Modification of dredged sediments to produce useful product by heating to high temperatures

The disposal of large quantity of dredged sediments on land has long been an issue, particularly when significant contaminants are present. Heating fine-grained dredged sediments to high temperatures can convert these problematic sediments into materials that can be used beneficially. It can both improve considerably the geotechnical engineering properties of the sediment, and neutralise contaminants. Furthermore, this process can reduce the amount of sediment disposed of on land. The sand, silt, and clay fractions of dredged sediments are essentially different in their mineralogical composition, becoming more complex with decreasing particle size. The clay fraction has the greatest influence on the effects of thermal stabilisation. Consequently, a broad knowledge of the quantities and types of sediments produced and the range of their geotechnical engineering properties and their contaminants is a prerequisite for successful, cost-efficient modification of dredged sediments by heating. Numerous alternative methods of soil stabilisation have been used with dredged sediments, but none offers the range of benefits afforded by thermal stabilisation. It is the most reliable method of soil improvement, and the only method that gives immediate results. The laboratory and industrial-scale research presented here shows that there is definite potential for dredged sediment to be incorporated cost-effectively in a variety of industrial manufacturing processes, and in engineering applications following high-temperature heat treatment. Potential products include sand and gravel for road base, solid fill and bricks, among others. Firing fine-grained marine or riverine sediments to high temperatures can convert them to good quality sand and gravel with high frictional shear strength and high permeability. Artificial gravel has been produced that satisfies the Queensland Main Roads Department requirements for use as Subtype 2.5 unbound pavement material. Heat treatment destroys or stabilises organic contaminants such as tributyltin, common salt, which is converted into other minerals when heated to high temperatures, and other inorganic contaminants. Although the cost of heat treatment can be higher than that of other common treatment methods, this can be offset by the sale of the end products. No other single treatment method is capable of both improving the engineering properties of dredged sediment and eliminating or immobilising its contaminants. The industrial-scale testing verified that the use of sediment as raw material for brick manufacturing is possible and is not only the cheapest way to recycle the dredged sediment, but also replaces source raw material for brick manufacturing. Dried sediment in reclamation ponds can now be recycled easily and maintenance costs are thereby reduced. The use of recycled sediment as raw material for brick manufacturing also reduces the cost of raw materials by about 20%. It is thus a long-term partial solution to problems faced by both port authorities and brick manufacturers. Analytical and numerical solutions of the desiccation of the dredged sediment disposed of on land have been presented that enable the moisture content profile of the sediment at particular time to be estimated. However, further validation is necessary. The solutions, when fully validated, will provide a useful tool for the management of dredged sediment in disposal ponds and for the desiccation of sediments prior to thermal stabilisation processes. The estimated direct cost of thermal stabilisation is competitive with that of alternative methods, and it should be possible to reduce the overall cost of sediment disposal by the sale of stabilised material for beneficial uses, and the reuse of existing sediment holding ponds. Overall, it has been shown that thermal stabilisation has the potential to reduce considerably the problem of the disposal of ever-increasing volumes of dredged sediments that have problematic engineering properties and/or are significantly contaminated.