Abstract:
An apparatus and method for chemically and physically stabilizing contaminated soil is disclosed. The soil is homogenized in a homogenizer; dropped into a mixer below the homogenizer and mixed with an additive; then dropped to a processing terminus located below the mixer. The apparatus and method are useful for processing highly clumped and/or acidic soil, e.g., soil contaminated with mining waste.

Description:
This application is a continuation of Ser. No. 08/193,449 filed on Feb. 8, 1994 now U.S. Pat. No. 5,542,614. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to remediation processing of contaminated soil. 
     In remediation processing, soil which is polluted, toxic, or otherwise contaminated is rendered stable--either chemically, physically, or both--by mixing the soil with additive. U.S. Pat. No. 5,007,590 and No. 5,028,010, which are incorporated by reference, disclose examples of remediation processing apparatuses and methods. 
     SUMMARY OF THE INVENTION 
     In general, in a first aspect, the invention features processing waste material by homogenizing the waste material in a homogenizer; dropping the waste material after mixing into a mixer located below the homogenizer; mixing the waste material with an additive in the mixer to form a mixture; and dropping the mixture to a processing terminus located below the mixer. 
     Embodiments of the invention may include one or more of the following features. The waste material may be solid or semi-solid. Lumps of waste material larger than a predetermined size may be removed before the waste material is processed. Waste material may be loaded into the homogenizer using an excavator, a conveyor, or a bulldozer. The waste material may be mixed with a pretreatment additive in the homogenizer. A batch of waste material may be accumulated in the mixer. The waste material may be weighed in the mixer, to determine an amount of additive to be added. The processing terminus may be a vehicle. 
     In general, in another aspect, the invention features apparatus for processing waste material. The apparatus includes a homogenizer; a mixer located below the homogenizer to receive waste material from the homogenizer by gravity feed; and a processing terminus located below the mixer to receive waste material from the mixer by gravity feed. 
     Embodiments of the invention may include one or more of the following features. The apparatus may include a loading conveyor with a discharge end disposed to deliver waste material to the homogenizer. The waste material may be solid or semi-solid. The homogenizer includes homogenizing augers which counter-rotate. The apparatus may include an additive receptacle and means for transferring additive from the receptacle to the homogenizer. The mixer includes mixing augers which counter-rotate, as well as weight sensing elements. The apparatus includes an additive receptacle and means for transferring additive from the receptacle to the mixer. The processing terminus includes space below the mixer which allows entry of a vehicle below the mixer to receive and transport the processed waste material from the apparatus. 
     The invention provides a method and apparatus useful for processing soil which is heavily clumped and/or acidic, e.g., soil contaminated with mining waste. Mining waste tends to cause the soil to form clumps which can abrade and &#34;jam&#34; the transfer pumps and augers which may be used in soil processing apparatuses. The acidic nature of such soil further degrades the transfer mechanisms. The present invention alleviates such problems. 
     Other advantages and features will be apparent from what follows. 
    
    
     DESCRIPTION 
     FIGS. 1A and 1B are side elevational and plan views, respectively, of a waste processing apparatus; 
     FIGS. 2 and 3 are side elevational views of alternative schemes; 
     FIG. 4 is a perspective view of a vibrating screen box; 
     FIGS. 5A and 5B are a side view and an end view, respectively, of the waste processing homogenizer. 
    
    
     Solid or semi-solid waste material (e.g., contaminated soil) to be processed is loaded into a loading hopper 10 of an homogenizer 12 (FIGS. 1A, 1B). By solid or semi-solid, it is meant that the soil consistency may range from dry and totally solid to flowable--e.g., sludge-like--with as little as 5%, by weight, solid chunks. The waste material may be scooped up from a supply dump 14 previously deposited near the loading hopper, or from a sludge pit (not shown), and loaded into the loading hopper using an excavator 16. Alternatively, waste may be loaded into a loading hopper 18 (FIG. 2), which deposits the waste material onto a conveyor 20. Conveyor 20 transports the waste material from loading hopper 18 up to loading hopper 10 of the homogenizer 12. In another alternative, a soil processing facility can be built to take advantage of the natural slope of a hill 22 (FIG. 3), at the top of which is contaminated soil 26. The homogenizer 12 would then be positioned in a recess 24 in the hillside and waste material 26 would be pushed directly into loading hopper 10 using a bulldozer 28. 
     Soil contaminated with mining wastes typically contains about 60% metal, 20% moisture, and 20% clay or other organic constituents. The contaminated soil does not mix well with water and tends to form large, aggregate chunks which can &#34;jam&#34; the homogenizer. Thus, it may be necessary to pre-screen these large chunks of material from the contaminated soil. 
     A vibrating screen box 30 (FIG. 4) prescreens the contaminated soil. The vibrating screen box, caused to vibrate by a mechanical shaker (not shown), has a slightly sloped mesh bottom 32 with openings of a desired size, e.g., six inches across. Chunks of waste material 34 which are larger than the openings bounce off to the side of the screen box, and the remainder of the waste material 36 passes through the vibrating screen box when it vibrates. The vibrating screen box may be located on site such that the waste material which passes through it piles up immediately in front of the homogenizer, e.g., mound 14 (FIG. 1A) or mound 26 (FIG. 3). Alternatively, the vibrating screen box may be located over loading hopper 18 (FIG. 2) such that the waste material passing through the screen box 36 is loaded directly into the loading hopper 18. 
     As shown more clearly in FIGS. 5A and 5B, homogenizer 12 has a pair of side-by-side homogenizing augers 39. Each of the homogenizing augers includes homogenizing paddles 40 that are welded or otherwise mounted to a shaft 42 at appropriate angles to cause movement of the waste material through the homogenizer while chopping, reducing the size of lumps in, and homogenizing the waste material. The homogenizing augers are positioned within semicircular troughs 44, which facilitate the chopping action of the paddles as waste material is conveyed to the discharge end of the homogenizer. 
     The homogenizing augers are driven by hydraulic motors, 38, which are independently powered such that the homogenizing augers may be counter-rotated. Counter-rotation causes the waste material to circulate within the homogenizer until adequate homogenization is obtained. 
     Often, mining waste is highly acidic due to the metals which are present. To prevent corrosion of the homogenizer, a pretreatment additive such as CaO (hot lime) or other basic material is mixed with the waste material while it is in the homogenizer. Enough pretreatment additive is added to raise the pH from a value on the order of 4 to a value on the order of 8 or 9. 
     The pretreatment additive is stored in an additive silo 50 located above the homogenizer (FIGS. 1, 2). Screw augers 52 (hidden in the Figures) transport the pretreatment additive from the silo either directly into the homogenizer (FIGS. 1A, 1B), or into loading hopper 10 (FIG. 2). 
     Once the desired consistency and degree of homogenization have been achieved, the waste material is discharged from the homogenizer through a hydraulically operated slide gate 54 (FIG. 5A). The waste drops, by gravity, through discharge chute 46 into a mixer 58 located below the homogenizer. Waste material is dropped into the mixer until a batch weight has been loaded into the mixer, as determined by load cells 60 on which the mixer is mounted. 
     The mixer may be of the kind used for concrete mixing (e.g., a &#34;Compulsory Mixer&#34; available from BHS/Ross). The mixer has a pair of mixing augers 64 positioned side-by-side within the mixer (FIG. 2). The mixing augers are independently controlled for counter-rotation, like the homogenizing augers, to circulate the waste material within the mixer. 
     One or more silos 62 store treatment additive to be mixed with the waste material. The treatment additive is calcium oxide (hot lime), calcium carbonate, some other type of lime, or other basic material which neutralizes the acidity of the waste material (hidden in the Figure). Additive is transferred from silo(s) 62 to the mixer by transfer means such as conventional screw augers 66 (FIGS. 1A, 1B); a butterfly valve-controlled drop chute 68 (FIG. 2); or a slide chute 70 (FIG. 3). The type and amount of specific additive(s) needed for a given weight of waste material of a given type is determined by experimentation. Once the waste material is loaded into the mixer and weighed, the amount of additive necessary to treat the waste material is determined and added to the mixer. 
     Once the required amount of additive has been added to the waste material, the waste material/additive mixture is retained and mixed in the mixer until the desired consistency and degree of mixing have been obtained. The mixture is then discharged from the mixer by retracting slide gates located in the bottom of the mixer (not shown) and allowing the mixture to drop, by gravity, to a processing termination location 72. The processing termination location is a truck access pit which is large enough to permit a waste-hauling truck 74 to drive under the mixer and receive the waste/additive mixture as it drops from the mixer. The mixture is then hauled away to a permanent disposal facility. 
     Other embodiments are within the scope of the following claims. For example, many other additives known in the remediation art such as portland cement, sodium hydroxide, and sodium sulfide can be used, depending on the nature of the material being remediated, and the invention is not to be limited by the particular additive used.