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New technologies to allow beneficial reuse of silt from construction and demolition waste recycling washing plant - WR0204

Description
Fine silt particles are generated as filter cake from construction and demolition (C&D) waste and aggregate recycling plants that use washing systems to produce high grade product. Plants exist that produce up to 80 tonnes of silt per hour and it is estimated that at least 2.5 million tonnes of this silt is produced each year in the UK. This is removed as a waste filter cake and is normally landfilled or disposed of in lagoons. This seriously undermines the economics of C&D waste recycling and is particularly problematic when plants are operating in urban areas. The objective of this research is to develop innovative processing that can be used at C&D aggregate recycling plants that allows the silt to be reused as aggregate. This has policy relevance as the commercial viability of many C&D waste recycling is dependent on solving this problem. The aim is to produce viable aggregate containing high levels of silt by mixing and extruding a new material containing cement and polymer, allowing silt to be processed into construction products and avoiding the transport and landfill/lagoon disposal of waste.

The aim is to develop a commercially viable process to transform silt generated from C&D waste recycling plants using washing systems into viable aggregate. The work will optimise a process based on mixing the silt filter cake with cementitious binders and polymer. This will involve high shear mixing and extrusion, to form aggregate particles that will rapidly harden. The purpose of this is to beneficially reuse the silt and thereby improve the commercial viability of C&D waste washing recycling plants. Defra funding will enable unique collaboration between diverse partners with different skills and experience to undertake industrially relevant research. This group would not come together without Defra funding and this will accelerate transfer of the technology developed into the market place. The key industrial collaborators are SMEs with limited resources. They would not be able to support this type of research programme without external funding that is also required to offset the financial risk.

The aim is to develop a commercially viable process to transform silt generated from C&D waste recycling plants using washing systems into viable aggregate. The work will optimise a process based on mixing the silt filter cake with cementitious binders and polymer. This will involve high shear mixing and extrusion, to form aggregate particles that will rapidly harden. The purpose of this is to beneficially reuse the silt and thereby improve the commercial viability of C&D waste washing recycling plants. Defra funding will enable unique collaboration between diverse partners with different skills and experience to undertake industrially relevant research. This group would not come together without Defra funding and this will accelerate transfer of the technology developed into the market place. The key industrial collaborators are SMEs with limited resources. They would not be able to support this type of research programme without external funding that is also required to offset the financial risk.

With closed loop water treatment plants associated with aggregate washing, lagoons are not required. The dirty water is fed into a thickener tank and anionic flocculent is injected. The small particles are attracted together and sink to the bottom of the tank. The clean water is displaced over a weir at the top of the thickener and is collected in a large tank to be reused in the washing plant. The thickened sludge from the bottom of the thickener is pumped to either a plate press or belt press where the majority of the water is pressed out and a manageable cake is dropped on the ground.The silt filter cake produced in urban areas has to be disposed of to landfill. The cost of this is approximately £3 per tonne plus haulage. If the silt is contaminated in any way the cost is currently £20 per tonne plus £18 per tonne tax plus haulage. This can have a major impact on the commercial viability of aggregate recycling plants.

The WRAP Aggregates Programme funded by Defra from the Aggregates Levy Sustainability Fund, aims are to promote sustainable aggregates use by reducing the demand for primary aggregates through encouraging greater use of recycled and secondary aggregates. This is addressing a range of barriers to greater use of these materials but it does not fund the type of basic research and process development outlined in this proposal. There has been significant research and development on the production of aggregates from fine granular waste materials. This normally involves pan pelletising and sintering in a rotary tube furnace at relatively high temperatures. This is the type of process used by R.T.A.L. at Tilbury to convert waste materials into lightweight aggregate and is also used at the pilot plant situated at the Manufactured Aggregate Research Centre (MARC) at the University of East London and the group at the University of Leeds headed by Dr Peter Wainwright. The major commercial producer of lightweight aggregate is Lytag who produce aggregate from sintered pulverised fuel ash. This uses a sinter strand that exploits the residual carbon content present in the ash. We have ongoing research associated with Lytag.

Our view is that these types of processes are not appropriate for use at C&D waste recycling facilities. The most viable option is for the silt filter cake to be formed into aggregate by homogenising/mixing with cementitious binders and a polymeric rheology modifier, to allow the mix to be extruded and formed into rapid hardening aggregate particles that contain a high fraction of silt filter cake. As far as we are aware this is a novel approach to aggregate production from C&D waste silt. A similar process involving mixing and extrusion has been reported that manufactures high carbon fly ash/mixed thermoplastic aggregate for use in lightweight concrete by workers in the USA.

This research project compliments ongoing work by those involved at Imperial. This includes work on the use of secondary waste minerals as a novel cementitious landfill liner (Construction and Building Materials, 18,9, 2004, 689-699), co-utilisation of mineral and biological wastes in mine site restoration (Minerals Engineering, 17, 2, 2004, 131-139), and properties of lightweight aggregate produced by rapid sintering of incinerator bottom ash (Resources, Conservation and Recycling, 43, 2, 2005, 147-162).

The research also includes key industrial collaborators. Powerscreen Washing Systems Ltd., established in 1985, and its associated companies, Mineral Improvements Ltd., Powerscreen Hire Ltd., and LJH Group Ltd., are recognised as the No 1 supplier to the aggregate processing industry of specialist washing equipment, water treatment, plant hire, contract processing, bespoke plant design, fabrication and related services. They have access through their customer base to a wide range of silt filter cake wastes. Winkworth Machinery Limited is a leading UK company manufacturing specialist high shear mixing and extrusion equipment which we believe is necessary to use in processing silt filter cake.
Objective
The overall project aim is to solve the silt disposal problem associated with C&D recycling plants that use aggregate washing technology. Sometimes it is not economical to wash material because the cost of the water treatment plant is prohibitive relative to the return from the washed aggregate. If however the silt material could be turned into a saleable commodity then this would offset the cost of the water treatment plant and therefore many applications which are currently unviable would become viable. The project is therefore highly relevant to the objectives outlined in the Call as this is research into systems and their logistics to maximise recovery of high quality uncontaminated material and their subsequent processing to realise potential environmental and economic gain. Much more use of recycled aggregate would be encouraged as washing the material would produce clean reusable higher quality aggregate. This will be achieved by completing the following specific objectives:

a) Characterisation of silt particles produced from different C&D aggregate recycling washing plants including assessment of their pozzolanic properties; the objective is to thoroughly understand and appreciate the types of silts generated by UK C&D washing plants and the factors that control this variability.

b) Evaluation of the properties of cement/C&D waste silt composites containing high silt loadings; the objective is to understand how silt effects the hydration and properties of different types of cement. The work will include Portland cement, high alumina cements and novel MgO containing (Eco-cement) systems that harden by adsorption of CO2 from the atmosphere.

c) Production at pilot plant scale, testing and microstructural analysis of rapid setting aggregate particles manufactured by high shear mixing and extrusion of polymer modified cement/C&D waste silt composite aggregate; the objective is to complete pilot scale trials that will produce representative quantities and qualities of high silt-containing aggregate using high shear mixer-extruder equipment manufactured by a leading UK company.

d) Specification of commercially viable processing plant that can produce aggregate from C&D waste silt, identification of routes to exploitation and impact on Defra policy on C&D waste recycling; the objective is to have identified all equipment required to process silt to aggregate in order to achieve the stated project aim.

An aggregate washing plant adds value by removing the silt. A water treatment plant does not add value to the material and is seen as a cost by producers. However in many applications it would not be possible to wash without the addition of a water treatment plant as there is not sufficient area for lagoons. In many applications today producers of aggregate are realizing the benefits of using a water treatment plant as opposed to lagoons because the system is fully automatic, takes up a small area and produces a manageable pressed cake which can be stockpiled rather than incurring the costs of maintaining lagoons and health and safety issues.The project aim will have been attained when a commercially viable silt cake processing plant can be specified and a route to exploitation has been identified. It should be noted that the type of novel technology envisaged and to be developed in this project will have wider applicability to other fine grained wastes arising in the UK such as quarry dusts, other combustion residues and tunnel spoil.
Project Documents
• Final Report : New technologies for reuse of silt from C&D waste recycling washing plant   (624k)
Time-Scale and Cost
From: 2006

To: 2008

Cost: £201,742
Contractor / Funded Organisations
The Imperial College of Science Technology and Medicine
Keywords
incl. processing and reprocessing              
Aggregates              
Analysis              
Analytical Chemistry              
Carbon Dioxide              
Civil Engineering              
Cleaning Systems              
Database              
Encourage change in resource use/waste generation              
Environmental Protection              
Establishing a sound database for waste management              
Materials Science              
Policy Development              
Process Technology              
Processes              
Recycling              
Sorting systems              
Systems for Resource Recovery              
Understanding waste composition and trends              
Waste              
Waste Management              
Fields of Study
Waste Management