Process and apparatus for fixing, encapsulating, stabilizing and detoxifying heavy metals and the like in metal-containing sludges, soils, ash and similar materials

Abstract: Heavy metals and compounds thereof and other toxic materials in industrial wastes, sludges, soils, incinerated ashes and the like are fixed and stabalized in a char residue, obtained by critical region pyrolyzing techniques and appro­ priate proportions of cabonaceous materials intimately mixed with the sludge, to encapsulate the heavy metals with carbon bonded thereto which effectively detoxifies the residue and renders it immune to any substantial leaching out or later exposure to the toxic metals, such that the same is environ­ mentally safe for such uses as landfill and the like.

Description: The present invention relates to processes and apparatus for detoxifying heavy metals and the like contained in sludges, soils, incinerated ashes and similar materials; being more particularly directed to novel techniques for effecting the encapsulation, fixing and stabilizing of such residues to render them safe (by at least Environmental Protection Agency (EPA) standards) from the leaching out of, or later exposure to, heavy metal products therein, such that the residues may be directly used in landfills and other applications, if desired. Previous approaches to the solution of the above problem have been made through the use of chemical fixation and stabilization techniques, including the combining of silicates and Portland Cement to produce a stabilized, solidified material. While such processes have some effectiveness in binding heavy metals, the effectiveness in stabilizing volatile organics is questionable, and the solidification process unfortunately requires increasing the volume of the sludge by ten to twenty percent. The solidified product is landfilled or may be used for landfill cover material but, as compared with the later-described fixed product of the present invention, is at least twice as expensive, with the invention providing most advantageously for about eighty seven percent less weight and seventy percent less volume in some tests. In accordance with the techniques employed to attain the results of the present invention, a pyrolysis step is employed that of itself has been used in prior systems that are and have as their objectives the antithesis of the current invention; that is, the deliverate recovery of the heavy metal from the end product. One such is described, for example, in U.S. Letters Patent No. 4,332,584 where the resulting carbon and heavy metal residue can be centifuged to recover chromium, for example, and the chromium may be readily leached out by dissolving in sulfuric acid solution. Other metal recovery techniques, as distinquished from metal fixing techniques, are disclosed in my prior U.S. Letters Patent No. 4,086,319 and in Patent No. 4,215,989. Examples of materials with which the fixing, stabilizing and thus effective detoxifyzing of heavy metals and the like of the invention may be employed include sludges, wastes such as tannery waste with its high chromium content, soils, ash materials and the like (often hereinafter generically referred to as "sludges"); and typical heavy metals or metal compounds (often hereinafter generically referred to as "metals") include chromium, arsenic, barium, cadmium, lead, mercury, selenium, silver, nickel, zinc, copper and others. In accordance with the present invention, unlike prior pyrolysis systems, it has been discovered that the chrome or other heavy metal sludge, tannery wastes or other heavy metal-containing solutions and the like may be treated in such a manner that the heavy metals are effectively non-leachable and cannot be recovered; and, indeed, are fixed and encapsulated with carbon-bonded coatings that enable the residue to be safely used for landfill and other uses without fear of ultimate leaching out of, or other later exposure to, the heavy metals from the end product. While the step of pyrolysis or heating of the sludge or tannery waste or the like, in a substantially oxygen-free atmosphere, is a step common to other processes and the present invention, thus, the way in which this step is utilized and subsequent steps are performed is uniquely designed to obtain the very different and indeed opposite result of a heavy metal-fixed char residue that is safe to the environment and from which the heavy metals cannot effectively be leached out. An object of the present invention, accordingly, is to provide a new and improved process and apparatus for fixing, stabilizing and effectively detoxifying heavy metals in a metal-containing sludge, soil, ash or the like that is not subject to the above-described disadvantages of prior chemical fixing and other techniques, but provides vastly improved, less costly and more efficacious stabilizing not only of the resulting residue but of the volatile organics and solvents driven off in the process, as well. A further object is to provide such a new process in which a novel intimate carbon-coating is bonded to or encapsulated with heavy metal residues to render the same permanently substantially non-leachable. Other and further objects will be explained hereinafter and are more particularly delineated in the appended claims. In summary, however, the invention involves a volume reduction process that thermally stabilizes the heavy metals and volatilizes organic compounds. The resulting residue often has fifty to eighty percent less volume than the original sludge or composite feed. The process operates in a starved air combustion unit capable of controlling temperature and residence time; and resulting residue contains a acid-insoluble matrix. This process produces about fifty percent more residue than incineration, but admirably passes EPA's Extraction Procedure Tests for leaching, heavy metals, and is therefore well-suited for landfill disposal. Heavy metals and other potentially toxic compounds are stabilized, moreover, at temperatures below their volatilization temperature, along with an intimate mixture of organic compounds. The thermal residence time is controlled to produce minimum volume without significant decarbonization (the latter being causable by excess oxygen). Toxic, volatile off-gas and odors are destroyed within a high temperature afterburner designed for apppropriate (two-second minimum) contact time. This process, as later discussed, has been tested effectively on sewage treatment plant sludges, among other materials, containing potentially toxic concentrations of chromium, lead, cadmium, barium, nickel, zinc and other toxic heavy metals, with the resulting residues tested using both the EPA Leaching Test employing acetic acid to acidify the residue to a pH of 5, and the well-known Multiple Extraction ME) Test that utilizes a subsequent sulfuric and nitric acid mixture to acidify the residue to a pH of 4. While sludge incinerators and trash incinerations often produce ash residues containing toxic, leachable metal oxides and dioxin, the residue produced with the process of the present invention does not contain significantly soluble metal oxides (e.g., chromium), as later tabulated. The residue, furthermore, can be mixed at ten to fifteen percent, for example, of Portland Cement (on a weight basis), if desired, with the quantity of water equal to the weight of the residue, and some coarse sand. The resulting cemented residue material has a specific gravity of about one, it is fireproof, and it has sufficient structural strength to be safely used for lightweight building products. Another illustrative application of the invention is for stabilizing toxic metals within waste sludges from steel drum reconditionig plants, the empty drums containing a variety of residual materials which often contain toxic metallic and organic compounds. The resulting residues show a 50-percent reduction in volume with the toxic metals immobilized with carbon, and the solvents destroyed in the afterburner. The invention can also be used to process toxic waste and eliminate the need of disposing of these materials in hazardous waste landfills; with the residue used for landfill or solidified for use in building products such as cinder blocks, asphalt, concrete or other lightweight products, as before suggested. As still another use, the invention may aid in the recovery of chromium from the leather and metal plating industries since the char residue passes the EPA Extraction Procedure, indicating that the residue will not leach excessive quantities of chromium. Subsequent roasting of such residues containing chromium produces a chromate ash from which chromium can be extracted, as ins one of the above-cited patents, with the remaining insoluble residues treated by the invention to produce a non-toxic reside. When a carbon source is mixed with an ash containing toxic chromium, as still another example, the process of the invention involving starved air conditions, results in a residue that cannot leach any significant levels of chromium, such that the invention allows toxic ash to be effectively detoxified, as well. According to the invention there is provided apparatus for fixing and stabilising heavy metals in a metal and carbon containing sludge as characterised in claim 1. The invention will now be described with reference to the accompanying drawings wherein; Fig 1 is an illustration of apparatus for use in performing the invention omitting means for introducing carbon-containing material to the sludge to be treated; Fig 2 is a graph illustrating the critical zone of operation required for the novel results of the invention; Figs 3 and 4 are flow diagrams illustrating the process of the invention and Figs 5 and 6 add drying steps. Referring to Fig 1, sludge containing toxic heavy metals and volatiles is dewatered on client's own existing belt press 2, and is conveyed to a tumble dryer 4 (as of the Tumbler Dryer Type of Bartlett Snow Pacific Co., for example). This dries the material from the one-third solids coming from the sludge belt press 2 to approximately 65% solids, utilising waste heat generated by the afterburner 6, to be described later. The material dried to 65% solids then enters at chute 4' (by gravity) into a pyrolysis unit 8, the size of which can be reduced because of the effect of weight reduction caused by the at least partial drying or pre-drying operation. Although not shown in Fig 1, a carbon-containing material may be mixed with the sludge between the conveyor and the tumble dryer 4 such as by means of mixer 16 shown in Figs 5 later to be described, or alternatively by introducing the carbon-containing material between the dryer 4 and pyrolyser unit 8 at 8', as shown in Fig 6 later to be described herein. The unit 8 provides a largely airtight oxygen free combustion furnace environment for pyrolysis at temperatures, in this particular case, of around 482°C (900°F), below the volatilisation temperature of most heavy metals before mentioned (and also, the oxides of nitrogen and sulfur as well, to avoid their presence) but sufficient to drive off the volatile organics and solvents as off-gases, later described. The critical conditions of operation for attaining the fixings of the invention by carbon-bonding or encapsulating, will later be described; a suitable pyrolysis unit adjustable for such purposes being, for example, that if Shirco Infrared Company of Dallas, Texas. The before-mentioned afterburner 6 (also, for example, of the Shirco Type) further oxidises and volatilises the off-gases and volatiles at about 1000°C (1800°F); though if highly toxic wastes are used, the afterburner temperature may be raised to around 1315°C (2400°F). As earlier stated, the off-gases or heat from the afterburner 6, in such case, may be used as at 6' to dry the incoming sludge into the dryer 4. As illustrated, non-contact drying is used so that the hot gases do not come in contact directly with the incoming sludge. The hot gases and water (entered into a venturi), are intermittently mixed and applied to a wet scrubber system 18. Since there is a potential of hexavalent chrome being generated in small quantities in the afterburner, a reducing chemical is fed to reduce any hexavalent chrome to the trivalent state. The final unit is a stack 10 which exhausts the clean effluent gases to the atmosphere. Assuming the carbon-encapsulating conditions, later described, in the pyrolysis chamber 8, at the bottom of Fig 1 there is shown the ash removal system embodying in ash-cooling conveyor 12 having an airtight connection between the furnace 8 and the ultimate truck T going to the landfill. the ash is thereby water-cooled to reduce the temperature of the material from 482°C (900°F) to somewhere below 120°C (250°F) at which exposure to the air cannot result in combustion. Returning, now, to the features of the process necessary to obtain its novel results, underlying the invention is the discovery that there is a criticality to the pyrolysing or heating process in the absence of oxygen in the furnace 8 that brings the process to a point where the carbon-containing material in the sludge or the like, becomes bonded to and encapsulated with the dangerous heavy metal content. This has been discovered to reside in the region where substantially all the volatile organics and solvents in the original sludge have been driven off at 6'' in the substantially oxygen-free atmosphere, whereby the resulting char residue is at a point of substantially minimum weight. This minimum weight region is shown at the shaded region "Preferred Stabilisation Zone" of the graph of Fig 2, plotting percent of original weight of the sludge applied at 4' along the ordinate as a function of time of pyrolysis plotted along the abscissa in minutes for the approximate 482°C (900°F) of the above example. In this test it took about 24 minutes to pyrolyse the carbon-coated material to its minimum weight of about 8-9%. This is as distinguished from operation of the left-hand steep curve regions of prior art metal recovery, as contrasted from the fixing result of the present invention. At such a time, the char residue, again in the absence of air, is cooled down at 12 to a temperature where it will not ignite if exposed to air (about 120°C (250°F) or less). Thereafter, cooling continues to ambient temperature. Under such conditions, it is found that the carbon is bonded or fixed to the heavy metal residue in a state where it is substantially non-leachable and where it is readily usable with safety for such purposes as landfill and the like. To prevent dusting, water may be added in the utilisation of the residue product and/or it may be encapsulated in cement or the like for producing fireproof materials, as before explained. Alternatively, silicate and clay-type materials may be added to the sludge to result in a non-dusting cooled residue. It is important, however, that there be sufficient carbon material in intimate contact in the sludge or other material to enable this complete bonding, encapsulation, fixing and stabilising of the heavy metals in the residue, and this may be provided by adding hydrocarbon solvents to the sludge material. The following Table 1 presents experimentally obtained results for the above-described industrial sludge, demonstrating the insignificant leaching of heavy metal residual contents by EPA's Leaching Test procedure (centre column, in mg/L) in the char residue produced the process of the invention, as compared with the allowable (and much greater) EPA limits. As another example, Table 2 shows similar substantially non-leachable or fixed conditions in a residue prepared by the above process from a second POTW sludge with a high percentage of industrial contribution: METAL BULK ASSAY SLUDGE SOLIDS mg/kgm DRY WEIGHT INVENTION CHAR RESIDUE mg/L MAXIMUM EPA LIMITS mg/L ARSENIC 8 < 0.01 5 BARIUM 1,030 20 100 CADMIUM 309 0.27 1 CHROMIUM 1,490 0.04 5 LEAD 126 0.48 5 MERCURY 0.1 < 0.002 0.2 SELENIUM < 0.7 0.011 1 SILVER 28 < 0.005 5 NICKEL 910 2.2 ZINC 2,210 1.0 COPPER 4,400 1.3 Cost comparisons using waste water treatment plant dewatered sludge, moreover, indicate that the present invention, with residue disposal, costs from $30 to $40 per ton; landfill, about $50 to $60 per ton, and compost (assuming adequate controls on the concentration of toxic industrial waste contamination), about $35 to $45, after deducting the value of the compost. As still a further example of the efficacy of the invention, Table 3 prevents the percent fixation achievable by the process of the invention on a sample of treated wet inorganic sludge (chromium hydroxide) at 19% Cr on a dry solids basis: A further test of the efficacy of the process underlying the invention was made on both organic and inorganic sludges (Cr(OH)₃) and the latter both in the presence of insufficient carbon for adequate intimate admixing with the heavy metal sludge to effect the total fixing results of the invention (as when so moist as to cause a degree of decarbonization) and in the presence of sufficient carbon, adequately contacted and intermixed, under EP Extraction Procedure (0.5 N Acetic Acid at pH 5) and under severe 1 Molar sulfuric acid extraction or leaching (20 times the gram weight): These results again illustrate the importance of a sufficient carbon source added to intimately mix with the metal-containing sludge (such as added carboneous fuel, diesel, gas, coal, etc) to establish the results of the invention. In some cases, moreover, it is preferable not to dry the residue to the almost dry (>95%, for example) state used in heavy metal recovery techniques, as for recovering chromium from char ash; but rather, to limit the drying to keep sufficient moisture (such as the 65% solids earlier described, as an illustration) to insure the intimate and adequate encasing of the metal-containing sludge or ash or the like for the fixation purposes of the invention. The invention also is a one-step process as further distinguished from the antithetical recovery systems involving first oxidation and then reduction. In the process according to the invention as shown in the flow diagram of Fig 3, sludge is mixed at 16 with a hydrocarbon fuel (liquid or solid) first to provide a direct mixing contact of the carbonless feed with a sufficient amount of carbon for achieving the fixation process of the invention. These soils can also be used as fee stuff as well as any hazardous waste that may be necessarily required to be detoxified. If the mixing at 16 involves solids, various configurations, such as a pug mill, may be used for the solid/solid intimate mixing. Again, the thermo-processor off-gases, mixed with air, are applied to the afterburner 6 and the stream goes to scrubbing for emission control at the stack 10, and with the fixed char residue discharging to the ambient air after cooling at 12. In the modified flow diagram of Fig 4, on the other hand, the sludge is fired with carbonaceous fuel but, in this case, a gaseous or a liquid fuel is added directly at 8' to the process furnace 8. Since the processing of the invention at 8 is under starved air pyrolysis conditions, it presents an option of having the feed intimately enjoined with the available carbon in a gaseous state during thermo-fixation; and it would be used, in such cases, where the nature of the feed would allow for a gaseous carbon source that would generate a char residue demonstrating the fixation properties of the invention. The system of Fig 5 combines pre-drying with the mixing (16) of non-organic or organic feeds with carboneous fuel of Fig 4. In the modification of Fig 6, on the other hand, the pre-drying (4) of Fig 5 is combined with the application of carbonaceous fuel at 8' into the processor pyrolysis furnace 8 of Fig 4. As before mentioned, the dried metal-fixed char residue cooled at 12 and ultimately at ambient temperature, may have water added to prevent dusting, or may be encapsulated in cement, or, indeed, silicate and clay-type materials may have been added to prevent an ultimate product that can dust.

Claims: Apparatus for fixing and stabilising heavy metals in a metal-containing sludge, soil or ash feed material comprising pyrolyser means (8) for heating the sludge, and a conveyor means for feeding the sludge to the pyrolyser means (8) characterised by the provision of means for introducing carbon-containing material to the sludge and intimately contacting the sludge with the carbon-containing material, adjustment means for adjusting the temperature of said pyrolyser means (8) to heat the sludge in an oxygen-free atmosphere until a minimum weight char residue being of the order of 8-9% of the original weight of the sludge in the form of carbon bonded to the metal is produced when substantially all volatile organics and solvents have been driven off, first cooling means (12) for cooling said char residue received from said pyrolyser in the absence of air to a temperature less than about 120°C (250°F) to ensure that the residue will no longer combust if exposed to air, and second cooling means (T) for thereafter enabling further cooling the residue received from said first cooling means to ambient temperature to provide a non-leachable residue. Apparatus as claimed in claim 1 wherein dryer means (4) is provided for at least partially pre-drying said metal-containing material before feeding the same to the pyrolyser means. Apparatus as claimed in claim 2 wherein after-burner means (6) is provided for oxidising further the volatiles driven off and further heating the metal-containing material in said dryer means (8). A process for fixing and stabilising heavy metals in a metal-containing sludge, soil, ash material and the like characterised by introducing and mixing with the sludge a carbon-containing material and then heating the sludge in an oxygen free environment to at least 482°C (900°F) to drive off volatile organics and solvents, continuing said heating until a minimum weight of char residue is produced in the form of carbon bonded to the metal with substantially all volatile organics and solvents being driven off, cooling the char residue in the absence of air to less than around 120°C (250°F) to ensure that the residue will no longer combust if exposed to air, thereafter cooling further to ambient temperature to provide a non-leachable residue. A process as claimed in claim 4 wherein at ambient temperature, water is added to prevent dusting. A process as claimed in claim 4 wherein the cooled residue is encapsulated in cement rendering the same fire-proof. A process as claimed in claim 4 wherein silicate and clay-type materials are added to the sludge to condition the residue and prevent dusting of the cooled residue. A process as claimed in claim 4 wherein the carbon-containing material comprises an added hydrocarbon solvent. A process as claimed in claim 4 wherein the volatilsed organics and solvents are further oxidised to destroy same. A process as claimed in claim 4 wherein said metal-containing material is pre-dried at least in part before said heating in the oxygen-free environment. A process as claimed in claim 10 wherein the said volatiles driven off are after-burned and scrubbed before exhausting. A process as claimed in claim 11 wherein the after-burned volatiles are passed to provide non-contact drying in the pre-drying of said sludge. A process as claimed in claim 11 wherein wet air from the pre-drying of said material is condensed into a dry air and is combined with the said volatiles driven off for said after-burning. A process as claimed in claim 4 wherein said carbon-containing material comprises a carbonaceous fuel.

Inventors: JONES BRADFORD H, JONES, BRADFORD H
Applicants: JONES BRADFORD H, JONES, BRADFORD H
Application year: 1986
Main classification: C10B53
IPC codes: B09B3, C10B53, C02F11, B09C1