Synthesis of high-purity precipitated calcium carbonate during the process of recovery of elemental sulphur from gypsum waste.

We recently showed that the production of elemental sulphur and calcium carbonate (CaCO3) from gypsum waste by thermally reducing the waste into calcium sulphide (CaS) followed by its direct aqueous carbonation yielded low-grade carbonate products (i.e. <90 mass% as CaCO3). In this study, we used the insight gained from our previous work and developed an indirect aqueous CaS carbonation process for the production of high-grade CaCO3 (i.e. >99 mass% as CaCO3) or precipitated calcium carbonate (PCC). The process used an acid gas (H2S) to improve the aqueous dissolution of CaS, which is otherwise poorly soluble. The carbonate product was primarily calcite (99.5%) with traces of quartz (0.5%). Calcite was the only CaCO3 polymorph obtained; no vaterite or aragonite was detected. The product was made up of micron-size particles, which were further characterised by XRD, TGA, SEM, BET and true density. Results showed that about 0.37 ton of high-grade PCC can be produced from 1.0 ton of gypsum waste, and generates about 0.19 ton of residue, a reduction of 80% from original waste gypsum mass to mass of residue that needs to be discarded off. The use of gypsum waste as primary material in replacement of mined limestone for the production of PPC could alleviate waste disposal problems, along with converting significant volumes of waste materials into marketable commodities.

Conversion of calcium sulphide to calcium carbonate during the process of recovery of elemental sulphur from gypsum waste.

The production of elemental sulphur and calcium carbonate (CaCO3) from gypsum waste can be achieved by thermally reducing the waste into calcium sulphide (CaS), which is then subjected to a direct aqueous carbonation step for the generation of hydrogen sulphide (H2S) and CaCO3. H2S can subsequently be converted to elemental sulphur via the commercially available chemical catalytic Claus process. This study investigated the carbonation of CaS by examining both the solution chemistry of the process and the properties of the formed carbonated product. CaS was successfully converted into CaCO3; however, the reaction yielded low-grade carbonate products (i.e. <90 mass% as CaCO3) which comprised a mixture of two CaCO3 polymorphs (calcite and vaterite), as well as trace minerals originating from the starting material. These products could replace the Sappi Enstra CaCO3 (69 mass% CaCO3), a by-product from the paper industry which is used in many full-scale AMD neutralisation plants but is becoming insufficient. The insight gained is now also being used to develop and optimize an indirect aqueous CaS carbonation process for the production of high-grade CaCO3 (i.e. >99 mass% as CaCO3) or precipitated calcium carbonate (PCC).

Physicochemical properties and nutritional quality of raw cereals for newly weaned piglets.

The digestibility of the starch component of raw cereals in newly weaned piglets is highly variable. Reasons for this must be elucidated if the most suitable cereals are to be used. A novel approach was employed, which consisted of assessing the physicochemical properties (rapid visco analysis, water absorption and solubility indices, particle size distribution and in vitro amylolytic digestion) of eight raw cereals contained within piglet diets and subsequently relating this in vitro data to the biological responses of weaned piglets. Trial 1 examined soft and hard wheat, trial 2 - soft wheat, barley, rye and triticale, and trial 3 - soft wheat, naked oats, whole oats and maize. The initial observation was that in vitro testing prior to animal trials is recommended in nutritional evaluation since it indicated fundamental differences between raw cereals, such as for example the levels of endogenous amylase in wheat. Starch and nitrogen digestibility differed between cereals (apparent digestibility coefficients at the 0.5 site of the small intestine ranged from 0.10 to 0.69 for starch and from 0.17 to 0.68 for nitrogen). There is also a probable relationship between the coefficients of ileal apparent starch digestibility, at approximately halfway from the gastric pylorus to the ileocaecal valve, and the presence of endogenous amylase (mean values of 0.53 and 0.62 in trials 2 and 3, respectively, for the higher amylase wheat; 0.38 for the low-amylase wheat used in trial 1). This additional variable (i.e. the unforeseen presence of endogenous amylase) in wheat made it more difficult to draw a firm conclusion about the nutritional suitability of the different cereals. All raw-cereal diets caused atrophy of the villi during the initial week following weaning, but the soft wheat was associated with the highest comparative villi height and might therefore be considered the best of all raw cereals in minimising the post-weaning growth check. For wheat, this might also suggest a possible interaction between villus architecture and endosperm texture in the immediate post-weaning period.

Physicochemical changes to starch granules during micronisation and extrusion processing of wheat, and their implications for starch digestibility in the newly weaned piglet.

Two trials were performed to assess changes in the physicochemical properties of precisely processed (micronised v. extruded) wheats, prior to inclusion in piglet diets. The in vitro data obtained were subsequently related to biological responses of newly weaned piglets over 14 days. The effects of the severity of micronisation (Trial 1) or extrusion (Trial 2) on the nutritional value of two wheats (varying in endosperm texture) were examined. Extrusion, in contrast to micronisation, drastically disrupted the structural properties of wheat starch granules through melting of crystallites and macromolecular degradation of starch polysaccharides. These structural changes strongly improved the hydration characteristics of starch and its digestibility. The amount of starch digested in vitro was approximately 0.20, 0.70 and 0.90 for the unprocessed, micronised and extruded samples, respectively. This enhanced in vitro digestibility correlated well with, and helped to explain, the significant improvement in the apparent digestibility of starch at both the 0.5 region (mean coefficients for extruded wheat were 0.869 and 0.959 v. raw 0.392; P = 0.017) and 0.75 region (extruded 0.973 v. raw 0.809; P = 0.009) of the small intestine, when compared with piglets fed an unprocessed wheat diet. Extrusion and, to a lesser extent, micronisation lessened the reduction in apparent starch digestibility on day 4 post-weaning, typically seen at the 0.5 intestinal region in piglets fed an unprocessed wheat diet. Processing variables influenced both in vitro and in vivo data, with for instance, a positive relationship between specific mechanical energy (SME) input during extrusion and starch digestibility at the 0.5 region. The higher digestibility coefficient observed at the 0.5 region for the high SME diet suggests enhanced digestion and more rapid release of starch. However, it remains to be seen whether a diet containing rapidly digestible, as opposed to slowly digestible, starch is more beneficial for piglets. This rate of starch breakdown in the piglet is an important finding, which may have implications in helping to alleviate the post-weaning growth check, particularly in the absence of in-feed antibiotic growth promoters. Processing did not appear to offer any benefit over unprocessed wheat with regard to daily live-weight gain or the apparent digestibility of nitrogen in the small intestine over the 14-day period. Based on the enhanced in vivo starch digestibility, performance might be improved over a longer period, although future studies are required to confirm this. Precise processing variables for raw materials must be stated in all animal trials.

Direct and indirect identification of the formation of hydroxyaluminosilicates in acidic solutions.

Morin-aluminium fluorescence and membrane filtration were successfully applied to the indirect identification of the formation of hydroxyaluminosilicates (HAS) in acidic solutions of varying pH and of known concentrations of aluminium (Al) and silicic acid (Si(OH)(4)). It was proven to be especially useful in providing evidence of the strong competition between Si(OH)(4) and Al(OH)(3) to condense with hydroxyaluminium templates to form HAS in preference to Al(OH)(3(s)). The aggregation and stability of HAS and Al(OH)(3(s)) were dependent upon both the pH and the [Al] of the solution. The applicability of these indirect techniques was confirmed using the direct observation of HAS in solution by atomic force microscopy (AFM). AFM was also a powerful tool in providing valuable information on the morphology of colloidal HAS of various structures and stoichiometries. The results have provided further confirmation of both the mechanism of HAS formation and the form and stability of HAS in solution. This information is essential to our understanding of the biological availability and hence toxicity of Al in biota, including man.