FR2739793A1 - Treatment of ash or waste gases to render them inert - Google Patents

Abstract

The treatment of ashes or waste gases by the aqueous extraction of the particles with phosphoric acid or acidic calcium phosphate followed by formation of a phospho-calcic gel at a neutral or basic pH. The gel agglomerates the ash particles or washed waste gases, allowing filtration to form blocks of treated dust. The treated residues may be vitrified with or without addition of fluxes such as borosilicate glass, borax or sodium silicates, in an arc furnace or induction crucible, or incorporated in a solid by mixing with a hydraulic binder (cement) or an organic binder(acrylic polymer or bitumen).

Description

Recent public demands for environmental protection are leading more and more municipalities to opt for a household waste incineration system in order to avoid clogging up conventional landfills. However, incineration produces residues which are also problematic.

Clinkers, recovered relatively easily and recyclable, despite the presence on their surface of a variable amount of dust, are used in various materials and are not considered as ultimate waste. On the other hand, fly ash or particles trapped by filters and the washing of smoke produce refiom: smoke residues from the incineration of household waste. The legislation pushes to treat this ultimate waste in order to inert it or possibly to recycle it. Indeed, their transport and storage in class I landfills present risks. The standards also specify the conditions of resistance to leaching and the friability of inert products intended to be buried.

Refioms have been the subject of numerous studies aimed at transforming them or including them in various products. Thus, it has been proposed in certain countries to dilute them with ordinary waste, to compact them into briquettes by simple addition of water, to include them in gypsum, cement, glass, bitumen.

Many processes are the subject of patents, but there is no ideal solution which makes it possible to actually make the treated refiom completely inert.

A German patent describes a method of washing with water which makes it possible to introduce the refiom washed in a cement which resists leaching well, but the metal salts recovered by evaporation of the aqueous solution must be considered. The same goes for the Corning process where the treated refiom are analyzed and certain additions dosed in order to obtain a glass by melting the whole. The plasma torch vitrification processes also generate a lot of smoke and cause the evaporation of mercury and the release of chlorine. As for incorporation into cements, the presence of heterogeneous ionic elements does not promote the stabilization of the solid obtained which becomes porous with time and humidity.

We now describe our invention which makes it possible to economically solve the problem of inerting refiom. This process can also treat calcination residues from industrial products.

The method, known as the apatite process, generally consists in extracting a majority of the soluble chlorides from the refiom in the form of alkali salts and in trapping the remaining mineral species in an apatite matrix. This method adapts well to dry, wet refiom, neutralized with lime or sodium carbonate or even containing unusual impurities. There are therefore several ways of treating refiom by the apatitic process which are differentiated by the supply of phosphoric acid and calcium but which have in common the obtaining of an apatitic phase. We now explain the importance and scope of the apatitic process.

When phosphate ions are mixed with calcium ions, a solution is obtained, at acidic pH, which gives a gel at neutral or basic pH. This gel generally has a chemical composition determined by the precipitation kinetics of the tricalcium phosphate, Ca3 (PO4) 2. This gel transforms more or less rapidly, depending on the temperature, the pH and the excess of calcium, into hydroxylapatite, Cas ( PO4) 3 (OH) with an atomic ratio Ca / P of 1.667, or close to that of tricalcium phosphate which is 1.5. Even in the absence of excess calcium, an apatitic phase can be obtained by disproportionation due to the lower solubility product of apatite.

The construction of an apatite network is interesting for several reasons. First, the apatitic process is similar to a sol-gel process, comparable to those resulting from the hydrolysis of tetraethoxysilane or titanates or zirconates.

The advantage is that the apatitic process forms the gel without catalysis neither acid nor basic and without loss of volatile products. In addition, the gel is stable and formed at neutral pH. This calcium phosphate gel leads to a fine and stable dispersion and good coagulation of the refiom with a concentration which can go down to 5% by weight of alkaline-earth phosphate relative to the weight of the other mineral species. The amorphous gel allows easy filtration and solidification of the refiom with separation of the aqueous phase, with simultaneous or subsequent formation, during drying, of the apatitic matrix. This apatitic process is therefore sufficient in itself to collect the microscopic particles of smoke without require the use of organic or mineral flocculent surfactants or coagulants, iron hydrous alum or silicates but without excluding the possibility of using these additives. Refioms treated by the apatite process can be incorporated into conventional or special cements.

The second decisive advantage of the apatitic process is the progressive formation of hydroxylapatite, an insoluble mineral at neutral pH and stable in the natural geological environment. This apatitic phase is useful for the continuation of the operations thanks to the capacity of substitution in the hexagonal crystalline mesh. Indeed, the calcium ions and phosphates and hydroxides of the hydroxylapatite can be replaced, individually or simultaneously by other ions such as for example magnesium, lead, vanadates, sulphates, chlorides, fluorides, etc. These substituted apatites make it possible to trap unwanted ions in the crystal lattice or by coating the particles. Thus, the apatite process works on the atomic and microscopic scale simultaneously. It is known that apatite can decrease the aqueous concentration of lead ions even in the presence of nitrates and other anions.

We now continue with a description of the consequences of the apatitic process. The gel containing the refiom is hydrated, but is in the form of a cake resembling those obtained by the filter press decantation processes. The drying results in a macroscopic solid in which the refiom particles are agglomerated and purified from the majority of soluble chlorides. The residual chlorides are trapped in the apatitic matrix and do not interfere with the rest of the process. There are several outlets for refioms treated by the apatitic process. If a large quantity of calcium phosphate is produced compared to the rest of the minerals, one can consider, by dehydration followed by a slow crystallization, to inert this product.

Calcination can be carried out to ensure the insolubilization of the solid.

On the other hand, if a minimum of calcium phosphate is present at the end of treatment, the resulting solid lends itself well to transport or handling with the aim of carrying out vitrification by arc furnace. Indeed, the elimination of chlorides makes it possible to avoid corrosion of the graphite electrodes and to vitrify the refiom treated more efficiently and economically than by calcination or by a plasma torch. The arc furnace makes it possible to transform the melt into various vitrifiates in the form of fibers, porous beads, blocks, etc., which is not feasible by the other processes. We can therefore say that the apatite process is the first step in a process of upgrading or recycling refiom.

In the case of the minimum process, in addition to vitrification, the process of incorporation into a cement is facilitated. In fact, it is known that the apatite present in the cement protects it from degradation by aqueous carbonates or chlorides thanks to its amphoteric properties. Derived cement therefore has good mechanical properties over time and good resistance to leaching.

Another advantage of the apatitic process comes from the reuse, if necessary, of the lime used to trap gaseous hydrochloric acid in the production of the inerting matrix. The efficiency of the process also makes it possible to envisage incorporating, in the apatitic process, residues from the purification of washing water, such as toxic metal hydroxide sludges, even those containing small quantities of sulfides which will release sulfates. during aerial heat treatments. To avoid this, residues containing too much metal sulphides will preferably be incorporated into cements. Hydroxide sludges containing sulphides may be added to the phosphocalcic gel after its formation, i.e. at neutral or basic pH, and in special batches which will concentrate the metal ions. In other cases, to minimize the costs, the acidification can be done with sulfuric acid containing the minimum of phosphoric acid to be able to react with the calcium present or added in order to form the phosphocalcic gel.

Thus, the apatitic process globally solves all the problems: those posed by the presence of solid toxic metals such as those posed by the presence of toxic ions in the aqueous washing phase which are treated by the conventional methods of purifying the ions and recycled. with solid refiom. It is a complete process which does not generate new toxic residues. In addition to the possibility of generating a variable amount of apatite by the sol-gel process, it is possible to increase the acid or basic resistance to erosion of the treated refiom by mixing them with apatite powders of grain size included. between 1 and 20 micrometers.

The refiom obtained by treatment of the fumes with sodium carbonate pose a special case: they give off carbon dioxide in contact with acidulated water.

However, their basicity is useful and saves the soda used to neutralize the phosphoric acid and obtain the phosphocalcic gel. Thus, by combining two thirds of refiom treated with lime with a third of refiom treated with sodium carbonate, one obtains a final insoluble compact residue and elimination on the one hand of carbon dioxide and on the other hand in the aqueous phase of NaCl from the decomposition of CaCl2. In this case it is preferable to add, in order, the refioms treated with lime, phosphoric acid, and finally the refioms treated with calcium carbonate.

Finally, in the case of untreated fly ash, it may be useful to add lime, phosphoric acid and then soda or else to introduce lime and phosphoric acid in atomic ratio Ca / P 1.5, then an excess of lime to obtain a neutral or slightly basic pH with elimination of any chlorides in the form of soluble calcium salts. This procedure minimizes the number of products stored and used for processing.

We present a typical, but non-limiting example, which illustrates the steps of the invention.

For 1 kg of dry refiom, two liters of water are added and, with stirring, an acid phosphate is introduced until a pH <6 and preferably between 4 and 5 is obtained. In the case of a refiom beforehand dry treated with lime to trap chlorine, acid phosphate may simply be industrial phosphoric acid at a dosage of between 30 and 50 ml for the so-called minimum treatment. For a refiom previously untreated or dry treated with sodium carbonate, the acid phosphate can be calcium hydrogen phosphate or a phosphoric acid solution partially neutralized with lime. After mixing and homogenization of the acid suspension, a calcium dose is added in order to adjust the Ca / P atomic ratio so that the calcium concentration is not less than the phosphorus concentration, taking into account the analysis of the mobilizable species present before treatment by the process apatitic. This calcium intake can be made with a soluble salt such as chloride or nitrate, but preferably with whitewash. The pH is then adjusted in the range 6.5 to 10 and preferably 7 to produce the phosphocalcic gel. This neutralization can be done with an alkaline or alkaline earth base (NaOH or CaOH2).

After homogenization of the mixture by stirring, filtration easily separates the solid phase from the aqueous phase. Filtration can be done within minutes of gel formation. As the gel formation is fast, the whole process is controlled simply by the kinetics of the mixtures. The solid product obtained may contain 30% residual moisture before drying.

From this stage it can be incorporated into a hydraulic cement or a hydrophilic polymeric binder, but the most economical method consists in letting it harden by air drying.

 Subsequently1 the dry product easily crumbles, especially in the minimum process, so that it can easily be mixed with recycled glass cullet or borates in order to obtain a vitrifiable mass at low temperature. lkg of dry refiom produced from lkg to lkg200 of treated refiom, depending on the amount of phosphate added and lkg to 1.5kg of vitrifiate depending on the number of additions used.

Figure 1 summarizes the process as a whole.

Claims (9)

DREAM .N: DICATIONS 1- Apatitic inerting process for ash or industrial smoke characterized by the aqueous extraction of the particles with phosphoric acid or an acidic calcium phosphate followed by the formation of a phosphocalcic gel at neutral or basic pH. The gel agglomerates the particles of washed ash or smoke and makes it possible to filter them to produce blocks of treated dust. 2 - Process according to claim 1 wherein the phosphocalcic gel is produced using the calcium added to the fumes to neutralize the acid gases. 3 - Process according to any one of claims 1 or 2 wherein the dry dusty residues are treated simultaneously with the precipitates of wet washings of the fumes. 4 - Process according to any one of claims 1 to 3 wherein the phosphocalcic gel is mixed with silicates, alums, iron hydroxides, flocculents or organic polymers. 5 - Process according to any one of claims 1 to 4 wherein the amount of phosphocalcic solid varies from 5% to 30% by weight of the total mass. 6 - apatitic process according to any one of claims 1 to 5 wherein the soluble anions are extracted in the aqueous phase with an acid containing a part of phosphoric acid which allows by neutralization to form an apatitic phase which traps chlorides or halides residual. 7 - Method according to any one of claims 1 to 6 characterized in that the treated residues are vitrified with or without the addition of fluxes such as borosilicate glass, borax or sodium silicates, by arc furnace or induction crucible. 8 - Process according to any one of claims 1 to 6 characterized in that the treated residues are incorporated into a solid by mixing in a hydraulic binder like cement or an organic binder like hydrophilic acrylic polymer or bitumen kind. 9 -A method according to claim 5 characterized in that the resistance to erosion of the particles of incineration residues are increased by mixing them with apatite powders with a particle size between 1 and 20 micrometers.