of between 95 and 99% was produced.
Figure 1 Process flow-diagram for the sulphur-recovery processMaterials and methodsFeedstockGypsum: Pure CaSO 4 .2H 2 O (Merck, AR grade) and waste gypsum from Landau Colliery prepared from the desalination stages of a mine-water treatment pilot plant were utilised in the reduction experiments.

Figure 2
Figure 2 Schematic diagram of H 2 S-stripping and absorption process

Figure 3
Figure 3 Picture of the 5 ℓ jacketed, pressurised & continuously stirred reactor used in CaS stripping experiments give a titration Value B. The difference between Values A and B (C) is equivalent to the SO 3 2-concentration.

Figure 5
Figure 4Sulphide stripping with CO 2 at a flow rate of 520 mℓ/min (concentrations vs. time)

Figure 7
Figure 6Sulphide stripping with CO 2 at a flow rate of 1 112 mℓ/min (concentrations vs. time)

Figure 9
Figure 8Effect of CO 2 flow rate on H 2 S-stripping

TABLE 2 Results of XRD analyses of the reaction products of carbon and pure gypsum Expt. number Parameter Value Percentage CaSO 4 CaS CaO
Available on website http://www.wrc.org.zaISSN 0378-4738 = Water SA Vol.33 No. 5 October 2007 ISSN 1816-7950 = Water SA (on-line)

of CO 2 flow-rate, temperature, pressure and hydrodynamicsTable 6 and
Figs. 8to 11 show the effect of various parameters on the rate of sulphide stripping when carried out in a pressurised unit.It was noted that:• The rate of sulphide stripping increased with increasing CO 2