Abstract:
Fly ash from incineration plants is treated with an acidic aqueous solution for the purpose of removing heavy metals. The residual solid matter is filtered. The filtration residue is treated with a precipitating agent for heavy metals. The thereby obtained inert residue can be directly dumped.

Description:
This application is a continuation of application Ser. No. 08/144,615, filed on Nov. 1, 1993, now abandoned; which is a continuation of U.S. Ser. No. 07/793,952, filed Oct. 22, 1991, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention broadly relates to a process for treating or purifying flue gases or fly ash and, more specifically, pertains to a new and improved process for the removal of heavy metals from the fly ash from an incineration plant, especially a waste incinerator. 
     Generally speaking, the process of the present invention is of the type according to which the resulting fly ash is subjected to treatment with an acidic aqueous solution and the liquid is separated from the residual solid matter by filtration. 
     Flue particulates come as a result in heating installations and incineration plants and are removed from the flue gases by means of fabric-base filters or electrostatic filters. These flue particulates, particularly in waste incinerators, contain heavy metals. The presence of soluble heavy-metal salts prohibits direct dumping of the flue particulates. 
     A process of the aforementioned type is known, for example, from German Patent No. 3,320,466, published Dec. 13, 1984 and its cognate U.S. Pat. No. 4,617,180, granted Oct. 14, 1986. According to the process disclosed therein for purifying flue gases developed during combustion of waste materials, the heavy metals are removed from the flue particulates by treatment with the acidic aqueous solution resulting in the flue-gas washing or scrubbing process. The washing condensate from such washing process is combined with the flue ash to partially dissolve the heavy metals and extract them from the flue ash. Following solid-liquid separation, the acidic aqueous solution, essentially composed of the washing condensate and the dissolved heavy metals, is subjected to heavy metal precipitation. The residue remaining subsequent to the acidic extraction still contains soluble metal compounds and therefore cannot be directly dumped. Accordingly, the residue must be either previously bound within a matrix, e.g. cement or bitumen, or returned to the combustion system in order to bind it into the slag during the incineration process. 
     The binding or retention in a matrix does not prove satisfactory for various reasons. According to environmental conditions, cement is sooner or later hydrolyzed, whereby the metal salts are set free. Water is entrained while returning or recycling the filtration residue to the combustion furnace. This adversely affects the energy balance. 
     SUMMARY OF THE INVENTION 
     Therefore, with the foregoing in mind, it is a primary object of the present invention to provide a new and improved process for treating flue particulates and thereby removing heavy metals from the fly ash of an incineration plant, and which is not afflicted with the drawbacks and limitations of prior art processes. 
     Another and more specific object of the present invention aims at providing a new and improved process for the treatment of flue particulates and by means of which the direct dumping of the treated product is rendered possible. 
     Now in order to implement these and still other objects of the present invention, which will become more readily apparent as the description proceeds, the process of the present development is manifested, among other things, by the step of treating the filtration residue with an aqueous solution of a precipitant or precipitating agent for heavy metals. 
     By means of the process according to the invention the soluble heavy-metal salts, which have not yet been removed after the acidic washing or extraction, are converted to an insoluble form. 
     Subsequent to the treatment with the precipitant or precipitating agent, the filtration residue, which practically no longer contains any soluble heavy metals and can be directly dumped, can be washed with some water for the purpose of removing the precipitant or precipitating agent. 
     The filtrate and the liquids from the treatment of the filtration residue can be brought together. The heavy metals are precipitated from this aggregate or combined liquid. The resulting sludge is upgraded and conducted to metal recovery. Bringing together these liquids enables using in excess the precipitation agent during the treatment of the filtration residue, since non-spent precipitants are automatically spent while precipitating the heavy metals. 
     According to another variant of the inventive process, the washing water of the filtration residue can be conducted to the flue-gas scrubber. 
     The choice of the precipitant or precipitating agent for the treatment of the filtration residue depends on the type of the burned or incinerated material and is also determined by economic considerations. Examples of suitable precipitants are caustic alkalis or earthy base hydroxides, for instance calcium hydroxide Ca(OH) 2 , and inorganic as well as organic sulfides. 
     It is advantageous to add a soluble alkaline-earth compound, preferably a barium compound, to the mixture of fly ash and acidic washing water. In this manner, sulfates present in excess in the flue-gas washing liquid are precipitated. This contributes, on the one hand, to the inerting of the fly ash to be dumped or disposed and, on the other hand, to the reduction of the quantity of the dissolved solids present during heavy-metal precipitation as well as of the quantity of the resulting sludge. The latter leads to a more economical process in the following recovery of the heavy metals. 
     The removal of ammonia or ammonium salts from the fly ash can be effected by heating prior to carrying out the acidic scrubbing. On the other hand, removal of ammonia or ammonium salts from the neutralized washing liquid, i.e. the filtrate, can be effected by stripping. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process and the inventive steps thereof are now illustrated with the aid of the following example, which is included for illustration purposes only and is not necessarily meant to limit the invention. 
     EXAMPLE 
     On an average, the following materials result from incinerating or burning one ton of waste or garbage: 
     approximately 300 kg sludge, 
     approximately 25 kg fly ash, and 
     approximately 112 kg acidic washing water from the wet flue-gas purification, containing 4.5 kg hydrogen chloride (HCl) and 3.0 kg sodium sulfate (Na 2  SO 4 ). 
     To begin with, mercury is removed from the acidic washing water or liquid by means of an ion exchanger resin. The 25 kg fly ash are treated with the 112 kg of acidic washing water during 30 minutes and at a pH of 3 in a vessel or boiler provided with an agitator or agitating machine. Mobile and mobilizable heavy metals are thereby dissolved. The obtained suspension is given or conducted to a vacuum belt filter. The fly ash freed from the heavy metals and remaining on the filter as inert residue is washed first with a solution containing 0.5 to 2.0 g/l TMT 15 and 0.5 to 2.0 g/l Ca(OH) 2  and then washed with water, TMT 15 being a 15 percent solution of the precipitant trimercapto-S-triazine (C 3  H 3  N 3  S 3 ). There is thereby used a total amount of 3 liters liquid per 1 liter fly ash, 1 kg fly ash corresponding to approximately 2 liters. 
     The heavy metals are precipitated from the united or combined liquids, i.e. filtrate and washing liquids, in that 3400 g of Ca(OH) 2  (dry) are added in water. The resulting sludge is dewatered in a filter press. The filter cake, which is designated as the heavy-metal concentrate, is then conducted to the metal-recovery process. 
     The following Table I shows a material balance of the process. The indications of weight indicate the quantity of dry substance. All indications of percentage refer to the dry-substance quantity which is brought in with the fly ash and the acidic washing water from the wet or wet-chemical flue-gas purification, such dry-substance quantity representing 100%. 
     
                                           TABLE I__________________________________________________________________________                 heavy-metal                        total  in g  inert residue                 concentrate                        waste water  (= 100%)        in g in %                 in g                     in %                        in g in %__________________________________________________________________________dry substance  30500 16200             53.1                 1700                     5.6                        16000                             41.3 (*)Cl     6300  &lt;10  &lt;0.15                 17  0.26                        15973                             99.6SO.sub.4  2800  2500 89.3                 50  1.8                        250  8.9Zn     1070  380  35.5                 690 64.5                        &lt;0.5 &lt;0.05Pb     356   286  80.3                 70  19.7                        &lt;0.1 &lt;0.03Cd     14.4  1.6  11.1                 12.8                     88.9                        &lt;0.01                             &lt;0.05Hg     1.42  0.09 6.3 0.03                     2.1                        &lt;.001                             &lt;0.07 (**)__________________________________________________________________________ (*) 3400 g of CA(OH).sub.2 (computed as dry substance) are added during precipitation of the heavy metals. (**) 91.6% of mercury are removed by the Hgpreprecipitation. 
    
     The extraction or precipitation yields with respect to the starting material `fly ash` are thus as follows: 
     dry substance: 35%, cadmium (Cd): 89%, zinc (Zn): 65% and lead (Pb): 20%. 
     The composition of the fly ash designated as inert residue and freed from heavy metals is listed in the following Table II. 
     
                       TABLE II______________________________________                  Content according                  to the TVA       Total Content                  eluate test(*)Material    g/kg       mg/l______________________________________Cl          0.69SO.sub.4    173.0Zn          11.60      3.85Pb          8.40       0.24Cd          0.05       0.08Hg          0.008      &lt;0.001______________________________________ (*) The TVA eluate test (Technische Verordnung fur Abfalle, i.e. `Technical Regulations regarding Waste`, issued by the Swiss Federal Ministry of the Interior) is a standardized process for determining whether waste acceptance requirements for direct dumping are met or not. The values obtained by virtue of the inventive process for treating flue particulates are below the limiting values specified in the fourth appendix (Anhang 4) of the TVA. In other words, fly ash treated according to the inventive process is suitable for direct dumping. 
    
     The composition of the filter cake, which was obtained by precipitation of the heavy metals from the waste water, i.e. filtrate and washing water, is listed in the following Table III. 
     
                       TABLE III______________________________________Material     Content in g/kg______________________________________Cl           4.9SO.sub.4     27.0Zn           290.0Pb           44.0Cd           6.0Hg           &lt;0.01______________________________________ 
    
     This filter cake is conducted to the metal recovery process. 
     While there is shown and described the present preferred example of the inventive process, it is to be understood that the invention is not limited thereto, but may be otherwise variously practiced within the scope of the following claims. ACCORDINGLY,