Technical Market Support » Future Technologies
Coal production and export are important to the Australian economy. The economic benefits of coal production in Australia also come with a number of sustainability challenges. One of the key sustainability challenges is the generation and accumulation of waste and the associated environmental issues. A practical way to reduce waste is to turn it into value added products for beneficial uses. In this project, three approaches to utilising the coal handling and preparation (CHPP) waste in generating value added products were examined.
The first CHPP waste utilisation pathway examined involved the use of the washery waste in producing geopolymer concrete products for construction purposes. This part of the study examined the use of coal mine tailings as source material in alkali activated binder for geopolymer concrete production. It also studied the suitability of CHPP coarse and fine rejects as substitutes for aggregates and sand respectively in geopolymer concrete production. The work involved the formulation and testing of geopolymer mixes for their setting behaviour, workability and strength development. These geopolymer properties were assessed using equivalent ordinary Portland cement concrete as benchmark. It was determined that coal mine tailings with appropriate pre-treatment is a suitable precursor material for geopolymer binder formulations in terms of workability and strength development using a mixture of concentrated sodium hydroxide solution and sodium silicate solution as the alkali activator. The optimised 7-day compressive strengths of the geopolymer binder and concrete produced by alkali activation of coal mine tailings were comparable to N32 ordinary Portland cement (OPC).
The second CHPP waste utilisation pathway studied involved the use fine coal tailings to produce paste using appropriate type and amount of low cost binders to achieve the desired mechanical properties for the structural support of the longwall goaf area. This was achieved through utilisation of Basic Oxygen Steelmaking (BOS) slag as a replacement binder in cement. Testing was performed to assess the viability of this use. BOS slag has analogous properties to that of Portland cement and has a high lime content. The utilisation of this material can help in using this waste material to reduce the energy requirements and cost in the production of a cement when making concrete. Various percentage replacements of the cement with the BOS slag were tested to validate the viability of the material in a cement mixture and to determine the optimum design of this cement BOS slag binder. It was also shown that aggregate and sand could be partially replaced by coarse rejects of the same size and fine coal wash tailings respectively. Approximately 50% aggregate could be replaced without the significant loss of compressive strength.
The third approach to the utilisation of CHPP waste in making value added products that was examined in this project was the use of washery rejects as fill material in novel hybrid fibre reinforced polymer (FRP) support for longwall tailgates. Strata control underground is mainly concerned with maintaining the integrity of underground roadways. As a critical component of underground support systems, secondary standing support is important and usually applied together with bolting system. Among them, the tubular standing supports are the most common due to their ease of construction and cost effectiveness. The novel FRP tubular standing support system for tailgates examined can serve the twin purposes of reducing the amount of waste to rehabilitate and reducing the impact of subsidence on surface structures. The use of infill materials made from industry by-products such as coal rejects instead of conventional cementitious grout material in standing support not only reduces the total cost of the standing support but offer significant environmental benefits. A series of experimental tests, numerical simulation as well as theoretical analysis were conducted, from which the superior compressive behaviour of FRP tubular standing supports with their strain hardening behaviour has been demonstrated. The direct use of unscreened coal rejects together with high flowable cementitious grout material in FRP tubular standing support (FTSS) and FRP-PVC tubular standing support (FPTSS) columns not only simplifies the manufacture procedure, but also show large axial deformation ability under compression. Test results showed that all these FRP tubular standing supports have a strain-hardening behaviour, which are much different from conventional pumpable standing supports in the market. In addition, the developed backfill material made of coal rejects and high water-content cementitious grout material can also be used in backfill for underground mines or be filled into the steel tubular container to generate the modified CAN®support.