Production and utilisation of Bayer Process Red Sand™ for construction and its beneficiation by magnetic separation

Abstract

The extraction of alumina from bauxite produces a high volume by-product which is stored in secure impoundments. In Western Australia, typically half of the by-product has a particle size in excess of 90 microns and is called Red Sand™ by Alcoa of Australia Limited (Alcoa). This coarse fraction is unique to Western Australia due to the high quartz content of bauxite from the Darling Range. Alcoa has three Western Australian refineries and this coarse fraction represents a potential resource of up to 20,000 tonnes per day.Red Sand™ currently requires storage in large and expensive impoundment sites due to the entrained sodium hydroxide. This is both an expensive cost upon the production facility as well as a wasted resource to the community. A variety of uses for coarse bauxite residues have now been evaluated. Primarily, Red Sand™ has been deemed to have physical properties suitable for use as construction sand because it is little different from fine crushed rock (which it actually is). Some of the uses now evaluated include use as a general fill material, in concrete, use in geopolymers, for topdressing of turf, for drainage improvement of turf, as lower sub-grade base in road construction and as the valuable sub-base in road construction. The normal use of sand in Western Australia is about 9 million tonnes per annum. Significant infrastructure projects, however, could require several multiples of this; whereas the production of Red Sand™ could be as high as 20 million tonnes per annum. Clearly new markets for this material will be required to take advantage of this unutilised resource if it is to be fully commercialised.One method for opening markets and improving product value is to apply magnetic separation techniques. Magnetic separation of Red Sand™ has yielded a material that may be suitable for iron production and another that may be suitable for concrete manufacture. Other fractions appear to have new opportunities but are not covered here. Initial separations tended to provide a band of sand that resists separation and totals about 30% of total volume. Operational parameters and investigations were required to reduce or remove this fraction.This thesis looked at the assessment of sand sources and the production of clean sand for various applications. This was an iterative process as the production of clean sand (Red Sand™) ultimately required a process that would allow subsequent magnetic separation if required. The production of clean sand was tested in the laboratory and then on a 10t/hr pilot plant. Product from this plant was then used in the construction of a trial section of road.Assessments were also made on the magnetic separation products to remove alumina, both for recovery to the alumina process, but also to improve product quality of the high iron fraction.