This review describes the current status of waste treatment using thermal plasma technology. A comprehensive analysis of the available scientific and technical literature on waste plasma treatment is ...presented, including the treatment of a variety of hazardous wastes, such as residues from municipal solid waste incineration, slag and dust from steel production, asbestos-containing wastes, health care wastes and organic liquid wastes. The principles of thermal plasma generation and the technologies available are outlined, together with potential applications for plasma vitrified products. There have been continued advances in the application of plasma technology for waste treatment, and this is now a viable alternative to other potential treatment/disposal options. Regulatory, economic and socio-political drivers are promoting adoption of advanced thermal conversion techniques such as thermal plasma technology and these are expected to become increasingly commercially viable in the future.
This work aimed to develop novel cement systems for waste encapsulation that would form with a pH of around 10. The approach taken was to investigate the formation of brucite by hydration of a light ...burned periclase (MgO). Commercially available MgO powders often contain some CaO, and therefore silica fume was added to form C-S-H gel. Identification of the hydrated phases in MgO/silica fume samples showed that brucite formed in substantial quantities as expected. However, brucite reacted with the silica fume to produce a magnesium silicate hydrate (M-S-H) gel phase. After 28
days, the pH of systems rich in MgO tended towards the pH controlled by residual brucite (~
10.5), whereas when all brucite reacts with silica fume a cement with an equilibrium pH just below 10 was achieved.
Assessment of the pozzolanic activity of cement replacement materials is increasingly important because of the need for more sustainable cementitious products. The pozzolanic activity of metakaolin, ...silica fume, coal fly ash, incinerated sewage sludge ash and sand have been compared using the Frattini test, the saturated lime test and the strength activity index test. There was significant correlation between the strength activity index test and the Frattini test results, but the results from these tests did not correlate with the saturated lime test results. The mass ratio of Ca(OH)
2 to test pozzolan is an important parameter. In the Frattini test and strength activity index test the ratio is approximately 1:1, whereas in the saturated lime test the ratio is 0.15:1. This explains why the saturated lime test shows higher removal of Ca(OH)
2 and why the results from this test do not correlate with the other test methods.
The hydration of reactive periclase (MgO) in the presence of hydromagnesite (Mg5(CO3)4(OH)2·4H2O) was investigated by a variety of physical and chemical techniques. Hydration of pure MgO-water ...mixtures gave very weak pastes of brucite (Mg(OH)2), but hydration of MgO-hydromagnesite blends gave pastes which set quickly and gave compressive strengths of potential interest for construction applications. The strengths of the blends increased with hydration time at least up to 28days, and were not significantly decreased by increasing the hydromagnesite content up to 30%. Raman spectroscopy suggests that an amorphous phase, of composition between that of brucite, hydromagnesite and water, may form. Small amounts of calcite also form due to CaO in the MgO source. Thermodynamic calculations imply that the crystalline phase artinite (MgCO3·Mg(OH)2·3H2O) should be the stable product in this system, but it is not observed by either XRD or FTIR techniques, which suggests that its growth may be kinetically hindered.
The recovery of phosphorus from sewage sludge ash samples obtained from 7 operating sludge incinerators in the UK using a sulfuric acid washing procedure to produce a technical grade phosphoric acid ...product has been investigated. The influences of reaction time, sulfuric acid concentration, liquid to solid ratio and source of ISSA on P recovery have been examined. The optimised conditions were the minimum stoichiometric acid requirement, a reaction time of 120
min and a liquid to solid ratio of 20. Under these conditions, average recoveries of between 72% and 91% of total phosphorus were obtained. Product filtrate was purified by passing through a cation exchange column, concentrated to 80% H
3PO
4 and compared with technical grade H
3PO
4 specifications. The economics of phosphate recovery by this method are briefly discussed.
Chemical extraction of phosphorus (P) from incinerated sewage sludge ash (ISSA) is adversely influenced by co-dissolution of metals and metalloids. This study investigated P recovery and leaching of ...Zn, Cu, Pb, As and Ni from ISSA using inorganic acids (sulphuric acid and nitric acid), organic acids (oxalic acid and citric acid), and chelating agents (ethylenediaminetetraacetic acid (EDTA) and ethylene diamine tetramethylene phosphonate (EDTMP)). The aim of this study was to optimize a leaching process to recover P-leachate with high purity for P fertilizer production. The results show that both organic and inorganic acids extract P-containing phases but organic acids leach more trace elements, particularly Cu, Zn, Pb and As. Sulphuric acid was the most efficient for P recovery and achieved 94% of total extraction under the optimal conditions, which were 2-h reaction with 0.2 mol/L H2SO4 at a liquid-to-solid ratio of 20:1. EDTA extracted only 20% of the available P, but the leachates were contaminated with high levels of trace elements under optimum conditions (3-h reaction with EDTA at 0.02 mol/L, pH 2, and liquid-to-solid ratio of 20:1). Therefore, EDTA was considered an appropriate pre-treatment agent for reducing the total metal/metalloid content in ISSA, which produced negligible changes in the structure of ISSA and reduced contamination during subsequent P extraction using sulphuric acid.
•Co-dissolution of P and metal (loid)s from ISSA were investigated by three kinds of extractants.•Sulphuric acid was the most efficient for P recovery.•EDTA was the optimal pre-leaching agent for reducing the total metal (loid)s content.•Leaching conditions by EDTA and sulfuric acid were optimized.•Treatment with EDTA has little effect on morphology and mineralogy characteristics of ISSA.
•Drivers behind sewage sludge incineration clearly demonstrated.•Technical detail on the incineration process and ash characteristics provided.•The 1st ever multi-disciplinary review of ISSA re-use ...is presented.•The optimum ISSA re-use routes are identified.•The first time acid insoluble ISSA residue has been considered as a resource.
The drivers for increasing incineration of sewage sludge and the characteristics of the resulting incinerated sewage sludge ash (ISSA) are reviewed. It is estimated that approximately 1.7 milliontonnes of ISSA are produced annually world-wide and is likely to increase in the future. Although most ISSA is currently landfilled, various options have been investigated that allow recycling and beneficial resource recovery. These include the use of ISSA as a substitute for clay in sintered bricks, tiles and pavers, and as a raw material for the manufacture of lightweight aggregate. ISSA has also been used to form high density glass–ceramics. Significant research has investigated the potential use of ISSA in blended cements for use in mortars and concrete, and as a raw material for the production of Portland cement. However, all these applications represent a loss of the valuable phosphate content in ISSA, which is typically comparable to that of a low grade phosphate ore. ISSA has significant potential to be used as a secondary source of phosphate for the production of fertilisers and phosphoric acid. Resource efficient approaches to recycling will increasingly require phosphate recovery from ISSA, with the remaining residual fraction also considered a useful material, and therefore further research is required in this area.
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•Oxalic acid achieved the highest extraction rates of P, Cu and Zn in ISSA.•The acids increased while chelating agents decreased the leachability of residual Zn and Cu.•Chemical ...extraction decreased the pozzolanic activity of residual ISSA.•Chelates extraction had little influence on the characteristics of ISSA.
The potential of six different extractants to recover phosphorus (P) from incinerated sewage sludge ash (ISSA) was evaluated. Secondary effects such as the co-dissolution of Zn and Cu were also considered. The residual ISSA from each study was assessed in particular detail, focusing on the leachability of remaining Zn and Cu, major element composition, crystalline phases and overall degree of crystallinity and particle size distribution. The residual ISSA was also evaluated as a pozzolanic material using a Strength Activity Index (SAI) test with mortars containing Portland cement with a 20% substitution by ISSA. All results were compared to tests with untreated ISSA. Overall, the use of 3 of the 6 extractants could be ruled out due to poor P recovery potential and/or a serious compromise of the potential reuse of residual ISSA in Portland cement-based materials. The results highlight the added value of considering the potential reuse of residual ISSA when trying to optimize P recovery from ISSA by wet methods.
•Metakaolin geopolymers need high water content to have appropriate rheology for some applications.•Extensive drying shrinkage and cracking occurs in high water content metakaolin ...geopolymers.•Geopolymer mortar samples with more than 10vol% sand do not crack on heating to 110°C.•Sand particles limit linear shrinkage by forming a network.•At higher additions micro-cracks form around non-shrinking sand particles.
Metakaolin geopolymers require a high water content to have appropriate rheology for use in some applications and these mixes exhibit extensive drying shrinkage and cracking. This work aimed to understand the effect of adding sand as inert filler to metakaolin geopolymers. It is found that the viscosity increases with sand addition and mortars become unworkable when more than 40vol% sand is added. Linear drying shrinkage was reduced from about 8% in the absence of sand to less than 1% with 38vol% sand addition. Moreover, geopolymer mortars containing more than 15vol% sand can withstand drying at 110°C without cracking. Because sand addition to metakaolin geopolymer mortars increases viscosity less than a reduction in water content it is possible to formulate mortars that flow but do not crack on drying and these materials have potential for use in a range of architectural and restoration applications.
The properties of lightweight aggregate (LWA) manufactured from the ash produced by fluidised bed incineration of sewage sludge have been investigated. The ash was mixed with a clay binder, formed ...into approximately spherical pellets and rapidly sintered in a rotary tube furnace at temperatures between 1020 and 1080
°C. Selected physical properties of sintered ash pellets relevant to use as lightweight aggregate have been determined, including density (apparent specific gravity), water absorption and compressive (crushing) strength. These have been compared to the properties of a commercially available lightweight aggregate (Lytag). Sewage sludge ash (SSA) pellets sintered over a range of temperatures were found to have lower densities than Lytag and low water absorption when fired between approximately 1050 and 1080
°C. Individual pellet strengths were comparable to Lytag. Major crystalline phases present in both as-received and sintered sewage sludge ash were quartz (SiO
2), the calcium magnesium phosphate mineral whitlockite (Ca
7Mg
2P
6O
24) and hematite (Fe
2O
3). Manufacturing lightweight aggregate from sewage sludge ash may become increasingly viable as landfill disposal costs continue to increase and the costs of alternative natural aggregates obtained from increasingly distant locations also increase. Sewage sludge incinerators tend to be situated in urban areas, close to where construction activity is occurring. The results indicate the potential for manufacturing high quality lightweight aggregate from the sterile, inert ash produced by sewage sludge incineration, using relatively simple processing and low temperature sintering.
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