Deliverables Stabilisation/valorisation of solid residues Stabilisation and land filling are actually the classic treatment. But a recycling route could be interesting as a treatment; particularly with two filtrations in the flue gas treatment separating ash and salts residues. In the land filling route; residues are treated with stabilisation followed by a solidification before the land filling. The stabilisation refers to the techniques that reduce the hazard potential of waste by converting the contaminants into their soluble; mobile or toxic form. The physical nature and handling characteristics of the waste are not necessarily changed by stabilisation. The solidification refers to techniques that encapsulate the waste in a monolithic solid of high structural integrity. Solidification does not necessarily involve a chemical interaction between the waste and the solidifying reagents; but may mechanically bind the waste into the monolith. Solidification processes use chemically reactive formulations that form stable solids. The control of pH remains by far the most common and simplest method of fixation of metal contained in Municipal Solid Waste Incineration residues. Any alkali might be used for the purpose of pH control; but the most common are lime; soda ash (sodium carbonate Na2CO3) and sodium hydroxide (NaOH). In addition of the alkalies; most of the solidification reagents are alkaline. Some can totally take the place of the traditional alkalies; acting both as pH control agents and as cement (e.g.; lime; sodium silicate). The present solidification processes are quite simple and uses standard mechanical equipment. The residues to be treated are conveyed into a surge tank; which in turn feeds the waste into the mixer where it is mixed with reagents. Today the use of dry lime as stabilising/solidifying reagent remains worldwide the most applied practice. Typically lime is added in excess to the ash residue forming a cement-like substance; which could be land filled. The other possibility is to recycle and reuse residues as much as possible what is. Fly ashes have been proposed as fine particulates to incorporate in asphalt or cement. Such an application has already been widely developed for the reuse of coal burning ashes. However; difficulties arise with use of APC residue due to poor performance or potential concerns for mobilisation of elements incorporated in products. High soluble metal and chlorine content are particularly concern regarding to the drastic standards required in the cement industry. Furthermore; fly ash from MSW incineration is not pozzolanic and thus cannot form a stabilised cement. APC residues of a semi-wet treatment with slurry MgO contain MgCl2 and MgSO4; which have both a high solubility. This characteristic implies that it is difficult to separate and purify each component. So the separation for solidification methods would need very expensive techniques. On the contrary; with slurry lime; the CaSO4 has not a good solubility; whereas the salt CaCl2 is highly soluble. Therefore the main aim of the valorization will be for the first time to reduce the volume of residues with elimination of soluble fraction (both Cl- and SO42-) and for the second time to recycle Mg(OH)2. A proposed procedure was to add NaOH until about pH10 to precipitate Mg(OH)2; then to add lime to obtain CaSO4; 2H2O (gypsum) not soluble. This component can be reused. The last step was to bubble through water of a part of flue gases released by stack; containing CO2. This procedure allowed obtaining some kilograms of solid CaCO3. But even if this solution could be considered as an ideal way; different points have to be mentioned: - This solution leads to a production of NaCl released in a river or in a sea; - The cost in the second case (recycling of MgO product) does not take into account the investment cost and the operational costs (except lime and NaOH costs); - This solution has no effect on Fluor elements (because of the solubility of MgF2). The MgF2 will stay in recycled Mg(OH)2 product and will increase more an more in the installation. Another additional treatment has to be found to remove the Fluor from the recycled MgO product. - APC residues are considered as hazardous waste. The only way to treat them in France and in other countries is the stabilisation and the disposal in hazardous waste landfill. Every new recycling method for these residues will have to prove to be healthy; what implies a long and costly period test before commercial use. It has to be noted that recycling studies for APC residues Stabilisation test were not possible because of the non-efficiency of MgO (residues samples not representative due to an excess of MgO). Physico-chemical study Control of hydrogen chloride and other undesirable substances (SO2; dioxins; heavy metals) is commonly required in combustion of municipal waste. The main sorbents that are used to lower the emission of HCl and SO2 are calcium-based products due to their low cost and abundance. Despite some interesting advantages of magnesium-based sorbents; resulting from their lower molecular weight and their milder basic character; seldom researches are reported on the use of these sorbents. To compete with the low cost calcium-based sorbents the magnesium-based sorbents used in this project have been produced from natural magnesite. The main target was the production of the most performing magnesium-based sorbents from natural magnesite and the testing of their performance-suitability for the above-mentioned application. The reactivity and therefore the chemical behavior of caustic magnesia and magnesium hydroxide depends on their physical properties; mainly the microstructure and grain size distribution. The reactivity is expressed by the neutralization speed of various acid reagents with the above sorbents while the contribution of the microstructure by the specific surface area (S.S.A) or the so-called �Iodine number�. The systematic investigation of all production parameters affecting the microstructure and other physical characteristics of caustic magnesia and its hydration to magnesium hydroxide leads to the following results: - Definition of production parameters and conditions to produce caustic magnesia with S.S.A > 60m2/g and low amount of non-decomposed magnesite. - Preparation at laboratory scale of a very reactive magnesium oxide (S.S.A > 200m2/g). 3. Development and pilot-scale production of Mg(OH)2 with S.S.A > 40m2/g. For the production of caustic magnesia with S.S.A > 60m2/g the feed size of magnesite and the burners were adjusted in order to achieve a better calcination control. Regularly the feed size of the rotary kiln is from 0 to 3-4cm and due to direct contact of magnesite with the flame there is a gradual calcination from the surface to the core of the grains which results to soft-burning of the core while the outer shell of grains and the finer fractions are almost high calcined. Selecting a narrower feed size (0;5-3cm) the above-mentioned results are minimized. In addition; thermal profile of the rotary kiln; which determines in high degree the thermal calcination cycle of raw magnesite; depends on the features and operating conditions of the burners. Adjusting the shape and the length of the flame and taking into account a better control of fuel consumption and combustion air; uniform caustic magnesia is produced concerning its physical characteristics. The trials for the production of Mg(OH)2 with high S.S.A (> 40m2/g) in comparison with commercial products which have a much lower S.S.A (< 25m2/g) were focused on the control of the hydration reaction in order to obtain a fully hydrated magnesia with fine particles and low agglomeration. During reaction of caustic magnesia with water a rapid hydration is observed in the beginning which slows down afterwards and the degree of conversion tends to a limiting value because a thin membrane of Mg(OH)2 is formed on the surface of magnesia particles which prevents the full hydration. This formation results to the blockage of active sites on the surface of magnesia and the final product is a low activity Mg(OH)2. In addition the rapid hydration leads to the agglomeration of fine particles decreasing also the activity. In order to overcome these difficulties the calcination of magnesite must be well controlled to produce caustic magnesia with the proper physical characteristics. The control of hydration is also of great importance while the use of hydration additives prevents the agglomeration of particles and enhances the degree of hydration. The above results constitute an innovation not only for Grecian Magnesite but also for magnesia market. Caustic magnesia derived from natural magnesite was out of applications; which require high activity; where the synthetic magnesia is used. Furthermore; to our knowledge the commercial products of Mg(OH)2 (even the synthetic ones) have S.S.A < 25m2/g. The acquired know-how will help our company to develop products in order to penetrate into new markets where it was absent until now and might give a new perspective for the products of natural magnesia. Of course it must be stressed the insuperable handicap of natural magnesia which is the lower chemical purity. Thermo-chemical study Control of hydrogen chloride is commonly required in combustion of municipal waste. Usually calcium-based sorbents are used to lower the emission of HCl in parallel to SO2 removal. Ca-based sorbents are widely utilized due to their low cost and high efficiency toward acidic gases. Therefore nearly all researches were aimed at using Ca-based sorbent for reducing the SO2 emission and with a small proportion discussing HCl removal. Employing magnesium-based sorbent for hydrogen chloride was seldom reported. However; there is an interest from the manufacturers of magnesium-based products to further investigate Mg-based sorbent for removal of acidic gases. The study of the relation between sorbent properties and removal efficiency as well as the reation kinetics will contribute in development and production of improved MgO and Mg(OH)2 based products. The reactivity of Mg-based sorbents for reduction of HCl and SO2 was investigated at low temperatures. The experiments were performed in a fixed-bed reactor with baseline conditions set at 0.5 gram sorbent; 1000 ppm HCl; and 10% water; at 100°C. It was found that specially treated Mg-based sorbents showed comparable reactivity with HCl and SO2 as a commercial Ca(OH)2 did. The superior capacity of the Mg-based sorbent was attributed to its large surface area; small particle size and small crystal size. The presence of 10% water improved sorbent utilization significantly. The effect of temperature was found limited in the range of 50 to 100C in the absence of water. The kinetics of the reaction between the sorbents and HCl was simulated by a fast reaction plus a slow reaction. The ratio of the sorbent reacting by the two mechanisms provided a good measure of sorbent�s reactivity. The present work aimed to study the capacity of Mg-based sorbent for acidic gases; mainly for HCl but also with consideration for SO2. Sorbents prepared by different methods were evaluated and compared with commercial Ca-based sorbents. The kinetics of the reaction between MgO or Mg(OH)2 and HCl was also studied. Fixed-bed reactor The experiments were performed in a fixed-bed reactor with an inner diameter of 20 mm. The reactor consisted of a vertical glass tube with a coarse glass plate as a support for the bed. Sorbent was dispersed into 20-gram inert sand bed with a weight ratio of 1:40; in order to eliminate the poor flow pattern. Desired synthetic flue gas was prepared by mixing air with HCl and SO2 in N2 from gas cylinders. During the early stage of experiments; synthetic flue gas containing HCl was also obtained by passing air through an HCl desorber. The gas was led through the bed in the reactor and the effluent gas was directed to an FTIR analyzer or to an HCl absorber. The concentrations of HCl and/or SO2 were measured by the FTIR analyzer. The concentration of HCl could also be determined by titration at PH=4.0 in the absorber. The reactor system was equipped with a bypass system to enable analysis of the inlet gas compositions for each experiment. Experiments were performed to investigate the capacity of sorbents for HCl reduction; with or without water; in the temperature range of 50 to 200°C. Water in the reactor system was rather problematic. In our fixed-bed reactor system; water was added to the synthetic flue gas with a motor-driven syringe and evaporated in a heated humidifier packed with glass fiber and rings to ensure good mixing and even evaporation of water. Since HCl is very soluble in water; condensation of water in the pipes could lead to unpredictable drops and peaks in the breakthrough curves. Thus the pipes were heated to avoid uncontrolled condensation of water; and drying of the gas was employed both before and after the reactor by means of three CaCl2 dryers. Sorbents: The samples tested were ten Mg-based sorbents from Grecian Magnesite (GM) in Greece; three Ca-based sorbents from Dankalk in Denmark; and one NaHCO3 sample in analytical purity. The Mg-sorbents were seven MgO samples and three Mg(OH)2 samples derived from these MgO; and Ca-sorbents were CaCO3; CaO and Ca(OH)2. The Mg-sorbents were mainly provided by Grecian Magnesite. Two Mg(OH)2-samples showed very high bed utilization and were very effective absorbents. Pilot depollution results In order to validate the results obtained from the laboratory scale; efficiency tests of flue gas treatment using the different samples of Mg-based product (from the current production and the modified ones) were conducted on a pilot-scale rotary kiln. A mixture of synthetic wastes was formulated and prepared in order to compose a gas emission mixture containing acidic pollutants; dioxin and heavy metals. The results obtained; concerning the efficiency of flue gas treatment using the downstream dry abatement; confirmed the results obtained from laboratory scale. However; the best results for acidic pollutants abatement was obtained using the semi-wet treatment and the modified sample Mg(OH)2 thin slurries. The characteristics of Mg(OH)2 retained were a sample with 40m2/g of S.S.A; fine grains size with lack in very fine fractions. This magnesium hydroxide was produced from reactive caustic magnesia under well-controlled hydration conditions. Searching for OpenAIRE data... There was an error trying to search data from OpenAIRE No results available
