Method and apparatus for treating waste materials

Abstract

A method and apparatus for treating waste materials comprising, particulating the waste materials into discrete particles, heating and drying the particles in a non-oxidizing atmosphere in a drier at a temperature in the range of 800 to 860 C. for carbonizing the particles, crushing the carbonized particles and leaching the crushed carbonized particles in an acid solution for dissolution of heavy metals into the solution, separating the leach solution containing heavy metal from the carbonized particles, adding to the carbonized particles particulate sodium hydroxide, silica, feldspar and limestone in a ratio of 100:0.3-0.5:8-12:2-4, mixing said particles with 15 to 18% by weight water to form a wet mixture and continuously extruding the wet mixture to form an elongated continuous extrusion, severing the elongated extrusion into blocks or planks of predetermined length, drying the blocks or planks and heating the dried blocks or planks in a kiln at a temperature in the range of 1200 to 1300 C. for a time sufficient in an oxygen deficient atmosphere to sinter the blocks or planks and to form carbides, and separating and recovering CO.sub.2 gas from combustion gases in the kiln.

Claims

1. A method for treating waste materials comprising organic and inorganic material, the method comprising, particulating the waste materials into discrete particles, heating and drying the particles in a non-oxidizing atmosphere in a drier at a temperature in the range of 800 to 860 C. for carbonizing the particles, crushing the carbonized particles into a size range of about 0.2 to 0.5 mm, leaching the crushed carbonized particles in a bath of an acid solution for dissolution of heavy metals into the solution, separating the leach solution containing heavy metal from the carbonized particles, adding to the carbonized particles particulate sodium hydroxide, silica, feldspar and limestone in a ratio of 100:0.3-0.5:8-12:2-4, mixing said carbonized particles and sodium hydroxide particles, silica particles, feldspar particles and limestone particles with 15 to 18% by weight water to form a wet mixture and continuously extruding the wet mixture to form an elongated continuous extrusion, severing the elongated extrusion into blocks or planks of predetermined length, drying the blocks or planks and heating the dried blocks or planks in a kiln at a temperature in the range of 1200 to 1300 C. for a time sufficient in an oxygen deficient atmosphere to sinter the blocks or planks and to form carbides, and separating and recovering CO.sub.2 gas from combustion gases in the kiln.

2. A method as claimed in claim 1, particulating the waste material to discrete particles about 1.5 to 2.5 mm in size, spreading the discrete particles into a thin layer and heating the thin layer of discrete particles for preliminary drying the particles before carbonizing.

3. A method as claimed in claim 1, in which the carbonized particles are leached in a nitric acid solution for dissolution of heavy metals, and passing the carbonized particles in the leach solution through and out of the leach solution on a permeable belt for separating the particles from the leach solution containing the heavy metals.

4. A method as claimed in claim 3, in which the carbonized particles are sprayed with sodium carbonate for neutralizing nitric acid thereon.

5. A method as claimed in claim 3, in which the carbonized particles are leached in the nitric acid solution for about 10 to 30 minutes.

6. A method as claimed in claim 1, in which the carbonized particles are mixed with sodium hydroxide, silica, feldspar and limestone in a ratio of 100:0.3-0.5:10:10:3.

7. A method as claimed in claim 4, in which the leach solution containing dissolved heavy metals is neutralized with sodium carbonate for precipitation and recovery of heavy metals as metal carbonates.

8. A method as claimed in claim 1, in which the blocks or planks are heated in the kiln in an oxygen deficient reducing atmosphere for about 6 to 7 hours for formation of metal carbides.

9. A method as claimed in claim 1, applying a plasma thermal spray coating to the blocks or planks.

10. A method as claimed in claim 1, continuously extruding the wet mixture by vacuum extrusion.

11. A method as claimed in claim 1, adding a particulate inorganic colour additive to the wet mixture while mixing the wet mixture and prior to extrusion to provide a marble or streak effect to the extrusion.

12. A method as claimed in claim 1, recycling at least a portion of combustion gases from the kiln to the drier.

13. A method as claimed as claim 12, heating the recycle combustion gases from the kiln to the drier.

14. A method as claimed in claim 1, applying ultrasonic agitation to the bath of acid solution to assist dissolution of the heavy metals into the acid solution.

15. The method of claim 1 wherein the waste materials comprise any of food waste, sewage, garbage, construction waste and industrial waste.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the process of the invention will now be described with reference to the drawings, in which:

(2) FIG. 1 is a schematic illustration of a flowsheet of the first half of the method of the present invention;

(3) FIG. 2 is a schematic illustration of a flowsheet depicting the second half of the method of the present invention;

(4) FIG. 3 is a perspective view of an embodiment of secondary crusher of the invention;

(5) FIG. 4 is a longitudinal sectional view of an embodiment of extruder of the invention;

(6) FIG. 5 is a sectional view of a vacuum piston extruder known in the art;

(7) FIG. 6 is a sectional view of a vacuum auger extruder known in the art;

(8) FIG. 7 is a sectional view of the pressure zones in an auger extruder known in the art;

(9) FIG. 8 is a longitudinal sectional view of a high-temperature kiln or furnace of the invention;

(10) FIG. 9 is a longitudinal sectional view of a carbon dioxide extraction unit of the invention;

(11) FIG. 10 is a graph illustrating viscosity of sludge feed slurry heated for tape casting;

(12) FIG. 11 is a graph illustrating effect of residual time during pyrolysis and carbonizing of dried sludge using recycle heat; and

(13) FIG. 12 is a graph illustrating heavy metal removal in a nitric acid leach with and without the acid of ultrasonics.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(14) With reference to FIGS. 1 and 2, waste materials P1 including food waste, sewage sludge, construction and industrial waste, plastic bags and the like garbage are fed to the hopper 11 of crusher 12 of crushing stage S10 onto screw conveyor 14 for preliminary crushing to particles of about 1.5 to 2.5 mm in size, preferably about 2 mm. The particulate material is advanced as a thin wet slurry to tape injector 21 of heating stage S20 wherein burner 22 heats the slurry to reduce viscosity of the slurry, as indicated in FIG. 11, to about 4000 MPas for effective preliminary drying as a thin layer of discrete particles.

(15) The waste feed typically has a solids content in the range of 23-27 wt % solids, the balance water. The physiochemical characteristics of the waste feed is typified in Table 1 below for a solids content of about 23 wt % and a liquid content of about 77 wt %.

(16) TABLE-US-00001 TABLE 1 Physiochemical characteristics of sludge input Item Unit Dewatered Sludge Water Contents wt % 77.0 Solid Contents wt % 23.0 Ash Contents wt % 50.0 Organic Compound wt % 50.0 Contents Calorific Value cal/g 2,497 PH 6.0

(17) The thin layer of heated particles P3 is transferred by belt conveyor V1 onto metal conveyor V2 and into primary drier 31 of stage S30 which receives recycle hot gases through inlet 32 from downstream unit S120, to be discussed, the gases having a temperature in the range of about 800 to 860 C. The waste particles are dried in an essentially oxygen-free atmosphere and pyrolysed for carbonization of the organic waste materials. The thermal cracking of organic waste materials causes thermal distillation of hydrocarbons leaving a residue of carbon such as charcoal and the like carbonaceous material and ash. After decomposition of water, hydrocarbons and hydrogen sulphide inside waste sludge drier 31, it is believed there is a cracking of saturated ring-bonds at 340 C., carbonization at 80 C. transformation of bitumen components to the tar or heavy-oils in the range of 400-600 C., and decomposition of organics into carbons and low-molecule gaseous hydrocarbons at 350-400 C.

(18) The waste from this carbonization process does not contain any dioxin components because they are thermally decomposed under reducing condition at over 800 C., dioxin components being cracked in the middle of the carbonization range.

(19) The carbonaceous material has fine pores providing a large surface area with high adsorption capability. A substantial volume reduction to about one-twelfth of the original volume results with sterilization by destruction of bacteria and viruses. Almost complete thermal destruction of dioxin results with trace amounts of dioxin in the exhaust gases which are completely destroyed in the downstream carburizing step S120 to be described.

(20) Table 2 and Table 3 below indicate the yield and Iodine Number at various carbonization temperatures for 30 minutes and various times at 860 C. respectively.

(21) TABLE-US-00002 TABLE 2 Effect of carbonization temperature on yield and iodine adsorption (Carbonization time: 30 min) Carbonization Temp Item 400 C. 500 C. 600 C. 860 C. Iodine Number 116.9 99.5 113.9 123.5 (mg/g) Yield 64.0 50.5 46.5 45.0

(22) TABLE-US-00003 TABLE 3 Effect of carbonization time on yield and iodine adsorption (Carbonization temperature: 860 C.) Carbonization Time Item 15 min 30 min 60 min Iodine Number (mg/g) 109.4 123.5 104.6 Yield (%) 46.3 45.0 44.0

(23) FIG. 11 illustrates the effect of agitation time on residual concentration.

(24) Table 4 below indicates the composition of a typical sludge before and after the drying carbonizing process in stage S30, in which DB=dry basis.

(25) TABLE-US-00004 TABLE 4 Composition of Carbonized sludge output through Drying Process (S30) Results Carbonized Item Unit Dried Sludge Sludge Water Contents wt % 22.5 2.9 Ash Contents wt %-DB 50.0 78.0 Calorific Value cal/g-DB 2,949 1,152 Element Analysis C wt %-DB 23.3 11.3 H wt %-DB 4.0 0.59 O wt %-DB 18.1 .505 N wt %-DB 3.5 0.52 S wt %-DB 0.57 0.06 Content Pb mg/kg-DB 114.4 174.1 Cu mg/kg-DB 219.4 235.7 As mg/kg-DB 0.9 1.8 Cr mg/kg-DB 34.7 44.5 Cd mg/kg-DB 1.9 1.4

(26) Particles P3 are conveyed by slatted metal conveyor V3 which is vibrated by vibrator 34 over hopper 41 causing the discrete particles to fall through the open slats of the conveyor into enclosed hopper 41 of crusher stage S40, illustrated in more detail in FIG. 4, under a partial vacuum to collect and separate dust particles. Rotating radial agitator 42 journaled for rotation across hopper 41 advances the particles onto auger 43 which concurrently advances and crushes the dried particles P4 to a size in the range of 0.2-0.5 mm.

(27) Belt conveyor V4 transfers the particles P4 to tank 51 of an acid leach bath S50, preferably having a nitric acid solution with a pH of 2-3, density of 1.115 and a concentration of 3.54 mol/L for a retention time of about 10 to 30 minutes; preferably about 20-30 minutes, and most preferably about 30 minutes, for leaching of heavy metals such as copper, lead, zinc and cadmium. Polypropylene belt conveyor V5 having a dialyser membrane layer conveys the particles through tank 51 over ultrasonic transducer 53 for ultrasonic agitation of the bath for about the preferred 20-30 minutes, most preferably about 30 minutes, with infiltration of the acid solution into the porous and permeable carbonized waste particles. The metals (M) in the waste particles dissolve into the solution as typified by copper as: 2HNO.sub.3+Cu.fwdarw.Cu(NO.sub.3).sub.2+H.sub.2(gas). The M(NO.sub.3).sub.2 cations in solution are separated from the solid particles by passage of the metal-bearing solution through the polypropylene chain conveyor V5 having the dialyser membrane. The solid particles are conveyed through and out of tank 51 onto belt conveyor V6 of drying stage S70. The acid solution with the cations in tank 51 which passes through the dialyzer filter can be isolated and neutralized by the addition of sodium carbonate solution to precipitate the metals as metal carbonates, typified for example by copper as Cu(NO.sub.3).sub.2+NaCO.sub.3.fwdarw.CuCO.sub.3+Na(NO).sub.3. Heavy metal removal, with and without ultrasonic assistance, is typified in Table 5.

(28) TABLE-US-00005 TABLE 5 Heavy metal removal process with nitric acid solution and ultrasonic equipment 10 min (mg/kg) 20 min (mg/kg) 30 min (mg/kg) input Without With Without With Without With element (mg/kg) ultrasonic Ultrasonic ultrasonic Ultrasonic ultrasonic Ultrasonic Pb 5.00 3.00 1.45 1.28 0.50 0.87 0.35 As 3.00 2.48 0.67 1.35 0.07 0.65 0.06 Hg 0.20 0.16 0.07 0.11 0.01 0.04 0.01 Cd 0.30 0.19 0.11 0.19 0.02 0.07 0.01

(29) FIG. 12 illustrates graphically the leaching of heavy metals with time as indicated in Table 5, with and without ultrasonic agitation, optimum leaching occurring at about 20-30 minutes.

(30) The leached waste particles P4 are conveyed from tank 51 to high pressure scrubber 61 of neutralizing stage S60 in which sprayer 62 directs sodium carbonate solution at high pressure and velocity onto the waste particles travelling on belt conveyor V6 to neutralize the nitric acid on the particles.

(31) The neutralized particles are surface dried by passage on belt conveyor V6 under a forced draft fan 71 in stage S70 and fed to feed hopper 81 of mixing stage S80 wherein the waste particles P4 are blended with sodium hydroxide particles, silica particles, feldspar particles and limestone particles in a ratio in the ranges of 100:0.3-0.5:8-12:8-12:2-4; preferably in the range of 100:0.3-0.5:10:10:3. The mixture is mixed by propeller mixer 82 with the addition of 15-18 wt % water and conveyed by conveyor V8 to hopper 97 of vacuum extruder 90 at stage S90, shown in more detail in FIG. 4. Extruder 90 comprises a primary screw auger 91 journaled tot rotation below waste feed hopper 97 and feed inlet 98 for particulate additives such as colour powders to provide a marble or streak effect to the extrusion. Auger 91 has a large diameter screw portion 93 for the particulate waste material and relatively small diameter screw portion 94 for mixing of the colour additives 98a, 98b to enhance marbling effects. Typical inorganic colour additive's are kaolin for white colour, cobalt oxide for blue colour, chrome oxide for green colour, copper oxide for green, red or black colours, and iron oxide for reddish brown.

(32) Secondary auger 92 has screw 95 for advancing the mixture with the 15-18 wt % water under pressure through die orifice 96, preferably having a rectangular cross-section opening. FIGS. 5 and 6 show prior art vacuum extruder dies for piston and auger extruders respectively. FIG. 7 illustrates the pressure zones of the secondary auger extruder of the invention, with Table 6 typifying flow properties determined by an extrusion rheometer.

(33) TABLE-US-00006 TABLE 6 Flow properties determined by using extrusion rheometer Electrical Cordierite + Property Porcelain Cordierite Lube Die-entry .sub.b (kPa) 160 850 520 k.sub.b kPa/(mm/min).sub.n 200 30 13 n Shear thin 0.3 0.3 0.4 Die-land .sub.f (kPa) 6 57 6 k.sub.f kPa/(mm/min).sub.m 0.7 1.4 0.1 m Shear thin 0.5 0.5 0.8

(34) The extrusion G1 is severed transversely into blocks or planks G2 by a saw at stage S100 and dried sequentially, such as by microwave unit 112 in drying stage S110 to minimize shrinkage, prior to loading in stacks in kiln or furnace 121 in high temperature sintering stage S120.

(35) The kiln 121, shown schematically in more detail in FIG. 8, comprises a base 120 on which the planks 62 are stacked. Furnace 121 is tired by combustion of natural gas with air at inlets 127, 128 to a temperature in the range of 1200-1300 C. Gas inlet 122 receives hot gases from outlet 32 of upstream drier 31. A portion of combustion gases containing CO.sub.2 and NO.sub.X are discharged through outlet 123 for CO.sub.2 recovery and the remainder recycled through outlet 124 by exhaust fan 125 to inlet 32 of drier 31. The stacked planks are sintered in kiln 121 for about 6-7 hours under controlled reducing conditions for the production of metal carbides from the remaining metals. The furnace is initially fired by combustion of natural gas to achieve the desired temperature in the range of 1200-1300 C. with combustion products discharged to CO.sub.2 collection stage S130 and to drier stage S30. The kiln discharge damper is partially closed at the end of the firing phase to provide a reducing atmosphere in the presence of carbonized Waste particles for carburizing the metals and silica.

(36) The hot exhaust gases discharged through outlet 123 are passed through a dust collector, not shown at a temperature of about 420-560 C. with a heat loss of about 70 celsius degrees and heated up to the desired reducing temperature of 800-860 C. by combustion of low pressure gas (LPG) or liquefied natural gas (LNG) with a deficiency of oxygen to produce CO and these hot gases fed to drier stage S30 and maintained oxygen deficient for suitable carbonizing conditions.

(37) CO.sub.2 collection stage S130 shown in FIG. 9 comprises enclosure 200 having inlet 210 communicated by ducting with outlet 123 of kiln 121 for receiving a portion of combustion gases containing carbon dioxide. A series of primary 211, secondary 212 and tertiary 213 absorbent units absorb carbon dioxide from the hot combustion gases. Fan impellor 215 circulates the combustion gases within enclosure 200. Primary absorber 211 preferably is dry-absorbent NHCO.sub.3, secondary absorber 212 preferably is MHCO.sub.3, and dry-absorbent 213 preferably is M.sub.3CO.sub.3. A fourth CO.sub.2 absorber 217 in exhaust stack 218 preferably is a mixture of Al.sub.2O.sub.3 and TiO.sub.2.

(38) The sintered and carburized planks G3 from furnace 121 preferably are passed through a plasma thermal spray coating stage S140, well known in the art, to seal the plank exterior surfaces to ensure leaching of residual heavy metals is prevented.

(39) The present invention provides a number of important advantages. Wet slurries of waste materials containing organic materials and heavy metals heretofore deposited in landfill sites to pollute the environment can be efficiently treated by carbonizing organic materials to render the heavy metals easily soluble in leach solutions for separation and recovery of the heavy metals from the solids, and the carbonized solids with inorganic additives extruded to form planks or blocks which are carburized to convert residual heavy metals to carbides in insoluble planks or blocks.

(40) It will be understood that other embodiments and examples of the invention will be readily apparent to a person skilled in the art the scope and purview of the invention being defined in the appended claims.