| One metal whose growth in the past century has been unsurpassed is aluminium. Its strength and light weight guarantees its demand, especially in transportation where fuel efficiency is paramount. The Bayer Process The alumina process used today is essentially the one invented by Karl Bayer in 1887. This uses sodium hydroxide to selectively dissolve the Al203. This produces a sodium-aluminium solution from which pure Al(OH)3 'S precipitated, later to be calcined to produce oxide, from which the metal is recovered. The remaining iron, titanium, sodium, silica and other impurities are all discarded. This waste product is known from its oxidised iron content as red mud, and because it has been subjected to sodium hydroxide treatment, is highly caustic with pH values in excess of 13.2. Consequently it gives rise to storage problems, is an on-going environmental liability and an uncertain cost overhang. Dealing with the Waste Products – Red Mud The cost of this waste management can be ameliorated if it is turned into a useable and saleable product. This logic has been applied by an Australian company, Virotec, to the treatment and disposal of red mud. Virotec scientists looked at the problem from the perspective of how best red mud could be neutralised, and if possible, turned into a usable product. To neutralise red mud and turn it into a saleable product, only those neutralisation procedures that involve the conversion of basicity (mainly sodium hydroxide) and soluble alkalinity (mainly sodium carbonate) into alkalinity are worth considering. Virotec’s answer was to mix the red mud with seawater. Neutralising Red Mud When seawater or other Ca- and Mg-rich brines (e.g. salt lake brines) are added to caustic red mud, the pH of the mixture is reduced causing hydroxide, carbonate or hydroxycarbonate minerals to be precipitated. The neutralising effect of the calcium and magnesium ions is initially large but decreases rapidly as complete neutralisation is approached. Neutralisation is considered to be complete when the liquid that can be separated from the treated red mud has a pH less than 9.0 and a total alkalinity less than 200 mg per litre (as calcium carbonate equivalent alkalinity) such water can be safely discharged to the marine environment. Products from Red Mud Neutralisation All the precipitated minerals are low in solubility while still retaining acid-consuming capabilities, including the ability to reduce acid mine wastes. They also exhibit strong metal-binding characteristics. The Mechanics of Seawater Neutralisation The mechanics of seawater treatment are simple. The mud is actively mixed with the seawater for a period of around 30 minutes to enable the reactions to take place. Seawater is added until the liquid phase of the precipitates can be decanted and reduced in alkalinity from pH 9.5 to 9. To meet marine discharge standards the liquid is treated with acid to bring the pH to below 9. This would normally require 0.05 litres of concentrated sulphuric acid per 1,000 litres of red mud after treating with seawater. Alternatively, the liquid fraction can be solar evaporated to salts, with the remainder retained as a slurry or dried for further use. The Effect of Degrees of Neutralisation If red mud is completely neutralised, 15-20 times the volume of sea water is required. This requires extensive and expensive pumping and although results in an inert product, it is completely worthless. However, Virotec has found that slightly less than complete neutralisation can be achieved with much less seawater. For example, if untreated red mud has a pH of about 13.5 and an alkalinity of about 20,000 mg/l, the addition of about 5 volumes of average seawater will reduce the pH to between 9.0 and 9.5, and the alkalinity to about 300 mg/l. Neutralisation for Commercial Gain Although not sufficiently neutralised for marine or land disposal (this would take the addition of a further 8‑12 volumes of sea water) this initial treatment is the first step in the conversion of the mud into a useful and saleable product. Post mixing, the mud/sea water is allowed to settle into a liquid and solid liquid phases. The liquid phase can be brought to discharge standards by the addition of a relatively small amount of acid, equivalent to 0.05 litres per 1,000 litres of the original red mud. Without seawater treatment this value would rise to 17 litres per 1,000 litres of mud. What this means is that greater than 95% of the neutralising treatment can be completed using about one third of the amount of sea water required for complete treatment, thereby resulting in substantially reduced water handling and storage costs. Increasing the Efficiency of the Process Virotec scientists have extensively modelled the geochemistry of this treatment with waste mud from refineries worldwide. They found that if the salinity of the sea water can be raised to about 1.5 times the ocean average, then the process is more efficient, requiring only 3.5 litres per 1,000 litres of red mud, as opposed to 6 litres of average salinity sea water. This is easily achieved by solar evaporation in coastal ponds. Applications of the Produced Minerals The partly neutralised red mud produced will consist of a cocktail of very fine grained minerals (about 80% have a particle size of less than 10 microns) that includes hematite, boehmite, gibbsite, sodalite and quartz. The exact composition depends on the composition of the original bauxite, but none of the minerals are known to be environmentally hazardous. Irrespective of its exact composition, the mineral mixture has a high acid neutralising capacity (up to 7.5 moles of acid/kg) and it is this feature that makes it a useful commodity. It can be used as the raw material base for commercial reagents with acid neutralising capacities in excess of 15 moles of acid/kg. Partly neutralised red mud also has a very high trace metal trapping capacity (greater than 1,000 milliequivalents of metal per kg) and as such, it is now being used as the base of commercial reagents with metal trapping capacities in excess of 2,000 milliequivalents of metal per kg. It also has a high capacity to trap and bind phosphate and is an excellent flocculent. Commercial reagents based on these characteristics have widespread applications in the sewage treatment industry. Case Study – Metal Trapping Ability of the Bauxsol Technology This metal trapping ability forms the base of Virotec's Bauxsol technology. An example of its effectiveness can be seen in trials on mine water from the old Captains Flat mine, near Canberra in New South Wales. This mine operated during the latter half of the 19th century and left a residue of acid-generating waste from sulphide and gold mineralisation. The following contemporary report of the workings indicate the scale of the pollution. A tram line was built to dispose of waste materials. The smelter, built on the southern edge of the town between the Molonglo River and Jerangle Road, spewed smoke and soot every hour of the day and night and the red hot slag looked like lava as it oozed down the slopes. The mines yielded 860,795 ounces of silver, 3,781 tons of copper and 16,140 ounces of gold from 205,707 tons of ore, between 1887 and 1899. The vast copper yields made world headlines and attracted great interest, seemingly assuring Captains Flat as a major industrial centre.’ Table 1. The effect of treating water from Captain’s Flat with Bauxsol Technology reagents, by direct addition methods and by passing it through a permeable reactive barrier (PRB). Metal values are in µg/L. | | | pH | 2.8 | 8.2 | 8.4 | | Aluminium | 12340 | 4.8 | 3.1 | | Cadmium | 94 | 0.3 | 0.2 | | Copper | 314 | 4.4 | 1.3 | | Iron | 38120 | 0.9 | 0.8 | | Manganese | 10000 | 1027 | 31 | | Nickel | 60 | 22 | 1.4 | | Zinc | 122400 | 18 | 6.3 | Effect of Time
Equally importantly, the metals that are bound when the material is used to treat contaminated water are held very tightly and only a small proportion can be released even if the solid residue is leached at a pH of 2.88. Furthermore, the longer the residue is left after use, the more tightly the metals are held as new minerals are formed. For example, if the residue is left in a tailings dam after the completion of treatment, the metal concentrations in the water will continue to decrease for at least 12 months. Conclusion Virotec’s Basecon and Bauxsol technologies offer a process treatment and an environmental product. By neutralising the caustic liability resulting from alumina production a product can be made to resolve the worldwide problem of acid mine drainage and metal contamination of rivers. To quote Brian Sheeran, Virotec’s executive chairman, “As we enter the 21st century, environmentalism is no longer a philosophy preached by a passionate minority It is a core business objective. Poor environmental performance damages a company’s efficiency, its sustainability and its prosperity.” |