Stabilization/solidification of MSW incineration residues from facilities with different air pollution control systems. Durability of matrices versus carbonation

This paper discusses the stabilisation/solidification process with Portland cement applied to municipal solid waste incineration residues. Two types of residues were considered: fly ash (FA) produced in an electrostatic precipitator, and air pollution control (APC) residues from a semi-dry scrubber process. Cement pastes with different percentages of FA and APC residues were characterised according to their physical properties, the effect of the hydration products and their leaching behaviour. Portland pastes prepared with APC residues showed a rapid setting velocity in comparison with setting time for those pastes substituted with FA residues. Portland cement hydration was retarded in FA pastes. Leaching test results showed that heavy metals (such as Zn, Pb and Cd) and sulphates are immobilised within the paste, whereas chlorides are only partially retained. The carbonation process increases the leachability of S04(2-) and heavy metals such as Zn and Cr.     

铬渣水泥固化体稳定性研究

采用水泥固化的方法对铬渣进行处理。在水泥与铬渣、砂、水、硅酸钠的质量比为1：0．6：0．45：0．15：0．1时固化效果较好。固化体经28d的养护后,表面Cr^6 的浸出率为10^-5数量级,即使破碎至5mm以下的粒度,其Cr^6 的浸出质量浓度仍在国家标准以下。模拟酸溶试验和固化体抗压强度测试结果表明,固化体用于填埋是长期安全的。     

Inertization of pyrite cinders and co-inertization with electric arc furnace flue dusts by pyroconsolidation at solid state

The viability of a pyroconsolidation process to render pyrite cinders inert and to co-inert pyrite cinders with a hazardous polymetallic residue such as electric arc furnace flue dusts (EAF) containing Pb, Cu, Zn, As, Cr, Ni and Mo were investigated. The effects of pyroconsolidation temperature (800-1200 degrees C), milling pyrite cinders and additions of both CaO and EAF on the resulting microstructure of the pellets were determined. The microstructural changes were then compared with the results of the standard leaching tests. Full inertization of pyrite cinders was achieved after milling to < 100 micron followed by a pelletization and pyroconsolidation process at a temperature of 1200 degrees C. This process also allows co-inertization of pyrite cinders with controlled additions of EAF (up to approximately to 10%). Following pyroconsolidation at 1200 degrees C, the metallic elements were inert components in the four main phases: traces of Cr in hematite; Cr, Cu, Zn and Ni in spinel-phase; traces of Cr and Zn in calcium ferrites; and Pb and traces of Cu, Zn and Ba in K-Ca-Al-Fe glassy silicate.     

Modelling the effects of waste components on cement hydration

Ordinary Portland Cement (OPC) is often used for the solidification/stabilization (S/S) of waste containing heavy metals and salts. These waste components will precipitate in the form of insoluble compounds on to unreacted cement clinker grains preventing further hydration. In this study the long term effects of the presence of contaminants in solidified waste is examined by numerically simulating cement hydration after precipitation of metal salts on the surface of cement grains. A cement hydration model was extended in order to describe pore water composition and the effects of cement grain coating. Calculations were made and the strength development predicted by the model was found to agree qualitatively with experimental results found in literature. The complete model is useful in predicting the strength and leaching resistance of solidified products and developing solidification recipes based on cement.     

Reuse of residues arising from lead batteries recycle: a feasibility study

The performance of products arising from the stabilization/solidification of slags from lead batteries recycle into a Portland cement matrix has been evaluated not only in order to get a stabilized waste to be disposed of according to the current legislation, but also to obtain a recyclable material, with both economic and environmental benefits. Under this respect a detailed characterization of raw slags has been performed and different slag-cement samples have been prepared by varying the slag content. The parameters related to the cementation process have been evaluated and a series of tests on the final waste forms have been carried out, aimed at assessing both mechanical performance and leaching behaviour. In spite of the acceptable values for flexural, compressive and tensile strength, however, the high release of lead from the solidification products seems to be a limiting factor for a reusable material. While explanations of such phenomenon are given (high alkalinity of Portland cement; early "doping" of cementitious components by lead in the amorphous state), the main conclusion of the research work is that further efforts should be addressed to the adoption of a different or a modified incorporation matrix.     

Use of waste ash from palm oil industry in concrete

Palm oil fuel ash (POFA), a by-product from the palm oil industry, is disposed of as waste in landfills. In this study, POFA was utilized as a pozzolan in concrete. The original size POFA (termed OP) was ground until the median particle sizes were 15.9 microm (termed MP) and 7.4 microm (termed SP). Portland cement Type I was replaced by OP, MP, and SP of 10%, 20%, 30%, and 40% by weight of binder. The properties of concrete, such as setting time, compressive strength, and expansion due to magnesium sulfate attack were investigated. The results revealed that the use of POFA in concretes caused delay in both initial and final setting times, depending on the fineness and degree of replacement of POFA. The compressive strength of concrete containing OP was much lower than that of Portland cement Type I concrete. Thus, OP is not suitable to be used as a pozzolanic material in concrete. However, the replacement of Portland cement Type I by 10% of MP and 20% of SP gave the compressive strengths of concrete at 90 days higher than that of concrete made from Portland cement Type I. After being immersed in 5% of magnesium sulfate solution for 364 days, the concrete bar mixed with 30% of SP had the same expansion level as that of the concrete bar made from Portland cement Type V. The above results suggest that ground POFA is an excellent pozzolanic material and can be used as a cement replacement in concrete. It is recommended that the optimum replacement levels of Portland cement Type I by MP and SP are 20% and 30%, respectively.     

Immobilisation of heavy metal in cement-based solidification/stabilisation: a review

Heavy metal-bearing waste usually needs solidification/stabilization (s/s) prior to landfill to lower the leaching rate. Cement is the most adaptable binder currently available for the immobilisation of heavy metals. The selection of cements and operating parameters depends upon an understanding of chemistry of the system. This paper discusses interactions of heavy metals and cement phases in the solidification/stabilisation process. It provides a clarification of heavy metal effects on cement hydration. According to the decomposition rate of minerals, heavy metals accelerate the hydration of tricalcium silicate (C3S) and Portland cement, although they retard the precipitation of portlandite due to the reduction of pH resulted from hydrolyses of heavy metal ions. The chemical mechanism relevant to the accelerating effect of heavy metals is considered to be H+ attacks on cement phases and the precipitation of calcium heavy metal double hydroxides, which consumes calcium ions and then promotes the decomposition of C3S. In this work, molecular models of calcium silicate hydrate gel are presented based on the examination of 29Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). This paper also reviews immobilisation mechanisms of heavy metals in hydrated cement matrices, focusing on the sorption, precipitation and chemical incorporation of cement hydration products. It is concluded that further research on the phase development during cement hydration in the presence of heavy metals and thermodynamic modelling is needed to improve effectiveness of cement-based s/s and extend this waste management technique.     

Effects of competitive ions,humic acid,and pH on removal of ammonium and phosphorous from the synthetic industrial effluent by ion exchange resins

The removal of the ammonium and phosphorous from the synthetic industrial effluent by the ion exchange resins was studied in this paper, aiming at the determination of the effects of competitive ions, humic acid, pH and resin amount. The kinetic experiments show that the equilibrium time for the removal of both contaminants in the absence and presence of the competing matters was 4 h. Na+ and K+ significantly reduced the ammonium removal percentage, while the existence of Mg2+, Ca2+ and humic acid also had a negative influence. Adsorption of ammonium ions in both absence and presence of Na+ and K+ observed the linear isotherm, however, it did not follow commonly used isotherms in the presence of Na+, K+, Mg2+, Ca2+ and humic acid. The phosphorous removal decreased in the presence of the competitive matters, such as Cl-, CO3(2-), SO4(2-) and humic acid. Higher pH can cause higher phosphorous removal percentage. A decrease in the solution pH was observed in the phosphorous removal experiments, possibly due to the ion exchange and the adsorption of OH-. Uptake of humic acid by the resins was observed. Finally, a series of fixed-bed experiments were performed, showing that the performance was dependent on the empty bed contact time (EBCT). Higher EBCT would cause higher bed volumes of both treated ammonium and phosphorous.     

Microstructural study of carbonated cement-solidified synthetic heavy metal waste

Synthetic wastes have been widely employed to help elucidate the complex interactions between real wastes and hydraulic binders during solidification. In this work, a laboratory produced metal waste mixed with Portland cement and immediately carbonated it using an accelerated method. The microstructures of carbonated and non-carbonated control samples were distinct despite both being dominated by unusually large phenograins derived from the waste. In the carbonated sample waste phenograins remained unaltered, whereas cement grains were largely decalcified. As a consequence of decalcification, observable porosity was significantly reduced by the formation of precipitated carbonates.     

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.     

Evidence of Al-Cr interactions affecting Cr-leaching from waste incineration ashes

Leaching tests on flue gas ashes from waste incineration showed low leaching of Cr from ashes that under moist anaerobic conditions also produced hydrogen gas. In some cases, also the redox levels (aerobic/anaerobic conditions) during the leaching test affected Cr leaching. Aerobic ashes tested in an open batch leaching test leached Cr up to 2-3 orders of magnitude more than the ashes kept moist under anaerobic conditions and tested in a closed leaching test. Model experiments showed that metallic Al could reduce Cr and at the same time produce H(2). The hydrogen gas per se could not reduce Cr. Laboratory experiments with ashes provided evidence that metallic Al, present naturally in the ashes or amended, could reduce Cr under moist anaerobic, but not under aerobic storage. Significant Cr reduction was linked to, but not caused by hydrogen formation. The reduced Cr seemed to be partially reoxidizable upon aeration or drying. The observations presented provide a basis for understanding the complexity of Cr leaching from waste incineration ashes, as for example, why some chemical stabilization methods increase Cr leaching, and point out the need for standardizing leaching test conditions with respect to Cr.     

Evaluation of the homogeneity of stabilised/solidified wastes by a video imaging technique

A video imaging technique is described for the homogeneity assessment of wastes that have been treated by stabilisation/solidification (S/S). The method incorporates a fluorescent tracer into the S/S reagent. A test “waste” consisting of an artificial soil was stabilised/solidified with varying degrees of mixing using Portland cement as the S/S reagent. The tracer distribution was monitored with a video camera, and the cement distribution was determined by chemical analysis for calcium. Measurement of the homogeneity of the products by the video imaging technique gave results comparable to those obtained by the chemical analysis. The results warrant use of the video imaging technique in field applications since it is easier, cheaper and faster than traditional chemical methods.     

A thermodynamic approach to the hydration of sulphate-resisting Portland cement

A thermodynamic approach is used to model changes in the hydrate assemblage and the composition of the pore solution during the hydration of calcite-free and calcite-containing sulphate-resisting Portland cement CEM I 52.5 N HTS. Modelling is based on thermodynamic data for the hydration products and calculated hydration rates for the individual clinker phases, which are used as time-dependent input parameters. Model predictions compare well with the composition of the hydrate assemblage as observed by TGA and semi-quantitative XRD and with the experimentally determined compositions of the pore solutions. The calculations show that in the presence of small amounts of calcite typically associated with Portland cement, C-S-H, portlandite, ettringite and calcium monocarbonate are the main hydration products. In the absence of calcite in the cement, however, siliceous hydrogarnet instead of calcium monocarbonate is observed to precipitate. The use of a higher water-to-cement ratio for the preparation of a calcite-containing cement paste has a minor effect on the composition of the hydrate assemblage, while it significantly changes the composition of the pore solution. In particular, lower pH value and higher Ca concentrations appear that could potentially influence the solubility and uptake of heavy metals and anions by cementitious materials.     

Removal of Cr(VI) Ions from Aqueous Solutions Using Poly 3-Methyl Thiophene Conducting Electroactive Polymers

This paper deals with a new application of poly 3-methyl thiophene synthesized chemically onto sawdust (termed as P3MTh/SD) as an effective adsorbent for removal of Cr(VI) ions from aqueous solutions using column system. Chemical synthesis of poly 3-methyl thiophene was performed by addition of ferric chloride (in chloroform) as oxidant to sawdust which had previously been soaked in monomer solution. All the sorption experiments were conducted using dynamic or column system at room temperature. The effect of important parameters such as pH and initial concentration on uptake of Cr(VI) was investigated. In order to find out the possibility of the regeneration and reuse of the exhausted adsorbent, desorption studies were also performed. The currently introduced adsorbent was found to be an efficient adsorbent for removal of highly toxic and hazardous Cr(VI) ions from aqueous solutions. As our breakthrough analysis has indicated, each gram of P3MTh/SD is able to remove more than 95% of Cr(VI)ions from 300 mL of Cr(VI) polluted solution with the initial concentration of 25 mg L⁻¹ in column system. Sorption/desorption of Cr(VI) ions was found to be a highly pH dependent processes.     

Role of aluminous component of fly ash on the durability of Portland cement-fly ash pastes in marine environment

The durability, of mixtures of two kinds of Spanish fly ashes from coal combustion (ASTM class F) with 0, 15 and 35% replacement of Portland cement by fly ash, in a simulated marine environment (Na(2)SO(4)+NaCl solution of equivalent concentration to that of sea water: 0.03 and 0.45 M for sulphate and chloride, respectively), has been studied for a period of 90 days. The resistance of the different mixtures to the attack was evaluated by means of the Koch-Steinegger test. The results showed that all the mixtures were resistant, in spite of the great amount of Al(2)O(3) content of the fly ash. The diffusion of SO(4)(2-), Na+ and Cl- ions through the pore solution activated the pozzolanic reactivity of the fly ashes causing the corresponding microstructure changes, which were characterized by X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). As a result, the flexural strength of the mixtures increased, principally for the fly ash of a lower particle size and 35% of addition.     

Effects of a composition change of ordinary Portland cement on waste utilization and CO2 emissions in Japan

Journal of Material Cycles and Waste Management - This paper estimates the effects of a composition change of ordinary Portland cement on waste utilization and CO2 emissions in the Japanese cement...     

Solidification with water as a treatment method for air pollution control residues

The process of solidification with water was studied on air pollution control (APC) residues from incineration of refuse-derived fuel (RDF) regarding mechanical strength and leaching behaviour of solidified material. Factorial design in two levels was applied to investigate the impact of water addition, time, and temperature to mechanical strength of solidified material. Factors time and temperature, as well as the interaction between the addition of water and time significantly (alpha=0.05) influenced the mechanical strength of solidified material. The diffusion-leaching test NEN 7345 was performed to investigate if the leaching behaviour of elements from solidified material was determined by diffusion. Since it was found that leaching is not diffusion controlled, the long-term leaching behaviour was not assessed. However, the investigation showed that some of the studied components (Al, Hg, Mn, Pb, Si, and Zn) could be considerably demobilised by solidification with water. Concentrations of As, Cd, Co, Cu, Fe, and Ni were either below or not quite above the detection limits to be included in the analysis of leaching behaviour. The elements least demobilised by solidification were Cl, Cr, K, and Na.     

Heavy metal leaching from hydroxide, sulphide and silicate stabilised/solidified wastes

A synthetic, mixed-metal solution has been stabilised by treatment with sodium hydroxide, sodium sulphide, and sodium silicate, respectively. The three stabilised filter cakes have subsequently been solidified using additions of ordinary Portland cement and pulverised fuel ash (PFA) which are typically used in UK solidification operations. Both the stabilised filter cakes and the solidified wastes have been subjected to an equilibrium extraction test, a modified TCLP test, and a series of single-extraction, batch leach tests using an increasingly acidic leachant. Metal release was found to be primarily dependent on the pH of the leachate. Under mildly acidic conditions, the percentages leached from the stabilised and the stabilised/solidified wastes were comparable for most metals. A high-volume fraction of these solidified wastes is occupied by the stabilised filter cake. When they are broken up and tested in single-extraction leach tests, the primary effect of the cementitious additives is to increase the pH of the leachate so that most heavy metals remain insoluble. When tested under acidic leachate conditions, copper, lead, and mercury were found to be particularly well retained within sodium sulphide stabilised wastes. Under similar test conditions, cadmium was leached at very low levels from the sodium silicate stabilised waste.     

Leaching of mercury-containing cement monoliths aged for one year

A directive from the Swedish Government states that waste containing more than 1% of mercury shall be permanently deposited. The stabilization of mercury by conversion to a sparingly soluble compound like the sulphide is crucial to ensure long-term immobilization in a permanent storage. Immobilization by the solidification/stabilization (S/S) method and possible formation of HgS from mercury oxide or elemental mercury by reaction with a sulphur source (S or FeS) is investigated by a modified version of the NEN 7345 Dutch tank-leaching test. The diffusion of mercury during 11 months from 1-year-old mercury containing monoliths of Portland and slag cement is demonstrated. In a geologic repository under conditions representative of deep granitic bedrock (bicarbonate buffered to pH 8.6), a favourable monolith combination is slag cement with addition of the iron sulphide troilite. The apparent diffusion coefficient of mercury is estimated.     

中孔活性炭对水溶液中Cr3+的吸附

采用模板法和氢氧化钾化学活化法制备出不同中孔率的中孔活性炭并用于水中Cr3+的静态吸附,探讨了中孔活性炭吸附Cr3+的影响因素.实验结果表明,当溶液pH为6.0、吸附温度为50℃、吸附时间为120min、活性炭加入量为2.0g/L以及活性炭中孔率为80.0％～90.0％时.中孔活性炭对溶液中Cr3+的去除率达到98.5％.分别采用Langmuir和Freundlich方程拟合活性炭对Cr3+吸附的等温线,发现Langmuir等温吸附模型对活性炭吸附Cr3+拟合程度更好.活性炭中孔率的增大有利于提高Cr3+的平衡吸附量,但同时还受到活性炭表面酸总量的影响.吸附Cr3+前、后活性炭的FTIR谱图表明,Cr3+与活性炭表面含氧官能团发生了离子交换反应.