Gypsum treated fly ash as a liner for waste disposal facilities

Fly ash has potential application in the construction of base liners for waste containment facilities. While most of the fly ashes improve in the strength with curing, the ranges of permeabilities they attain may often not meet the basic requirement of a liner material. An attempt has been made in the present context to reduce the hydraulic conductivity by adding lime content up to 10% to two selected samples of class F fly ashes. The use of gypsum, which is known to accelerate the unconfined compressive strength by increasing the lime reactivity, has been investigated in further improving the hydraulic conductivity. Hydraulic conductivities of the compacted specimens have been determined in the laboratory using the falling head method. It has been observed that the addition of gypsum reduces the hydraulic conductivity of the lime treated fly ashes. The reduction in the hydraulic conductivity of the samples containing gypsum is significantly more for samples with high amounts of lime contents (as high as 1000 times) than those fly ashes with lower amounts of lime. However there is a relatively more increase in the strengths of the samples with the inclusion of gypsum to the fly ashes at lower lime contents. This is due to the fact that excess lime added to fly ash is not effectively converted into pozzolanic compounds. Even the presence of gypsum is observed not to activate these reactions with excess lime. On the other hand the higher amount of lime in the presence of sulphate is observed to produce more cementitious compounds which block the pores in the fly ash. The consequent reduction in the hydraulic conductivity of fly ash would be beneficial in reducing the leachability of trace elements present in the fly ash when used as a base liner.     

Possibility of using waste tire rubber and fly ash with Portland cement as construction materials

The growing amount of waste rubber produced from used tires has resulted in an environmental problem. Recycling waste tires has been widely studied for the last 20 years in applications such as asphalt pavement, waterproofing systems and membrane liners. The aim of this study is to evaluate the feasibility of utilizing fly ash and rubber waste with Portland cement as a composite material for masonry applications. Class C fly ash and waste automobile tires in three different sizes were used with Portland cement. Compressive and flexural strength, dry unit weight and water absorption tests were performed on the composite specimens containing waste tire rubber. The compressive strength decreased by increasing the rubber content while increased by increasing the fly ash content for all curing periods. This trend is slightly influenced by particle size. For flexural strength, the specimens with waste tire rubber showed higher values than the control mix probably due to the effect of rubber fibers. The dry unit weight of all specimens decreased with increasing rubber content, which can be explained by the low specific gravity of rubber particles. Water absorption decreased slightly with the increase in rubber particles size. These composite materials containing 10% Portland cement, 70% and 60% fly ash and 20% and 30% tire rubber particles have sufficient strength for masonry applications.     

Modification of hydraulic conductivity in granular soils using waste materials

This paper evaluates the use of waste products such as silica fume and fly ash in modification of the granular soils in order to remove some environmental problems and create new useful findings in the field of engineering. It is known that silica fume and fly ash, as well as clay material, are used in geotechnical engineering because of their pozzolanic reactivity and fineness to improve the soil properties needed with respect to engineering purposes. The main objective of this research project was to investigate the use of these materials in geotechnical engineering and to improve the hydraulic properties of soils by means of grouting. For this reason, firstly, suitable grouts in suspension forms were prepared by using silica fume, fly ash, clay and cement in different percentages. The properties of these cement-based grouts were then determined to obtain the desired optimum values for grouting. After that, these grouts were penetrated into the soil samples under pressure. The experimental work indicates that these waste materials and clay improved the physical properties and the fluidity of the cement-based grouts and they also decreased the hydraulic conductivity of the grouted soil samples by sealing the voids of the soil. The results of this study have important findings concerning the use of these materials in soil treatment and the improvement of hydraulic conductivity of the soils.     

Effects of calcium hydroxide and calcium chloride addition to bentonite in iron ore pelletization.

Pyrite ash is created as waste from the roasting of pyrite ores during the production of sulphuric acid. These processes generate great amounts of pyrite ash waste that is generally land filled. This creates serious environmental pollution due to the release of acids and toxic substances. Pyrite ash waste can be utilized in the iron production industry as a blast furnace feed to process this waste and prevent environmental pollution. The essential parameters affecting the pelletization process of pyrite ash were studied using bentonite as a binder. Experiments were then carried out using bentonite and a mixture of bentonite with calcium hydroxide and calcium chloride in order to make the bentonite more effective. The metallurgical properties of pyrite ash, bentonite, calcium hydroxide, calcium chloride, a mixture of these and sintered pellets were studied using X-ray analysis. The crushing strength tests were carried out to investigate the strength of pyrite ash waste pellets. The results of these analyses showed that pyrite ash can be agglomerated to pellets and used in the iron production industry as a blast furnace feed. The crushing strength of the pellets containing calcium hydroxide and calcium chloride in addition to bentonite was better than the strength of pellets prepared using only bentonite binder.     

Potential use of lignite fly ash for the control of acid generation from sulphidic wastes

In the present paper, the potential use of lignite fly ash in the control of acid generation from sulphidic tailings disposed of at Lavrion, Greece was studied. Long-term laboratory column kinetic tests were performed on tailings containing 27% S, which were homogeneously mixed with various amounts of fly ash, ranging from 10 to 63% w/w. The drainage quality of the columns was monitored over a test period of 600 days. Chemical and mineralogical characterisation of column solid residues was performed after a 270-day test period. The hydraulic conductivity of the mixtures was also measured to evaluate the potential of fly ash to form a low permeability layer. Based on the results, the addition of fly ash to sulphidic tailings, even at the lower amount, increased the pH of the drainage at values of 8.6-10.0 and decreased the dissolved concentrations of contaminants, mainly Zn and Mn, to values that meet the European regulatory limits for potable water. Higher fly ash addition to tailings, at amounts of 31 and 63% w/w also reduced the water permeability of material from 1.2 x 10(-5) cm/sec to 3 x 10(-7) and 2.5 x 10(-8) m/s, respectively.     

Percolation of clay liners of ash landfills in short and long time perspectives.

Top covers of waste landfills conventionally contain a drain layer over a 1(low-permeable clay liner usually containing smectite minerals. The rate of percolation of the clay liner, which may require tens of years to become water-saturated, determines the downward transport of ions released from the underlying waste to and through the bottom clay liner. The percolation rate is controlled by the composition and density of the tipper liner, which should be as tight as possible. This implies a high density and therefore a high swelling potential which must be moderated by proper design. The bottom clay liner is a less effective and reliable barrier since cation exchange will increase the hydraulic conductivity and cause a significant rise in percolation rate and risk of chemical attack by the percolate. The top liner will undergo very moderate strain if the ash fill is effectively compacted and undergoes little self-compaction. Processes that may cause degradation are freezing and drying and require proper design. In this paper the authors examine the performance of ash-fills isolated by clay liners and conclude that the most important issue is to design and construct the top liner to be as impermeable as possible paying less attention to the tightness of the bottom layer.     

Leachate injection using vertical wells in bioreactor landfills

Leachate recirculation or liquid injection in municipal solid waste landfills offers economic and environmental benefits. The key objective of this study was to carry out numerical evaluation of key design variables for leachate recirculation system consisting of vertical wells. In order to achieve the objective, numerical modeling was carried out using the finite-element model HYDRUS-2D. The following design parameters were evaluated by simulating liquid pressure head on the liner and the wetted width of the waste under steady-state flow conditions: (1) hydraulic conductivities of the waste and vertical well backfill; (2) liquid injection rate and dosing frequency; (3) well diameter, screen height and screen depth; and (4) hydraulic conductivity of the leachate collection system, slope of the leachate collection system and spacing of the leachate collection pipes. The key findings of this study are as follows. The well diameter, hydraulic conductivity of the well drainage pack, and screen height and screen depth of the well have very little effect on the wetted width for a given liquid flux. The wetted width and the injection pressure for a given liquid flux decrease with the increase in the hydraulic conductivity of the waste. The pressure head on the liner increases with the decrease in the vertical distance between the bottom of the well screen and the top of leachate collection system. The liquid injection flux increases with the decrease in hydraulic conductivity of the leachate collection system. Unlike sand (k approximately 10(-4)m/s), pea gravel (k approximately 0.01 m/s) resulted in less than 0.3m pressure head on the liner for all simulations carried out in this study.     

Potential use of densified polymer-pastefill mixture as waste containment barrier materials

Mining activities generate a large amount of solid waste, such as waste rock and tailings. The surface disposal of such waste can create several environmental and geotechnical problems. Public perception and strict government regulations with regards to the disposal of such waste compel the mining industry to develop new strategies which are environmentally sound and cost effective. In this scenario, recycling of such waste into mining or civil engineering construction materials have become a great challenge for the mining and civil engineering community. Hence, in this study, taking advantage of the inherent low hydraulic conductivity of paste tailings (pastefill), small amounts (0.05, 0.1, 0.2, 0.5%) of a super absorbent polymer (SAP) are added to the latter after moisturizing the tailings. The resulting densified polymer-pastefill (PP) materials are compacted and submitted to permeability tests at room temperature and performance tests under cyclic freeze–thaw and wet–dry conditions to evaluate their suitability as a barrier for waste containment facilities. Valuable results are obtained. It is found that the hydraulic conductivity of the proposed barrier material (PP) decreases as the amount of SAP increases. Hydraulic conductivity values as low as 1 × 10^?7 and 6 × 10^?9 cm/s are obtained for PPs which contain 0.1–0.5% SAP, respectively. The PP material also shows relatively good resistance to cyclic freeze–thaw and wet–dry stresses. The results show that negligible to acceptable changes in hydraulic conductivity occur after five freeze–thaw and six wet–dry cycles. None of the changes reach one order of magnitude. As a final step, a cost analysis is undertaken to evaluate the economical benefits that could be drawn from such a proposed barrier material. When compared to a conventional compacted sand–bentonite barrier with 12% bentonite concentration, it is found that the benefit realized could be estimated to 98, 96 and 90% when using PP material that contains 0.10, 0.20 and 0.5% of SAP. From this study, it can be concluded that PP materials show encouraging performance properties for barrier design.     

Hydrological performance of MSW incineration residues and MSW co-disposed with sludge in full-scale cells

Water flows were analysed for the filling phase and the first 4 years after closure of two types of full-scale landfill cells: 'special cells' containing mostly fly ash from municipal solid waste (MSW) incineration disposed with other special/hazardous waste, and 'biocells' (biological cells) containing co-disposed MSW and food industry sludge. The landfill cells were constructed about -1.5 m above sea level (masl) at Lomma Bay, southern Sweden. The hydrological effects of water intrusion into the special cells from surroundings and sludge moisture within the biocells were studied. HELP modelling of hydrological processes predicted delay in peaks of leachate generation from uncovered special cells following rain, which was not confirmed. Faster leachate production as a response to rainfall from special cells than from biocells was observed. It was inferred that special waste has more intensive channelling, lower water absorption and higher hydraulic conductivity than mixtures of sludge/MSW. To avoid convergence problems in modelling uncovered special cells, the use of a 5 cm deep top layer with saturated hydraulic conductivity 1.7 x 10(-3) cm s(-1), porosity 0.437, and field capacity 0.105, is suggested.     

Assessing the environmental impact of ashes used in a landfill cover construction

Large amounts of construction materials will be needed in Europe in anticipation for capping landfills that will be closed due to the tightening up of landfill legislation. This study was conducted to assess the potential environmental impacts of using refuse derived fuel (RDF) and municipal solid waste incineration (MSWI) ashes as substitutes for natural materials in landfill cover designs. The leaching of substances from a full-scale landfill cover test area built with different fly and bottom ashes was evaluated based on laboratory tests and field monitoring. The water that drained off above the liner (drainage) and the water that percolated through the liner into the landfill (leachate) were contaminated with Cl^?, nitrogen and several trace elements (e.g., As, Cu, Mo, Ni and Se). The drainage from layers containing ash will probably require pre-treatment before discharge. The leachate quality from the ash cover is expected to have a minor influence on overall landfill leachate quality because the amounts generated from the ash covers were low, <3–30 l (m² yr)^?1. Geochemical modelling indicated that precipitation of clay minerals and other secondary compounds in the ash liner was possible within 3 years after construction, which could contribute to the retention of trace elements in the liner in the long term. Hence, from an environmental view point, the placement of ashes in layers above the liner is more critical than within the liner.     

Leaching behavior of polychlorinated dibenzo-p-dioxins and furans from the fly ash and bottom ash of a municipal solid waste incinerator

The leaching behavior of dioxins from landfill containing bottom ash and fly ash from municipal solid waste incineration has been investigated by leaching tests with pure water, non-ionic surfactant solutions, ethanol solutions, or acetic acid solutions as elution solvents for a large-scale cylindrical column packed with ash. Larger amounts of dioxins were eluted from both bottom ash and fly ash with ethanol solution and acetic acid solution than with pure water. Large quantities of dioxins were leached from fly ash but not bottom ash by non-ionic surfactant solutions. The patterns of distribution of the dioxin congeners in the leachates were very similar to those in the bottom ash or fly ash from which they were derived.     

An Investigation into the Influence of Filler Silanization Conditions on Mechanical and Thermal Parameters of Epoxy Resin-Fly Ash Composites

The insulation material of electronic devices should offers high thermal conductivity whilst retaining suitable mechanical properties. Epoxy resin is an example of a material that is commonly used by industry for electronic insulation, despite the fact that neither the thermal conductivity nor the mechanical properties are particularly satisfying. These properties can be enhanced by incorporating filler, with silica flour representing the most popular filler. An economically appealing solution is to replace silica flour with fly ash as filler material, however it must be remembered that compatibility of fly ash and epoxy resin is not ideal. In order to improve the coupling between these two materials, fly ash particles covered with [3-(2-Aminoethylamino)propyl]trimethoxysilane were obtained with six different conditions of the silanization process, where the amount of silane, the temperature and the time of the reaction were changed. The presence of the silane layer was confirmed via Fourier Transform Infrared Spectroscopy, Thermogravimetric Analysis and Scanning Electron Microscopy. The mechanical properties, including tensile strength, Young Modulus and fracture toughness, as well as the thermal conductivity of the final samples were investigated. In the case of composites with silanized fillers, all of the mechanical properties were improved, and an enhancement of thermal conductivity was observed for several composites. Moreover, the differences in coupling between the silanized fly ash and the untreated fly ash, and the epoxy matrix were precisely recorded by means of SEM. The presented studies confirm that an effective silanization process can significantly improve the properties of composites, while also verifying the usefulness of waste material. The results highlight that fly ash may be utilized to create a more economically affordable insulation material.     

Leaching behaviour of elements and evaluation of pre-treatment methods for municipal solid waste incinerator residues in column leaching tests.

Two new pre-treatment methods (water-washing/carbonation and carbonation/phosphate stabilization) of municipal solid waste (MSW) incinerator residues were evaluated by column leaching tests under aerobic conditions and anaerobic conditions (which were changed to aerobic conditions after 10 months). A mixture of bottom ash and fly ash (5:1 ratio) was pre-treated using each method. Shredded incombustible residues (SIR) were added to each ash preparation in proportions similar to the ratios present in landfills. For comparison, landfill wastes typical of Japan, namely, a mixture of bottom ash, chelating-pre-treated fly ash, and SIR, were also examined. Leachate samples were collected periodically and analysed over a 15-month period. When compared with chelating pretreatment, both water-washing/carbonation and carbonation/ phosphate stabilization reduced the leaching of Pb, Al, and Cu by about one to two orders of magnitude. Moreover, the initial concentrations of Ca and Pb in leachates from column of water-washing/carbonation were 56-57% and 84-96% less than those from the column of carbonation/phosphate stabilization. Therefore, water-washing/carbonation was considered to be a promising approach to obtain early waste stabilization and to reduce the release of heavy metals to near-negligible levels. The leaching behaviour of elements was also discussed.     

Cement solidification of simulated off-gas condensates from vitrification of low-level nuclear waste solutions

Solidification in a cementitious matrix is a viable alternative for low-level nuclear waste management; it is therefore important to understand the behavior and properties of such wasteforms. We have examined the cementitious solidification of simulated off-gas waste streams resulting from the vitrification of low-level nuclear waste. Different possible methods for scrubbing the off-gasses from a vitrifier give rise to three possible types of waste compositions: acidic (from aqueous dissolution of volatile NOx and POx carried over from the vitrifier), basic (from neutralizing the former with sodium hydroxide), and fully carbonated (arising from a direct-combustion vitrifier). Six binder compositions were tested in which ordinary Portland cement was replaced at different proportions by fly ash and/or ground granulated blast furnace slag. A high solution to binder ratio of 1l/1 kg was used to minimize the volume of the wasteform and 10% attapulgite clay was added to all mixes to ensure that the fresh mix did not segregate prior to setting. The 28-day compressive strengths decreased when a high proportion of cement was replaced with fly ash, but were increased significantly when the cement was replaced with slag. The heats of hydration at early age for the various solids compositions decreased when cement was replaced with either fly ash or slag; however, for the fly ash mix the low heat was also associated with a significant decrease in compressive strength. High curing temperature (60 degrees C) or the use of extra-fine slag did not significantly affect the compressive strength. Recommendations for choice of binder formulations and treatment of off-gas condensates are discussed.     

Calcium phosphate stabilization of fly ash with chloride extraction

Municipal solid waste incinerator by products include fly ash and air pollution control residues. In order to transform these incinerator wastes into reusable mineral species, soluble alkali chlorides must be separated and toxic trace elements must be stabilized in insoluble form. We show that alkali chlorides can be extracted efficiently in an aqueous extraction step combining a calcium phosphate gel precipitation. In such a process, sodium and potassium chlorides are obtained free from calcium salts, and the trace metal ions are immobilized in the calcium phosphate matrix. Moderate calcination of the chemically treated fly ash leads to the formation of cristalline hydroxylapatite. Fly ash spiked with copper ions and treated by this process shows improved stability of metal ions. Leaching tests with water or EDTA reveal a significant drop in metal ion dissolution. Hydroxylapatite may trap toxic metals and also prevent their evaporation during thermal treatments. Incinerator fly ash together with air pollution control residues, treated by the combined chloride extraction and hydroxylapatite formation process may be considered safe to use as a mineral filler in value added products such as road base or cement blocks.     

Trace element partitioning in ashes from boilers firing pure wood or mixtures of solid waste with respect to fuel composition,chlorine content and temperature

Trace element partitioning in solid waste (household waste, industrial waste, waste wood chips and waste mixtures) incineration residues was investigated. Samples of fly ash and bottom ash were collected from six incineration facilities across Sweden including two grate fired and four fluidized bed incinerators, to have a variation in the input fuel composition (from pure biofuel to mixture of waste) and different temperature boiler conditions. As trace element concentrations in the input waste at the same facilities have already been analyzed, the present study focuses on the concentration of trace elements in the waste fuel, their distribution in the incineration residues with respect to chlorine content of waste and combustion temperature.Results indicate that Zn, Cu and Pb are dominating trace elements in the waste fuel. Highly volatile elements mercury and cadmium are mainly found in fly ash in all cases; 2/3 of lead also end up in fly ash while Zn, As and Sb show a large variation in distribution with most of them residing in the fly ash. Lithophilic elements such as copper and chromium are mainly found in bottom ash from grate fired facilities while partition mostly into fly ash from fluidized bed incinerators, especially for plants fuelled by waste wood or ordinary wood chips. There is no specific correlation between input concentration of an element in the waste fuel and fraction partitioned to fly ash. Temperature and chlorine content have significant effects on partitioning characteristics by increasing the formation and vaporization of highly volatile metal chlorides. Zinc and cadmium concentrations in fly ash increase with the incineration temperature.     

Sintering characteristics of CaO-rich municipal solid waste incineration fly ash through the addition of Si/Al-rich ash residues

Thermal treatment is a promising technology for the fast disposal of hazardous municipal solid waste incineration (MSWI) fly ash in China. However, fly ash produced in grate incinerator (GFA) is rich in CaO and chlorides, which promote the formation of toxic hexavalent chromium [Cr(VI)] and ash agglomeration during the thermal process, inhibiting the thermal disposal of GFA. In this study, sintering characteristics of CaO-rich GFA were improved by adding Si/Al-rich MSWI ash residues. According to the results, ash agglomeration was well suppressed during thermal treatment of the mixed ash. Si/Al/Fe-compounds competed with un-oxidized Cr-compounds to react with CaO and suppressed Cr(VI) formation. Meanwhile, chlorides in GFA facilitated heavy metal volatilization from added ashes to the secondary fly ash, favoring the recovery of these metals. Ca-aluminosilicates was found as the main mineral phase in the thermally treated mixed ash, which has attractive potential for applications. The formation of the aluminosilicates made the heavy metals that remained in the treated mixed ash more stable than the thermally treated single ash.     

Quick monitoring of pozzolanic reactivity of waste ashes

This article proposes a quick method of monitoring for pozzolanic reactivity of waste ashes by investigating the electrical conductivity of the suspension at an elevated temperature. This suspension is obtained by mixing tested pozzolan with an ordinary Portland cement (OPC) solution produced by mixing ordinary Portland cement with water. For comparison, silica fume, metakaolin, rice husk ash and river sand – whose pozzolanic reactivities range from reactive to inert – were used in the experimental investigation. The electrical conductivity of the suspension was continually recorded by using an electrical conductivity meter and stored by using a personal computer for a period of slightly over 1 day. The indicative parameters that can be related to pozzolanic reactivity were discussed and analyzed in detail. It was found that it is possible to determine the pozzolanic reactivity of fly ash within 28 h by using the proposed technique, as compared to 7 or 28 days for the determination of strength activity index according to ASTM. This technique would help concrete technologists to speedily investigate the quality of fly ash for use as a cement replacement in order to alleviate pollution caused by cement production and solve disposal problems of waste ashes.     

Reducing volatilization of heavy metals in phosphate-pretreated municipal solid waste incineration fly ash by forming pyromorphite-like minerals

This research investigated the feasibility of reducing volatilization of heavy metals (lead, zinc and cadmium) in municipal solid waste incineration (MSWI) fly ash by forming pyromorphite-like minerals via phosphate pre-treatment. To evaluate the evaporation characteristics of three heavy metals from phosphate-pretreated MSWI fly ash, volatilization tests have been performed by means of a dedicated apparatus in the 100-1000 °C range. The toxicity characteristic leaching procedure (TCLP) test and BCR sequential extraction procedure were applied to assess phosphate stabilization process. The results showed that the volatilization behavior in phosphate-pretreated MSWI fly ash could be reduced effectively. Pyromorphite-like minerals formed in phosphate-pretreated MSWI fly ash were mainly responsible for the volatilization reduction of heavy metals in MSWI fly ash at higher temperature, due to their chemical fixation and thermal stabilization for heavy metals. The stabilization effects were encouraging for the potential reuse of MSWI fly ash.     

Potential use of lateritic and marine clay soils as landfill liners to retain heavy metals

The potential of a lateritic soil and a marine clay, typical of those found in hot and humid climatic regions, was assessed for use as a landfill liner material. A series of tests were conducted - physical and chemical, batch adsorption, column, hydraulic conductivity, etc., - to evaluate the heavy metal sorption capacity, chemical compatibility of hydraulic conductivity, and transport parameters of the soils. Experimental results showed that the marine clay had better adsorption capacity than that of the lateritic soil and that its hydraulic conductivity was an order of magnitude lower. In addition, the hydraulic conductivities of both soils when permeated with low concentration heavy metal solutions were below 1x10(-7)cm/s. When permeated with Cr, Pb, Cd, Zn, and Ni solutions, the retardation factors of the lateritic soil and the marine clay ranged from 10 to 98 and 37 to 165, respectively, while the diffusion coefficients ranged from 1.0x10(-5) to 7.5x10(-6) and 3.0 to 9.14x10(-7)cm2/s, respectively. For both soils, Cr and Pb were retained relatively well, while Cd, Zn, and Ni were more mobile. The marine clay had higher retardation factors and lower diffusion coefficients, and its hydraulic conductivity was more compatible with Cr solution, than that of the lateritic soil. In general, the properties of the marine clay indicate that it has significant advantages over the lateritic soil as landfill liner material.