Waste Resource Recovery

Developing circular economy and establishing recycling society is a key approach to the implement sustainable development strategy. To develop recycle economy is also the certainly choice for our country to establish an affluent society in a way of new type industrialization. The principal practical principle of recycle economy is the“decrement, reuse and recycle”of wastes.For reasons such as higher reclamation cost and lower quality after reclamation, etc., it limits the recycle of used sodium silicate-bonded sand for casting. A part of used sand must be given up which would cause environmental pollution for its alkalinity while piled up as waste. The residual of mulching films also is a comprehensive serious problem. The method adapted at present of reclaiming waste foundry sand and waste mulching films respectively is not too economical, simple and effective.The main target of this task is to explore a new way for the comprehensive utilization of waste foundry sand bonded with sodium silicate and waste mulching films with low cost and high efficiency. The main thinking of this task is to make the composites with waste foundry sand used as fillers and waste mulching films used as matrix for practical application in a simple and feasible mode.“The science and technology special action of resource comprehensive utilization”which was passed in“the national science and technology working conference about society development”as early as in 1995, would be brought into force from 1996 to 2010. One of the strategic research keys in this action plan was that“Studying the composite mechanism and technologies of different waste resources, exploiting serial products of polymer matrix composites, silicate matrix composites and metal matrix composites made of wastes”. And the key technologies requiring to be solved were pointed out in detail. In“A guide to important fields of prior developing high-tech industrialization”published jointly by the national development and reform commission and the ministry of science and technology in 2004, it was pointed out that“in the near future, the industrialization emphases are producing composites with wastes, etc.”.According to the research orientation initiated by the national developing circular economy, this paper carries out the applied basic research on comprehensive utilization of waste foundry sand and waste mulching films for preparing composites and promotes theoretic research to guide production practice. The research subject of this paper is the item“Basic Research on Foundry Waste Resource Recovery”supported by national natural science foundation of China in 2000. This task is an important research of the industry chain for circular economy in the plan--“Establishing Second Resource Valley of China”implemented by the National Engineering Research Centre of Waste Resource Recovery. It is also the component of two national key projects. One of which is national demonstration project of resource saving and environment protection–“Producing composites made of plastics and industrial wastes”assigned by the State Economic and Trade Committee. The other is national torch program—“Waste composites and production lines”assigned by the Ministry of Science and Technological.Through X-ray diffraction analysis, infrared spectral analysis, thermal analysis, scanning electron microscopy and energy dispersion X-ray analysis, as well as image analysis, it could determine that the main mineral substance of the soil adhered to waste mulching films is crystalline silica (quartz in particle level), which has characters similar to the casting quartz sand as the filling material in waste polymer matrix composite materials. Waste mulching films can, without cleaning, directly be used to make the composite material with casting waste sand, which would greatly decrease the processing cost of the composite material. This material actually is a ternary compound system composing matrix of PE, plentiful SiO2 in the adhered soil and SiO2 of the casting quartz sand.The interspaces of the compound of sand and soil is the volume of matrix plastic, and the lower volume it is, the lower waste mulching films it needs, furthermore the material cost of the composite material would be lower. The law from sand and soil pileup experiments is: The void ratio value of sand particles and soil particles decrease according to the increasing of the weight percentage of sand in the mixture. When the weight percentage of sand in the mixture is about 50%, actual space rate is lower than the theoretical value.The bending strength, tensile strength and elastic modulus of the soil/plastic compound system increase according to the increase of the weight ratio of adhered soil in waste film, while the elongation drops rapidly. When the soil content is over 50%, the bending strength of the soil/plastic compound system, while the elongation approaching to zero. Another law found by many experiments is: the soil content of mulching films could not exceed 60%, and over this ratio, the preparation of composite material would be very difficult. When the plastic content maintains at 40% in weight, along with the increase of the sand content and the decrease of the soil content, the bending strength and tensile strength grows firstly and then drops, with only a small scope drop of elastic modulus in tension, while little change in elongation. The soil content in waste mulching films purchased in Yunnan Province is generally among 60~75%, which could reduce to about 50% after simple processing, thus it is feasible to make the waste mulching film directly into the process flow without washing soil adhered to the film, and it could match casting waste sand of about 20%, being the filling material of the polymer based composite material together, to make an applied material with certain strength at a low price.Through Compare accelerated aging and natural aging of the mulching film in Kunming region, the tensile strength maintain ratio of polythene mulching film drops according to the increase of aging time, and the time for xenon lamp exposure test to reach a tensile strength maintain ratio of 50% is 22 days, equaling to 310 days of natural weather exposure test. Mulching films are generally used only for one season, so it is feasible to use current year recycled mulching films to produce the composite material. And other regions with ultraviolet radiance lower than that in Kunming, current year recycled mulching films are available.By heating for 5~10 minutes at a temperature of 600℃, and then vibrating for 4 minutes at the frequency of 50Hz, it could effectively remove most inert films covering the surface of waste sodium silicate-bonded sand for casting. By this method it could achieve a stripping rate of 50% with breaking ratio less than 6%. The surface of casting waste sand after stripping pretreatment can improve by coupling agent. After compare, it determines KH-560 as the practical coupling agent. Proper dosage of coupling agent is 1.5% of the composite material in weight, which makes the composite material bending strength increasing by 21.79%, while the tensile strength increasing by 36.06%. The test result proofed the mechanism analysis of silane coupler could be free of hydrolysis first, i.e. silane coupler can form the monomolecular layer of organosilicon together with the surface of sand particles through condensation reaction in condition of no water before it forms the homopolymer film covering surfaces of sand particles through the homopolymerization dehydration reaction of the coupling agent itself in condition of water and causes physical entanglement to macromolecules of polyethylene matrix in sequence.In basis of actually measured random load from vehicles passing the inspect cover and times of vehicle passing from on-site investigation, adopting a procedure composing methods and principles such as rain flow counting method, Goodman Equation, Miner Equation, S-N Curve, etc., it process the calculation on fatigue life of the inspect cover. Adopting this calculating method system, it further researches influence on inspect cover fatigue life from factors such as surface temperature of the road, failure load of inspect cover, the relation coefficient k for inspect cover limit strength and fatigue limit, vehicle load, vehicle flow rate, vehicle model array, etc.. Proved by lots of calculation and practice, it adopts the empirical relation coefficient for limit strength and fatigue limit as k=0.32. In practical applications, when there is lack of fatigue limit data for products of the same kind, it could use the empirical formula found in this thesis to estimate the fatigue life of the inspect covers made of composite materials.The researching findings of this thesis on waste sand/mulching film composite material has been industrialized, and it has built three process lines with waste processing capacity separately of 2500 tons/annum, 2000 tons/annum and 1500 tons/annum, the saleroom is more then RMB 40 million. The researching result on the composite inspect cover made of waste foundry and sand waste mulching film of this thesis has been adopted in industrial standards issued by State Ministry of Construction: Polymer Based Composite Material Manhole Cover (CJ/T211-2005) and Polymer Based Composite Material Gully Grid (CJ/T212-2005).For urban road manhole covers made of foundry sand/ mulching film composite material, over 95% of the raw materials are of wastes, thus the mass generation could protect the environment as well as save raw material resources, having obvious environmental, social and economic benefits.