BACKGROUND OF THE INVENTION Society has become increasingly aware of the dangers posed by improperly disposing of wastes. Improper disposal threatens humans, wildlife and the environment.

This increased awareness has prompted local, state and federal officials to draft regulations and ordinances for the proper treatment and disposal of wastes.

In passing the Resource Conservation and Recovery Act of 1976 (commonly referred to by the acronym RCRA), the United States government instituted a system for classifying wastes. For example, solid waste is classified as"hazardous waste"if the waste has been so identified by the United States Environmental Protection Agency.

Waste may also be classified as hazardous if it satisfies the criteria of"ignitability," "corrosivity,""reactivity"or"toxicity"as these terms are defined in applicable regulations.

Whether a waste meets the"toxicity"characteristics depends upon whether the waste leaches an unacceptably high level of a hazardous substance during a leaching test.

Contaminants considered to have toxic characteristics include the so-called"heavy metals,"including for example, arsenic, barium, cadmium, chromium, lead, mercury, selenium and silver.

Land disposal regulations prohibit the land disposal of wastes which exceed the maximum allowable concentrations when the waste is subjected to a leaching test.

Wastes failing the test must be treated appropriately to reduce leaching below the established limits or a permit or variance obtained. Given its relationship to the land disposal regulations, leaching tests generally attempt to mimic the slightly acidic leaching conditions encountered at the interior of a municipal landfill site.

In addition to satisfying the foregoing leaching tests, it is often necessary that treated wastes satisfy additional requirements. For example, the United States Environmental Protection Agency has established a rule which requires that waste to be deposited in a landfill have an unconfined compressive strength of 50 pounds per square inch; this is to ensure adequate structural support for landfills.

Several general strategies have been developed to meet the foregoing requirements or at least to partially stabilize wastes. The method probably most infrequently used is a method in which the waste is washed with a reagent capable of dissolving contaminants so they can be flushed from the waste. The solubilized contaminants are subsequently removed from the rinse solution by precipitation or filtration. In addition to the obvious drawback of generating a liquid waste, this method suffers from lengthy processing time and cost. Methods employing a washing step are described in U. S. Patents 5,009,793 to Muller and 5,045,115 to Ginunder.

A second method involves solidification. This approach utilizes binders to produce an end product having low permeability characteristics, thereby significantly reducing the rate at which contaminants leach from the waste. The solidification approach often involves the use of grout, cement, lime and/or silicates as the solidifying agent. This treatment approach is limited by the fact that such treatments are costly, significantly increase the volume of the waste and often require long curing periods.

Examples of this approach are U. S. Patents 4,049,462 to Cocozza; 4,375,986 to Pichat;

4,615,643 to Gouvenot and 5,150,985 to Roesky. Some methods utilize starting materials, such as fly ash, which have inherently high silica concentrations. In this case, by pressurizing a mixture containing water and fly ash it is possible to encapsulate waste materials (U. S. Patents 5,425,807,5,405,441,5,374,307 and 5,366,548 to Riddle). Other related methods using fly ash are described in U. S. patents 5,154,771 to Wada, et al.; 4,659,385 to Costopoulos, et al.; 3,625,723 to Sicka; 5,534,058 to Strabala; 4,615,809 to King; and 5,627,133 to Nelson.

Chemical stabilization or fixation comprises a third method. These methods involve addition of one or more chemical additives to the waste so that contaminants are converted into an insoluble form. A common chemical stabilization method for treating wastes contaminated with heavy metals is to add an alkaline reagent to produce insoluble metal complexes. This approach can be problematic in that certain metals may only form insoluble complexes within a narrow pH range. Lime and sodium carbonate are reagents typically used in this approach. Several U. S. Patents utilize this method, including U. S.

Patents 4,737,356 to O'Hara and Surgi; 4,671,882 to Douglas; 4,701,219 to Bonee; 4,443,415 to Queneau; and 4,645,651 to Hahn.

Another chemical stabilization technique is to use phosphate compounds, usually in conjunction with a buffering agent or a secondary complexing agent, to stabilize heavy metal-containing wastes. Several U. S. Patents are directed at using phosphate compounds solely to treat lead-contaminated materials. Illustrative of this group are U. S.

Patents 5,193,936 and 5,569,155 to Pal; 5,536,899 to Forrester; 5,545,805 to Chesner; 5,512,702 to Ryan; and 5,5162,600 to Cody.

Other U. S. Patents describe the use of phosphates, again generally with a buffering component, to treat wastes containing heavy metals besides just lead. Patents describing this approach include: U. S. Patents 5,527,982 and 5,397,478 to Pal; 5,037,479, 5,202,033 and 5,637,355 to Stanforth; 5,591,116 and 5,674,176 to Pierce; and 5,645,518 to Wagh. In some cases these approaches specifically require the formation of a slurry (U. S. Patent 5,645,518 to Wagh) or note that dry mixing of treatment agents may not provide the correct conditions to enable the necessary stabilizing chemical reactions to

occur (see for example, U. S. Patents 5,037,479 and 5,202,033 to Stanforth). Related methods utilizing phosphate compounds are described in U. S. patents 3,959,975 to Graf; 4,334,029 to Naito, et al., 3,640,021 to Grafmuller ; 3,201,268 to Hemwall; 4,919,711 to Banyai, et al.; and 4,231,984 to Hofman.

There remains a need in the field of waste treatment for a method of treating diverse waste forms, whether the waste be liquids, solids or combinations thereof by a simple process utilizing a minimal amount of stabilizing agents which are inexpensive and readily available. The method should also achieve significant volume reduction of the treated waste. The current invention described herein addresses this need.

SUMMARY OF THE INVENTION The current invention provides a method for treating or stabilizing a variety of wastes. In form, the wastes may be solids, liquids or combinations thereof. Thus it is possible to treat, for example, soils, sludges, slurries and mill and mine tailings. With regard to composition, the wastes may include ordinary debris, including but not limited to, aggregate, glass, metal, plastic, paper, concrete, asphalt, wood, ceramics and combinations thereof. The waste may also be a hazardous or toxic waste containing heavy metals, contaminants, or pollutants, or be a radioactive or mixed waste. Thus, the method can be used to stabilize hazardous wastes containing, for example, contaminants, pollutants, asbestos, pesticides, herbicides, polychlorinated biphenyls, metals and combinations thereof.

The invention provides a method for treating such wastes by combining the waste with a phosphate agent and then compacting the amended waste mixture to form a solid material which encapsulates and stabilizes the waste. If desired, other additives such as fly ash, a metal oxide agent, an oxidizing agent or combinations of these can be added.

It is even possible to add debris to the mixture so that it too is encapsulated in the final product.

This method has the advantage of utilizing inexpensive and readily available materials and can be carried out at room temperature. The method produces a solid

monolith which stabilizes contaminants by both chemical and physical means such that the final product reduces the leaching of contaminants to below current regulatory limits.

The final product also exhibits high compressive strength and can be formed into any number of predetermined shapes.

The current invention also provides construction forms or materials prepared from a starting material, wherein the starting material includes contaminated soils and soil- containing compositions, mine tailings, mill tailings and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a method for producing treated and compacted waste from solid waste according to the present invention.

FIG. 2 is a schematic diagram of a method for producing treated and compacted waste from liquid-containing waste according to the present invention.

FIG. 3 is a graph illustrating how compressive strength increases as the concentration of phosphate agent is increased.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention addresses the need to develop new methods for the treatment or stabilization of various waste forms such that the end product meets regulatory and disposal requirements for a number of waste streams. The terms"treating"or "stabilizing"wastes are meant to refer to processes in which the waste is physically and/or chemically modified so that pollutants and/or contaminants located in the waste or material do not leach from the treated or stabilized material; the terms may also include processes which result in treated materials that can withstand physical and/or chemical degradation or forces.

As used herein,"waste"is meant to generally refer to materials which are no longer deemed to be of value or to have use in their current form, including

environmental media and other materials. Waste may more specifically include hazardous waste, radioactive waste and mixed waste. Such waste can include metal- contaminated waste, for example waste which is contaminated with heavy metals such as arsenic, cadmium, chromium, lead, mercury, selenium, silver and nickel. Wastes may also contain contaminants, pollutants, asbestos, pesticides, herbicides, polychlorinated biphenyls, metals and combinations thereof. Such constituents may be defined in federal and state environmental and safety statutes, such as RCRA, CERCLA, TSCA or AEA.

Solid waste does not mean solid waste as defined under RCRA, but refers to waste which is neither a liquid or gas.

"Radioactive waste"as used herein refers to wastes that are Naturally Occurring or Accelerator Produced Radioactive Material (NORM/NARM) or which contain radioactive contaminants as defined in the Atomic Energy Act (AEA)."Mixed wastes" are meant to refer to wastes which contain hazardous and radioactive contaminants. It is possible using the method of the present invention to encapsulate hazardous, radioactive and mixed wastes into a compacted and solid monolith which facilitates disposal.

This invention is also applicable to treating secondary waste streams from other processes such as soil washing and thermal treatment processes. Additionally, the method described herein can also be used to encapsulate debris including, but not limited to, paper, plastic, concrete, asphalt, wood, metal, glass, aggregate, ceramics and combinations thereof. This capability allows the disposal of secondary wastes from processing and construction which would otherwise require other treatment and disposal options.

While many other systems rely on the use of hazardous phosphoric acid additions during treatment, this invention can utilize phosphate agents other than just phosphoric acid. In some cases it is possible to use dry phosphate powders, which, when combined with water, or when combined with water already present in the waste, can be safely and easily added to the waste form, thus making the process much more amenable to field operations.

The method has the additional advantages of being able to be performed at room temperature and can be accomplished economically with commonly used soil handling equipment at minimal capital cost. Further, this method is capable of using only a phosphate agent in conjunction with compaction to stabilize wastes; unlike many other methods, it is not necessary to add other additives. In particular, it may not be necessary to add sulfate agents (including, for example, gypsum, anhydrite, alum and halites), carbonate agents (including, for example, simple carbonate salts as well as lime and Portland cement), complexing agents (including, for example, Fe (II), Fe (III) or A1 (III)), metal oxides (including, for example, MgO, CaO, FeO, Fe203 and Fe304), or hydroxides, in order to sufficiently stabilize wastes so that the end product is capable of passing leach tests. The result is a simpler and less expensive method of treating wastes.

Of particular importance, the phosphate agents used in the present method effectively react with the contaminants in a waste, even wastes having very high concentrations of contaminants, to form stable compounds in which the leach rates are below regulatory limits. In part, this stabilization is a consequence of the phosphate agent reacting with contaminating metals to form highly insoluble metal phosphate complexes. In these complexes, the metallic ion is inactivated and no longer participates in its usual chemical reactions. In addition, the phosphate binder reacts with other constituents in the waste to form a matrix which further impedes leaching of the metal contaminants. Decreased permeability is also achieved by the compaction step. The result is a hard ceramic-like material which binds contaminants and constituents in the waste by both chemical and physical means and thus has long-term stability for disposal.

The volume reduction also permits increased waste loading at disposal sites and the potential for using the compacted amended waste as a construction material, including, but not limited to, disposal pit liners.

While some of the other phosphate stabilization methods which have been described require the formation of slurries, the present invention can be accomplished using waste forms with minimal moisture content. It is also not necessary to grind the starting material to a powder.

Treatment of Solid Wastes The method of the invention is shown schematically in FIG. 1. The broken lines indicate optional steps. Initially, solid waste feed material is preferably size reduced to less than 4 centimeters in all dimensions. Typically, such size reduction is achieved by grinding and then screening the ground waste. Grinding or pulverizing of the solid waste can be accomplished using equipment such as hammermills, ballmills, jawcrushers and shredders appropriate to the waste form. Screening is done using stationary or vibratory screen systems. The grinding step, however, does not require formation of a powder in which the average particle size is at the micron level.

Separately, a solution containing phosphate agent is prepared from a concentrated form of phosphate agent diluted with water. Although the use of a solution is preferable, it is possible to apply the phosphate agent as a dry powder, especially if the waste feed has sufficient inherent moisture.

The"phosphate agent"provided for in the invention is meant to broadly include chemicals capable of supplying a phosphate anion, including polymeric compounds. A non-exhaustive list which illustrates the scope of such chemicals includes phosphoric acid and its salts. The phosphoric acid salts may be of the monobasic, dibasic or tribasic form.

The counterion of these salts may include, but is not limited to, sodium, potassium, magnesium, calcium, aluminum, iron, zinc and ammonium ion, or combinations thereof.

The phosphate agent may come from a mineral source such as fluorapatite [Ca, 0 (PO4) 6F2], hydroxyapatite [Cal0 (PO4) 6 (OH) 2] (also synthetic forms), and phosphate rock (used here to mean the naturally occurring rock which consists primarily of calcium phosphate). The phosphate agent may also be in the form of single superphosphate, superphosphate and triple superphosphate, forms of phosphate which can be obtained from standard commercial fertilizers."Superphosphate"refers to a mixture of calcium sulfate and calcium phosphate; it is typically prepared by adding sulfuric acid to phosphate rock, bone ash or basic slag."Triple superphosphate"is defined as the P205 such as can be found in commercial fertilizers.

It is also possible for the phosphate agent to be a metaphosphate compound, a pyrophosphate or polyphosphoric acid or a salt thereof."Polyphosphoric acid"is defined to mean chemicals having the general formula of Hn+2P n°3n+1 where n is greater than 1.

Polyphosphates include the salts of polyphosphoric acid. The phosphate agent may also be a long chain compound including, for example, sodium hexametaphosphate (available from Monsanto in powder or granular form; CAS No. 68915-31-1). In the case of metaphosphate, pyrophosphate, polyphosphate and hexametaphosphate, the counterions associated with these chemicals may include those listed above for the phosphoric acid salts. Combinations of the foregoing chemicals may also be used.

In determining the effective amount of phosphate agent to employ in order to achieve a compacted amended waste which significantly reduces metal leaching, it is useful to conduct an initial screening test with a small but representative sample of the waste to be treated. The amount of phosphate agent required will depend upon the amount of contaminant in the waste and on the characteristics of the waste. In general, however, when the phosphate agent is phosphoric acid, 5 to 20 percent of phosphoric acid by weight is sufficient (unless stated otherwise, all phosphate agent weight percentages are expressed relative to the weight of the initial solid waste). In the case of triple super phosphate (TSP), 3 to 20 percent of TSP will effectively immobilize metal contaminants.

Higher concentrations of hydroxyapatite and fluorapatite tend to be required; typically, 10 to 50 percent by weight is sufficient.

In the case where the phosphate agent is hexametaphosphate (HMP), dry powdered sodium HMP is added to water, preferably in the concentration of 1.75 kg per liter. The amount of HMP added to the waste varies but, based upon the dry weight of the powder, 0.5 to 8 percent by weight relative to the initial waste is typical and 1 to 3 percent by weight is preferred.

In a combining or mixing step, the solid waste feed material is then preferably mixed with the phosphate agent solution to yield an amended waste. It is possible to add one or more additives to the solid waste in the combining step as well. These additions may be prior to, at the time of, or after the phosphate agent is mixed with the solid waste.

Like the solid waste, the debris is preferably size reduced to 4 centimeters or less in all dimensions prior to addition.

For example, secondary waste or debris may also be mixed with the solid waste.

As noted above, debris may include, but is not limited to, paper, plastic, concrete, asphalt, wood, metal, glass, ceramics and combinations thereof. Preferably, the amount debris added is 10 percent or less of the solid waste by volume.

Fly ash may also be added to the solid waste. As used here,"fly ash"is meant to include by-products in the combustion of coal in large power plants. Preferably, the fly ash is Class C or Class F fly ash which can be purchased from a variety of sources. The fly ash is generally added to provide additional strength to the compacted product. The amount of fly ash added preferably does not exceed 25 weight percent based on the initial weight of the waste, and may be less than 15 weight percent in some instances.

It is also possible to include a metal oxide agent with the solid waste. The term "metal oxide agent"is meant to include metal oxides generally, and MgO, CaO, FeO, Fe203 and Fe304 in particular. The amount of metal oxide agent added is typically less than 50 percent by weight relative to the initial solid waste; in some cases, the percentage by weight may be less than 15 percent.

An oxidizing or reducing agent may also be included in order to achieve the desired oxidation state of a particular metal. Examples of suitable oxidizing agents include salts of hypochlorite (for example, Ca (OCl) 2 and NaOCI) and hydrogen peroxide.

The mixing or combining of the solid waste, phosphate agent and additive (s), if any, can be performed in any type of mixer which can adequately blend the components into a uniform mix, including, for example, a pug mill or other standard mixing equipment. If prepared as a solution, the phosphate agent, can be pumped using conventional pumping into a spray bar which is located in the mixer and the phosphate agent sprayed over the mix. The phosphate agent is typically added in minutes and mixed with the solid waste and any additives until the phosphate agent is uniformly distributed throughout the mix, thus yielding an amended waste. This combining step can often be accomplished in as little as five minutes or less.

The moisture content of the amended waste is preferably 10 percent or less by weight. Based on the moisture content of the amended waste, additional water may be added as necessary to achieve the preferred moisture levels. The preferred water content will also depend upon the physical characteristics of the solid waste and can be varied so that after compaction a physically stable product is obtained.

After the amended waste has been adequately mixed, it is compacted using conventional or harmonic compaction techniques. The general term"compaction"as used herein is defined to mean not just the addition of pressure, but the application of force which results in densification and volume reduction of the original material.

The term"harmonic compaction"refers to a method for fusing particles by rapidly accelerating and decelerating particles within a mold having at least one moveable side. Accelerations of at least 25 G's to 50 G's, and preferably several hundred to several thousand G's, are required. These rapid accelerations and decelerations are preferably accomplished by rapidly impacting opposing ends of mold between an oscillating member and a pneumatic ram. Typically, the oscillating member is a table which rapidly reciprocates upward and downward and the pneumatic ram is part of an under damped pneumatic system adjusted to oscillate out of phase with the oscillating member.

Under operating conditions, the oscillating member propels the mold from itself toward the pneumatic ram. The pneumatic ram is initially compressed backward but then rebounds, forcing the mold back toward the oscillating member. Deceleration and impact occurs as the pneumatic ram moves the mold toward the oscillating member which is simultaneously moving in the direction of the returning mold. The oscillating impacts are repeated over and over in rapid succession, resulting in the desired compressive forces.

The frequency of the reciprocating movements of the oscillating member and the pneumatic ram must be appropriately timed to achieve the necessary impacts. Movement of the mold may be at the harmonic frequency of the oscillating member, hence the name harmonic compaction. U. S. Patents 4,531,903 and 4,456,574 to Frey and Wier describe an example of a harmonic compactor and methods for achieving harmonic compaction.

The compaction step can also be accomplished using conventional compaction methods, i. e., non-harmonic compaction methods which do not involve the rapid accelerations and decelerations which characterize harmonic compaction. Conventional compaction techniques and compactors may include vibratory compaction using vibratory tampers, plate compactors and rollers. Compaction may also be accomplished using kneading compaction utilizing, for example, tamping rollers, sheepsfoot rollers, mesh or grid pattern rollers, and rubber tire rollers. Static compaction may also be employed, including, for example, the use of compaction presses and hand-operated tampers. Dynamic compaction is another option, including, for example, the dropping of weights. Any of the compactors, whether they be conventional or harmonic, may be at a fixed site or part of a mobile facility.

Compaction forces will vary depending on the composition of the amended waste and on the intended use of the compacted amended waste, but typically a force up to 3,000 pounds per square inch is applied. Volume reductions up to and greater than 50 volume percent are achieved depending on the composition of the original waste feed and the compaction force which is applied. Typically, compaction achieves a volume reduction of at least 20 percent.

After compacting, the compacted amended waste is allowed to cure at room temperature. Curing times vary and, in general, compressive strength increases with increasing cure time. However, adequate product strength is typically obtained within about one hour to approximately one month; generally, the cure period is approximately 7 days..

Using the above method, ingredients and additives, it is possible to prepare construction materials or forms from contaminated starting materials, including contaminated soils, soil-containing compositions, mine tailings, mill tailings and combinations thereof (see FIG. 1). The contaminant may include, but is not limited to, asbestos, pesticides, herbicides, polychlorinated biphenyls, metals and radioactive material. Importantly, the process described above effectively encapsulates such contaminants, thereby resulting in low leach rates from the construction form.

During the compaction step, it is possible using many of the compacting devices listed above to compact the mixture (starting material, phosphate agent and additive (s), if any) into any number of desired shapes. Thus, for example, bricks having a square, rectangular, hexagonal or"T"shape can be formed. A variety of presses designed specifically for manufacturing bricks have been patented and could be used in the current invention, including the presses described in U. S. Patents 5,145,692 to Hereford; 4,640,671 to Wright; 4,559,004 to Augier; and 4,035,128 and 4,050,865 to Drostholm, et al.

A water-proofing agent may also be added to the construction form to enhance its durability when exposed to water. The agent can be included during the combining/mixing step or applied after the construction form has been produced. The water-proofing agent may include, but is not limited to, polyvinyl alcohol, polyurethane and asphalt emulsions.

Treatment of Liquid-Containing Wastes The foregoing discussion has focused on describing a method for treating solid wastes. Advantageously, the method can also be used to provide a way for treating liquid-containing wastes. The term"liquid-containing waste"is meant to include a liquid waste or a waste of which a significant portion is a liquid. More specifically, the term is meant to include wastes in which the water composition of the waste by weight is approximately 10 percent or more. Thus, the term is meant to encompass sludges, slurries and liquids.

In treating liquid-containing wastes, the general method described above for solid waste applies (see FIG. 2 where the broken lines indicate an optional step). A primary difference is that in a combining step the liquid portion preferably is initially reduced to approximately 10 percent by weight by adding an additive to absorb part of the liquid.

This additive may include any number of materials, but preferably includes soil, a quantity of debris, fly ash, a metal oxide agent, an oxidizing agent or combinations of these (terms are as defined above). After combining one or more of these additives with

the liquid-containing waste and adding phosphate agent, a compressible amended waste is formed which can be compacted using the procedures and apparatus described above.

Phosphate agent can be supplied in either solid or liquid form. The type and concentration of phosphate agent is as described for solid waste treatment. If fly ash is added, it typically is added in an amount less than 25 percent by weight relative to the liquid-containing waste, and may be less than 15 percent. A metal oxide, if added, is typically less than 50 percent by weight that of the initial waste; it may be less than 15 percent by weight. If debris is included, the amount added is typically 10 percent or less of the liquid-containing waste by volume.

Compaction may be accomplished using either the conventional compaction methods (e. g., vibratory, kneading, static or dynamic methods) or harmonic compaction method described earlier in the section on treating solid wastes. The compacted amended waste is sufficiently stabilized through chemical and physical means so that contaminants and pollutants are immobilized, thereby satisfying regulatory leaching criteria, especially for metals.

In-situ Treatment of Waste It is possible to treat and compact waste at its original site rather than compacting the waste at a site separate from the treating site. This method of treating is referred to herein as"in-situ"waste treatment. The initial step in this process is to obtain a representative sample of the waste to assess the amount of phosphate additive necessary.

Multiple samples may be required if the site to be treated is large.

The treatment site is then divided into sections. The appropriate amount of phosphate agent is then applied. The phosphate agent may be applied in liquid or solid form. In the combining step, phosphate agent may be evenly distributed over the treatment site. For large sites, where for example, soil is to be treated, the combining step may be accomplished, for example, by raking in the phosphate agent to an approximately uniform depth or by using standard rotary tillers that are either manually operated or attached to a tractor. If a quantity of debris, fly ash, metal oxide agents

and/or oxidizing agents are also to be added, these materials may be combined with the waste in a like manner.

For sites in which the contaminated zone runs more than a few feet below the surface, it may be necessary to apply the phosphate agent as a solution or slurry. Liquid application alone may be adequate for sufficiently porous soils. In other cases, it may be necessary to apply the phosphate agent and any other materials by first perforating the soil or by the use of injection nozzles which can be inserted to sufficient depths.

Once the phosphate agent and any other materials have been added and combined with the waste, compaction may be performed in a number of ways. For example, kneading compaction may be employed by using a variety of rollers. Dynamic compaction methods utilizing pneumatic tampers or methods in which weights are dropped may also be used. Compaction can also be achieved by driving heavy machinery over the treatment site, in which case the tires provide the necessary compressive force.

Composition of Construction Materials or Forms In addition to the foregoing methods, another aspect of the present invention is a composition comprising a contaminated starting material and a phosphate agent, with or without the additives described above. This composition has numerous applications including, for example, forming construction materials, especially bricks. The composition has additional utility as a mortar to bond the construction forms provided for by this invention as well as other construction materials.

In its most basic form, the construction materials or forms provided by the present invention comprise a compacted mixture including a phosphate agent and a contaminated starting material. However, the construction material may also include debris, fly ash, a metal oxide agent and/or an oxidizing agent. The contaminated starting material or waste may include contaminated soils, soil-containing compositions, mine tailings, mill tailings and combinations of these. A non-exhaustive list of potential contaminants includes, asbestos, pesticides, herbicides, polychlorinated biphenyls, metals and radioactive waste.

The phosphate agent included in the composition includes those chemicals described above in the waste treatment section and is present at the levels listed earlier.

Likewise, the composition may include one or more of the additives (debris, fly ash, metal oxide agent and/or oxidizing agent) at the concentration ranges listed earlier.

The composition is a compacted mixture. Generally, the construction form is compressed to less than 3,000 pounds per square inch, and preferably between approximately 1,500 and 2,300 pounds per square inch, although the actual pressure will depend upon the nature of the starting material and the intended use of the construction form. When fly ash is a component in the composition, pressures may exceed 2,500 pounds per square inch. At 30 percent volume reduction, the construction form typically has a density of approximately 128 pounds/in2 ; at 40 percent volume reduction, the density is approximately 170 pounds/in2. Final densities are dependent upon the starting material and compressive pressure.

The construction form may be of a predetermined shape. The options with regard to the final shape are essentially limitless. In general, a shape is desired which enables multiple construction forms to be interlinked with one another, thereby enhancing their use in construction projects. Examples of construction forms having particularly useful shapes include, but are not limited to, rectangular, square, hexagonal, and T-shaped bricks.

The construction form may also include a first interlocking member. Such a member might include, for example, a hole running through the construction form through which a connecting rod might be inserted to assist in stabilizing a stack of construction forms. More preferably, the construction form includes a first and second interlocking member, such that an interlocking member on one construction form is capable of interlocking with an interlocking member on another construction form. For example, a construction form may have a first and second face which are on opposing sides of the construction form. A first interlocking member might be located on the first face; the second interlocking member might be located on the second face. The interlocking members would be sized so that a first interlocking member on one

construction form would be capable of interlocking with the second interlocking member on another construction form. In such a case, the first interlocking member might include a ridge, whereas the second interlocking member might include a depression sized to accommodate the ridge on another block. Numerous other variations on these general approaches could be successfully utilized.

The construction form composition may also include a water-proofing agent to increase the stability of the construction form when exposed to water for extended periods. Numerous water-proofing agents could be utilized, including, for example, polyvinyl alcohol, polyurethane and asphalt emulsions.

An important feature of the present composition of the construction forms is its effectiveness in binding the contaminants within the composition, thereby resulting in a composition which can be prepared primarily from waste materials yet which exhibits leach rates which are below regulatory limits, especially for metals. As described earlier, this stabilization or immobilization of contaminants is a consequence of the phosphate agent forming a complex with the contaminant and also binding to constituents in the soil to form a matrix which limits leaching. Furthermore, the construction forms have excellent strength, as well as good durability and workability. The forms have the additional advantage of being in a form which provides for facile handling and transport; they can also be produced at room temperature using standard soil handling equipment, thereby allowing the forms to be easily manufactured in a variety of settings, including field operations.

Since the construction forms comprise compacted mixtures, the overall volume of the contaminated starting material is reduced, thereby decreasing the space needed if the forms are placed in a landfill. However, because contaminants within the construction form are stabilized, the construction forms can also be utilized in various construction projects including, for example, pit liners, erosion control systems, construction barriers, road beds, embankments and retaining walls.

The foregoing sections have provided a general description of the method and compositions of the invention. The following are examples of how the general method

can be used to stabilize diverse waste forms so that leach rates for contaminants, particularly metals, are below regulatory levels. The examples also illustrate the chemical stability and mechanical strength of construction forms prepared using the method of the current invention. The laboratory data reported for the various examples are average results from several similar tests.

Example I Treatment of Sand/Clay Samples One waste type treated included soil samples composed primarily of sand and clay. Clay comprised 20% to 40% of the sample composition by weight and sand comprised the remaining 60 to 80%. The sand/clay samples were tested on a bench-scale (0.25 pounds of waste material/block) and at full-scale (36 pounds of waste material/block). In field tests, the sand/clay samples were screened to obtain material which was less than 4 centimeters in all dimensions.

The phosphate agent employed in this test was dry, powdered sodium hexametaphosphate (HMP) purchased from Monsanto. HMP was added to water in the concentration of 1.75 kg per liter of water; dissolution was achieved at room temperature.

Separate tests using the sand/clay samples were run at both bench-scale and full- scale under room temperature conditions. In one set of tests, the sand/clay samples were mixed only with the HMP solution. In the second set of tests, a quantity of debris (10% by volume relative to the sand/clay sample) was combined with the sand/clay samples after first being size reduced to 4 cm or less in all dimensions. In each set of tests, HMP was sprayed uniformly over the samples and the resulting amended waste thoroughly mixed.

Compaction was accomplished using a conventional compaction method, namely a hydraulic punch and die. For bricks sized 10 in. by 14 in., compaction forces were approximately 1,600 pounds per square inch. After compacting, the compacted amended waste was allowed to cure for approximately 7 days. Compressive strengths were determined using a point load machine (e. g., an Instron'machine) according to ASTM

methods (unless otherwise stated, all compressive strength values are reported in kg/cm2 ; these values can be converted to pounds per square inch by multiplying by 14.22).

Volume reduction, i. e. the extent of compaction, was determined by subtracting the volume of the compacted amended waste from the bulk volume of the starting waste (including debris, if any), dividing the difference by the volume of the starting waste and then multiplying by 100.

Samples were subjected to the Toxicity Characteristic Leaching Procedure (TCLP) test as described in 40 C. F. R. 260.11 and 261.24 (a) to determine the extent to which metals leached from the compacted amended waste. In general, the TCLP methodology involves agitating a 100 g sample, randomly obtained from the waste being tested, in 2 liters of a specified extraction fluid after first sizing the waste to smaller than 3/8 inches or 9.5 mm. The agitation step is continued for 18 hours using a rotating agitator which is operating at a speed of 30 revolutions per minute. A sample of the extraction fluid is then tested for the particular contaminant or contaminants of interest.

For the metals covered under current TCLP regulations (40 C. F. R. 261. 24 (a) (1996)), the maximum concentration at which these metals can leach into the specified extraction solution is (all concentrations expressed as mg/1) : arsenic, 5.0; barium, 100.0; cadmium, 1.0; chromium, 5.0; lead, 5.0; mercury, 0.2; selenium, 1.0; silver, 5.0. These limits are listed in the second to the last line of Table 1 and are similarly listed in the other Tables.

As Table 1 indicates, the TCLP results were within the regulatory limits established for the metals tested (cadmium, chromium and lead), except for cadmium at the bench-scale level when debris was included. Compressive strength was well above the test objective of 70 kg/cm2. Volume reductions of 28 to 44 percent were achieved.

TABLE 1 TREATMENT OF METAL-CONTAINING SAND/CLAY WASTE WITH PHOSPHATE AND COMPACTION: TCLP, COMPRESSIVE STRENGTH AND VOLUME REDUCTION RESULTS Total Metals TCLP Metals Compressive Surrogates & Strength Bulk Volume Waste Forms Cd Cr Pb Cd Cr Pb (kg/cm2) Reduction (%) Sand/Clav Bench-Scale w/o Debris 1067.0 21.0 1967.0 0.5 <0.01 <0.01 345 28 w/10% Debris 1067. 0 21.0 1967.0 4.5 <0.01 0.2 283 Full-Scale w/o Debris 1067.0 21.0 1967.0 0.2 <0.1 0.1 359 44 w/10% Debris 1067. 0 21.0 1967.0 0.0 0.0 0.1 Treatment Std. 1.0 5.0 5.0 Test Objective 70 Example II Treatment of Buried Waste Samples The efficacy of the present method was also tested with a second set of samples comprised of surrogate material to simulate buried waste from a DOE site, in particular, a Fernald OU-1 waste pit. HMP solutions were used to supply the phosphate agent and treatment and testing conditions were generally as described in Example I. Tests were performed at the bench-scale and full-scale level. Tests at the bench-scale level were done with and without added debris. As shown in Table 2, TCLP tests were conducted for cadmium, chromium and lead; the TCLP limits were met for each of these metals.

Compressive strengths were well above the test objective of 70 kg/cm2, with or without

the addition of debris, ranging from 318 to 409 kg/cm2. Significant volume reduction was also achieved, ranging from 43 to 64 percent.

TABLE 2 TREATMENT OF METAL-CONTAINING BURIED WASTE WITH PHOSPHATE AND COMPACTION: TCLP, COMPRESSIVE STRENGTH AND VOLUME REDUCTION RESULTS Total Metals TCLP Metals (mg/1) (mg/1) Compressive Bulk Volume Surrogates & Strength Reduction Waste Forms Cd Cr Pb Cd Cr Pb (kg/cm2) (%) Buried Waste Bench-Scale w/o Debris 1159. 0 32. 3 1045. 0 <0.01 <0.01 <0.01 318 43 w/10% Debris 1159. 0 32.3 1045.0 0.3 0.2 <0.01 409 FuH-Scate w/o Debris 1159.0 32. 3 1045. 0 <0. 05 0.2 <0.01 64 Treatment Std. 1.0 5.0 5.0 Test Objective 70 Example III Treatment of Mill Tailing Samples A set of tests was also conducted on surrogates to simulate mill tailings from a DOE Fernold Silo 3 waste site. Trials were conducted at the bench-scale and full-scale level. At the bench-scale level, separate trials were conducted with and without added debris. HMP was used as the phosphate agent and treatment and testing protocols were as described for Example I. TCLP leaching results for the metals tested were all well within the established limits except for arsenic and selenium, but even in these cases the metal concentration was reduced by nearly two orders of magnitude or more relative to the starting concentration (Table 3). Compressive strengths at both bench-scale and full-

scale were significantly above the objective of 70 kg/cm2. Bulk volume reductions ranged from 31 to 66 percent.

Example IV Treatment of Mine Tailing Samples The ability of the current method to stabilize metals in mine tailings was also examined. HMP was used as the phosphate agent and treatment and testing conditions were as described for Example I. As shown in Table 4, the TCLP leaching results were all under the regulatory limits. Furthermore, compressive strengths in each case were above, generally significantly above, the objective of 70 kg/cm2. Bulk volume reductions ranged from 33 to 44 percent.

TABLE 3<BR> TREATMENT OF METAL-CONTAINING MILL TAILINGS WASTE<BR> WITH PHOSPHATE AND COMPACTION: TCLP, COMPRESSIVE STRENGTH AND VOLUME REDUCTION RESULTS<BR> Total Metals TCLP Metals<BR> Compressive<BR> Surrogates & Strength Bulk Volume<BR> Waste Forms As Cd Cr Pb Hg Se Ag As Cd Cr Pb Hg Se Ag (kg/cm2) (°<BR> MillTailing<BR> Bench-Scale<BR> w/o Debris 2200.0 40.0 570.0 200.0 23.3 1. 1 <0.1 8.5 394 31<BR> w/10% Debris 2200.0 40.0 570.0 200.0 53.5 0.8 <0.1 9.4 1054 31<BR> Full-Scale<BR> w/o Debris 14, 100 574.0 92.0 <10 <0. 1 560.0 <2 84.0 0.4 0.0 <. 05 <. 02 10.0 <. 02 66<BR> w/10% Debris<BR> Treatment Std. 5. 0 1. 0 5. 0 5.0 0.2 1.0 5.0<BR> TestObjective 70 TABLE 4<BR> TREATMENT OF METAL-CONTAINING MINE TAILINGS WASTE<BR> WITH PHOSPHATE AND COMPACTION : TCLP, COMPRESSIVE STRENGTH AND VOLUME REDUCTION RESULTS<BR> Total Metals TCLP Metals<BR> Compressive<BR> Surrogates & Strength Bulk Volume<BR> Waste Forms As Cd Cr Pb Hg Se Ag As Cd Cr Pb Hg Se Ag (kg/cm') (°<BR> Mine Tailings<BR> Bench-Scalc<BR> w/o Debris 0.5 20.9 2.5 4309.0 0.0 0.0 20.5 2. 1 <0.05 <0. 1 0. 1 0.0 <0.5 <0. 1 203 44<BR> w/10% Debris 0.5 20.9 2.5 4309.0 0.0 0.0 20.5 <0.05 <0.05 <0. 1 <0. 1 0.0 <0.5 <0.1 327<BR> l nll-Scale<BR> w/o Debris 0.5 20.9 2.5 4309.0 0.0 0.0 20.5 1. 0 0.0 <0.01 <0.05 0.0 <0. 1 <0.01 78 33<BR> w/10% Debris 0.5 20.9 2.5 4309.0 0.0 0.0 20.5 0.8 0.0 <0.01 <0.05 0.0 <0.1 <0.01--<BR> Treatment Std. 5.0 1.0 5.0 5.0 0.2 1. 0 5.0<BR> Test70

Example V Dependence of Compressive Strength on Phosphate Agent Concentration The dependence of the compressive strength of the compacted amended waste as a function of phosphate agent concentration was also examined. Samples were composed of sand/clay mixtures. Different aqueous solutions containing increasing concentrations of HMP were prepared (0.70,1.0,1.20 and 1.75 kg/1). The waste samples were treated and compacted as described in Example I. Compressive strength was determined using a point load machine (e. g., an Instron machine in compressive mode) according to ASTM methods. As shown in FIG. 3, compressive strength increased with increasing binder concentration. In particular, compressive strength doubled (600 psi to 1200 psi) as the concentration of HMP was increased from 0.70 kg/1 to 1.75 kg/1).

As noted in the preceding examples, diverse sample types treated with HMP solutions having a concentration of 1.75 kg/1 resulted in compacted waste forms having compressive strengths which generally exceeded 70 kg/cm2 and often had compressive strengths ranging from 203 to 400 kg/cm2. When solutions containing 1.75 kg/1 of HMP were used, this meant that the concentration of HMP expressed as dry weight was approximately 6 percent of the waste being treated.

A second set of compressive strength tests were conducted on bricks having concentrations of HMP ranging from 1 to 8.75 percent of the starting material by weight.

Fly ash was also included at 15 weight percent. Compacted amended waste bricks were prepared as described in Example I, including the 7 day cure period. Compressive strength values were also determined as described in Example I.

The durability of the compacted amended waste bricks when submerged in water was determined using the CRUMB Test (USBR 5400). During these tests, construction forms were immersed in water having a temperature of 20 °C for 24 hours and the physical integrity of the forms monitored with time.

The construction forms were classified as Grade 1 or 4; grades 2 and 3 were not considered during the classification process because these intermediate situations are not taken into account in the construction industries. Grade 1 means the brick is

nondispersive under the test conditions just described. In particular, no clay suspension is noticed in the dish used for the test; thus, the brick is classified as insoluble in water after 24 hours of immersion. Grade 4 means the brick is dispersive. In this case, bricks subjected to the test disintegrate in water in less than 24 hours; thus, the bricks are classified as soluble in water.

Table 5 summarizes the results of the compressive strength and CRUMB tests.

The first number in the Sample I. D. in Table 5 represents HMP concentration; the second number represents fly ash concentration. For example, sample 1B-15A contained 1 percent HMP and 15 percent fly ash by weight, relative to the weight of the starting material. The results show that compressive strength increased with increasing phosphate concentration. Yet, even at very low concentrations of HMP, excellent compressive. strength was obtained. In all cases except one, the bricks were nondispersive as defined according to the CRUMB test.

TABLE 5 Dependence of Compressive Strength on Phosphate Concentrations Compressive Strength Sand-Clay Surrogate (7 days cure time) Grade (Sample I. D.) (psi) (CRUMB Test-USBR 5400) 1B-15A 1 3B-15A 1 <BR> <BR> 5B-15A1<BR> 7B-15A1 8.75B-15A 6060 4 Example VI Effect of Cure Time on Compressive Strength Tests were performed to evaluate the dependence of compressive strength with the amount of time the compacted amended waste was allowed to cure. Compacted amended

waste bricks were prepared and compressive strengths measured as described in Example I. Bricks prepared from the same batch of solid waste were allowed to air dry for either 7 or 40 days and then compressis e strengths determined. Results are shown in Table 6. In general, compressive strength increased with increasing cure time. However, even a 7 day cure time resulted in compacted amended waste bricks having high compressive strengths.

TABLE 6 Effect of Cure Time on Compressive Strength Sand-Clay Surrogate 7 Days Cure Time 40 Days Cure Time I B-15A 2300 2670 3B-15A 2430 3040 5B-15A 4070 5640 7B-15A 4710 6950 8.75B-1 SA 6060 6430 The foregoing examples are to be considered in all respects as illustrative of the current invention rather than to be restrictive. It will be appreciated by those skilled in the art that additions, modifications, substitutions and deletions not specifically described may be made without departing from the spirit and broad scope of the current invention.