Method for the Treatment and Stabilization of Hazardous Waste

1. Field of the Invention

This invention relates to a composition and a method for the treatment and stabilization of hazardous wastes so that they may be safely disposed or discarded in an environmental manner. Specifically, this invention relates to a composition and a method for the stabilization of hazardous radioactive wastes. Still more specifically, this invention relates to a composition and a method for the stabilization of low level radiation contaminated soil. Even more specifically, this invention relates to a composition and a method for encapsulating low level radiation contaminated soil so that said encapsulated soil may be safely disposed.

2. Description of the Prior Art

There are a host of hazardous waste materials that are difficult to dispose because of the possibility that they may contaminate the environment. These hazardous wastes include things like dioxins; polychlorobiphenyls; wastes containing heavy metals and the like; waste from sewage treatment plants; radioactive wastes such as that contained in contaminated soil or effluents from radiation producing plants and the like; and chemical wastes, among many others. As there are wastes generated by society, so there are processes described in the prior art that are said to either remove or stabilize these hazardous elements so that they can be safely disposed. Most of these processes are either too complicated to be economical or do not safely stabilize the hazardous wastes so that can safely be disposed environmentally.

It is said that hazardous waste products such as chemical precipitation sludges; waste ion exchange materials; metal cuttings, among others, can be stored in metal or plastic containers or mixed with a cement and then buried stored in the resulting cement container. Metal or plastic container storage is unsatisfactory since it is known that such containers can deteriorate with age and thus there is usually some leaking of the hazardous waste. Storage of hazardous waste materials embedded in cement has been somewhat successful but cement does not have good setting properties when mixed with most hazardous wastes. The containment of radioactive wastes within a cement matrix has also been reported. This too has been unsuccessful due to the poor mixing properties of cement and waste. Additionally, during the curing phase, the cement is known to form micro-channels and thus the radiation can leak through these channels into the environment.

It is also known to treat soil containing lead with a mixture of silicates. This process is said to effectively reduce the amount of detected lead in the soil by encapsulating the lead containing soil within a matrix of the silicate. Although this process has been tested successfully for lead in soil, it has not been used for the other commonly known wastes such as those listed above, in addition, considerable amounts of the silicate mixture must be used and this is costly.

Finally, it is also known to treat radioactive or toxic solid wastes with a polymerizable mixture of a styrene compound or mixtures of styrenes. It is said that large, solid blocks of this material are safe to handle and dispose. However, aging of this material is a problem and most styrene containing compounds are hazardous in and of themselves.

It is an object of this invention to provide a composition in which hazardous wastes can be safely contained and then disposed. Another object of this invention is to provide a quick and convenient process for the treatment and disposal of hazardous wastes. Yet another object of this invention is to provide both composition and method or processes for the safe containment and disposal of radioactive wastes. These and yet other objects are achieved in a mixture comprising, in parts by weight, of 0 to 50 parts sand, 5 to 75 parts cement, 2.5 to 30 parts water, 12.5 to 75 parts of hazardous waste, and minor amounts of an alkali metal silicate and a gelling agent.

In yet another embodiment, in the mixture of this invention, some of the cement may be replaced by blast furnace slag, in an amount of 0 to 60%, based on total cement present, fly ash, in an amount of 0 to 40%, based on total cement present, or fine kiln dust, in an amount of 0 to 40%, based on total cement present.

In a continuous process for the treatment and stabilization of hazardous wastes comprising the steps of intimately mixing 0 to 50 parts by weight of sand, 5 to 75 parts by weight of cement, 2.5 to 30 parts by weight of water, 12.5 to 75 parts by weight of hazardous waste, a minor amount of an alkali metal silicate and a minor amount of a gelling agent, allowing this mixture to harden in a format suitable for safe disposal thereof.

FIG. 1 is a schematic drawing which details a particular efficacious method for practicing the process of this invention.

Looking now specifically at FIG. 1 which is a schematic illustration of the process of this invention, shows a portable system for the safe disposal of contaminated material. In this figure, the contaminated material is added to a screener 1 via any method available, such as a front end loading device, for example. This material is shown being added at arrow 2. The material is then fed by a conveyor belt 3 and is then dropped into a continuous mixer 4 as shown by arrow 5. Sand (optional) and cementitious material (which may include blast furnace slag, fly ash or cement kiln dust) are then added from bins 6 and 7 as shown by arrows 8 and 9. This mixture is then carried by a conveyor belt 10 to the remainder of the processor. Simultaneously, the other ingredients (e.g. the alkali metal silicate and the gelling agent and the water) are added from source 11 via arrow 12 to another tank 13. This material also is carried by the conveyor belt to a continuous mixing auger 14 and the total mixture then falls into a pit 16 and is shown being accumulated at 17. When all of the hazardous material has been treated in this manner and dropped into the pit, the pit may be covered by dirt or other covering materials shown as 18 and 18a. It should be here noted that this is simply a schematic drawing or flow sheet of the process of this invention. When actually in use, there are a myriad of tanks and bins to hold or contain the various ingredients desired to form the mixture that can contain and safely dispose of the hazardous waste.

In a particular example wherein low level contaminated soil is being treated, this soil is excavated using a conventional back hoe, for example, and the soil is then screened and pulverized down to fines (e.g. 1/4 inch in size). This material is then fed into a continuous mixing device that has been taken directly to the site. This device dispenses the cement (which may include blast furnace slag, fly ash or cement kiln dust), sand, water, silicate and gelling agent to this hazardous waste in exactly the desired proportions as previously determined by laboratory tests. The dry ingredients are first layered and then taken by an internal conveyor belt to the rotating auger where water and the final ingredients are added. All of this mixing action is of the high sheer type. the flow of all components are constantly monitored as they are processed by the continuous mixer. The mixed material is then discharged out the opposite end of the mixing auger directly into a burial pit. Alternatively, these materials may be discharged in a transfer pit where a front end loader can pick up the processed material and move it to a final placement area. The placement of the stabilized material, either temporary or final, is simply a matter of choice. The stabilized hazardous material within this pit can then be covered over with the existing clean soil that was previously excavated from this pit.

Other systems for the complete mixing of the ingredients of this invention so as to form the resulting, impervious block containing the hazardous wastes, may also be used within the ambit of this invention. The processes shown and particularly described herein are only considered to be the best mode known at the time of this filing.

To fully understand the problems of the prior art that this invention solves, one must know that discarded hazardous materials are an environmental nightmare. There are a host of these materials known in the art and these include many chemicals, wastes from chemical plants, sewage sludge, heavy metals and radioactive materials, to name a few. It is essential that the public is able to safely contain and dispose of these materials in an efficient and environmentally safe manner. Among the most important of these hazardous wastes are those that are radioactive. There are a myriad of places in which low-level radioactive wastes have contaminated the soil and it is imperative to safely contain and dispose of these products.

Prior art elements and processes, including those that use a cement matrix to contain these materials, will not function correctly. In addition, none of the prior art references describe the modifications to cement and the cement matrix taught herein. I have found that it is possible to safely contain hazardous wastes, including the radioactive wastes, in a cement matrix when the cementitious materials (which may include blast furnace slag, fly ash or cement kiln dust) and the waste are simultaneously mixed with a silicate and a gelling agent. The composition of this invention, along with the associated process, works most conveniently with low level radioactive contaminated soil, although any of the conventional hazardous wastes mentioned herein, will work with this composition just as well.

To function most propitiously, the cementitious material (which may include blast furnace slag, fly ash or cement kiln dust), should be very finely ground. The level given to finely ground cementitious material is measured by the so-called Blaine Fineness method, which is taught in "Design & Control of Concrete Mixtures" by the Portland Cement Association. In this test, the higher the number, the finer the particle size. For example, Portland Type I Cement has a Blaine Finess of 2,500 sq. cm./gram and that of Portland Type III Cement, one of 6,570 sq. cm/gram. The other materials which may be added, such as blast furnace slag, fly ash or cement kiln dust, also possess very high Blaine Finess numbers (e.g. 6,000 to 10,000 sq. cm/gram).

Within the working ambit of this invention, I will assume that there are between 2,500 to 4,500 pounds of total material within a cubic yard. Using a typical cubic yard of material, there are:

1. 200 to 2,400 pounds (5% to 75%) of cement (which may include blast furnace slag, fly ash or cement kiln dust);

2. 500 to 3,000 pounds (12.5% to 75%) of hazardous waste;

3. 100 to 1,200 pounds (2.5% to 30%) of water; and,

4. 0 to 2,000 pounds (0% to 50% ) of sand.

Within this mixture, 0.1 to 14 gallons of silicate (as a 40% aqueous solution) are added. 0.5 to 12 gallons of a gelling agent (0.5 to 4.0% solids in an aqueous solution) may then be added if so desired, and such is preferred.

In a particularly preferred embodiment, the composition of this invention, assuming 4,000 pounds per cubic yard of output material, will contain 400 to 2,200 pounds (10.0 to 55%) of cement, 200 to 800 pounds of blast furnace slag (5 to 20%), 200 to 600 pounds of kiln dust (5 to 15%), 1500 to 2,500 pounds (37.5 to 62.5%) of hazardous waste, 200 to 1000 pounds (5.0 to 25%) of water, 0 to 1,000 pounds (0 to 25%) of sand, 0.25 gallons to 0.75 gallons (most preferred is 0.5 gallons) of the aforementioned silicate and 1.0 to 2.0 gallons (most preferred 1.25 to 1.75 gallons at 1.5% solids) of a gelling agent. All of this is per yard of cementitious material.

One, particularly preferred composition representing this invention is:

25% Portland Type Ill Cement

16% Blast Furnace Slag

13% Water

40% Radioactive Contaminated Soil or Sand

3% Potassium Silicate

2% Gelling Agent (Guar Gum)

1% Calcium Stearate

When a cylinder of this material is made, the radioactive contaminated soil will be safely contained and can be stored or buried without further problems. Small amounts (up to 2%) of finely divided carbon black may be added to the radioactive contaminated soil. It is surmised that the carbon black will absorb free particles of radioactive material, although the fly ash previously mentioned may also do the same.

To prepare a cementitious block or unit of material which will contain the hazardous waste to be disposed in a safe and convenient method, I prefer using a mixing device that is continuous in nature, as previously described. These devices are conventional and well-known in the cement mixing art. The hazardous waste material or materials and the sand (if necessary) are volumetrically proportioned into the device. Preferable, dry, Portland-type cement is then added. The cement may be any of the well-known types of Portland cement, such as Types I, II, III, IV or V. There are a myriad of other cements, some of the rapid-setting type, available in the prior art and all will function adequately within the metes and bounds of this invention. Some of this cementitious material may be replaced with blast furnace slag, fly ash or kiln dust, in fact it is so preferred. This replacement for the aforementioned cementitious material (which may include blast furnace slag, fly ash or cement kiln dust) will enhance the strength of the composition which is requisite to insure that the hazardous material is safely contained therein. I refer to the material added as "strength boosting material". The aforementioned waste, sand and cement (which may include blast furnace slag, fly ash or cement kiln dust) are all relatively dry and these are then conveyed together to the auger mixing device (e.g. a high sheer auger mixer), where water is immediately added. The silicate and the gellant may be added along with the water in some instances. in other instances where, for example, acid may be added to the water to help reduce the pH of the final product, the gellant and silicate may be added separately some distance down the mixing auger (prefer 1-4 feet). This then allows the initial mixing of ingredients to take place prior to the addition of gellant and silicate and will prevent any side reaction with the acid from occurring. There may be other times when the addition of the gellant and silicate may occur further down the mixing auger (e.g. 7 to 9 feet) to help blend a particular hazardous waste stream.

By gelling agent I mean compatible materials which, when added to the entire mix, will enhance the resulting matrix to become more pliable or plastic. In some instances, these materials are referred to as "plasticizers". For the purposes of this invention, they will include, for example, metal stearates such as calcium stearate; titanium dioxide; resins such as "VINSOL RESIN" or any other chemical commonly used to entrain air within concrete (see "Design and Control of Concrete Mixtures" by Portland Cement Association). Nylon polymer fibers or polypropylene fibers may also be added to reduce shrinkage and cracking of the final product. A metallic stearate (e.g. calcium stearate), or titanium dioxide, may be added to impart water proofing.

A number of commercially available alkali metal silicates are available in the prior art and most will work within the metes and bounds of this invention. These silicates include sodium and potassium silicates of varying viscosity's and ratios of alkali metal to silicate. I prefer using a potassium silicate. In particular, a potassium silicate with 8.3% K.sub.2 O and 20.8% SiO.sub.2, with a viscosity of 40 cps and a pH of 11.3, sold by PQ Corporation of Valley Forge, Pa. as "KASIL.RTM.-1". It is thought that as the hazardous waste material is dispersed thoroughly throughout the grout-cement matrix, this dispersion of hazardous waste is then coated with the alkali metal silicate which forms a so-called shell (encapsulate) about the particles of hazardous waste. This encapsulation of the hazardous waste within a cement matrix is very surprising. Since these silicates are infinitely dispersible within the grout-cement mixture, they are totally compatible with this material and cause no problem in the total mix. As previously mentioned, it is thought that the silicate covering forms a hard, impervious shell around the hazardous waste thus encapsulating this material within the grout-cement mixture. The gelling agent is thought to cause a "super" curing of the silicate covering so as to form a harder shell than that taught in the prior art.

The grout-cement containing the silicate treated hazardous waste material, may be allowed to harden into a block that can be disposed. However, the lightest material, and the one with the lowest specific gravity within a grout-cement mixture is the water. Immediately after mixing, a predictable phenomena occurs. The free water in the mix begins to squeeze in an upward movement, displaced by the weight of the other materials in the mix, which are of a higher specific gravity. As the water moves upward within the mix, so does the entrapped and entrained air present therein. This rising air is thought to create and form micro-capillary tubes within the forming grout-cement. These capillary tubes appear to be about 20 to 30 microns in diameter. Any free, hazardous waste present within the mix may travel to the surface of the hardening grout-cement inside of these capillary tubes. Thus, even though a large amount of the hazardous waste is contained deep within the block of cement, some can be entrained and move upwards to the surface through these capillary tubes. This is highly undesirable and perhaps represents what has been missing in the prior art.

To counteract this migration/capillary action and formation theory, one should be able to create a gel-like quality to the free water contained within this mix. To accomplish this stage, I add a gelling agent to the grout-cement, silicate encapsulated hazardous waste matrix. Gel led free water in this matrix does not move. Since there is no water movement, no capillary action takes place and the grout-concrete/silicate containing hazardous waste matrix becomes a closed cell system. No hazardous waste will find its way through this system and will remain entombed within the dried material on curing. Additionally, a gelling agent will also act as an integral curing agent and thus dramatically improve the quality of the hardened grout-cement. Other free water gelling agents useful within the ambit of this invention are legion in number and include hydroscopic chemicals including guars, bentonite, naturally occurring celluloses (e.g. methyl cellulose), water soluble polymers derived from cellulose (e.g. cellulose ethers), among others. Most of these compounds are infinitely miscible with cementitious materials and are highly efficient water retention agents.

It is surmised that the silicate starts forming a permanent shell around the contaminated material when contacted thereby. It is thought that the heat of curing, primarily due to the grout-cement reaction, will harden and create a glass-like capsule. When fully cured (usually within 24 to 48 hours, depending on ambient conditions) it should then permanently isolate the hazardous waste, including radioactive materials, which then appear to be impervious to all naturally occurring soil reactions. The block of cementitious material containing the so-called encapsulated hazardous materials can be safely disposed by burial. The burial pit may be lined or unlined and may then be covered with soil or other cementitious material for further surface use. Alternatively, the cementitious material containing the hazardous wastes may be disposed by other, conventional methods for disposal, such as land-filling, dumping or underground storage, for example.

In yet another embodiment of this invention, a vault made from the mixture of this invention can be constructed to store hazardous waste materials. This particular process may be efficacious where there are particularly large volumes of hazardous wastes to be stored.

This stabilized material complexes many of the commonly available hazardous materials or toxins known and present in contaminated soil. This complex appears to be stable to many of the known processes which have leached out this contaminants from a cementitious matrix in the past. The entire process can be carried out in situ which is highly advantageous. The movement of equipment from contaminated site to contaminated site is also advantageous since the hazardous material does not need to be moved in and of itself. The contaminated material is thus lifted in place, treated and can be safely returned to the same place in a proper burial pit, for example. There are a host of chemicals, toxins, heavy metals, chemical wastes, radioactive wastes, etc. that can be efficaciously treated using the novel elements and process of this invention. The ratio of cementitious material and other ingredients can be balanced according to the type of hazardous material to be treated by my invention. These ratios of elements can be easily determined by laboratory testing before going into the field to remove, treat and stabilize that particular hazardous waste. Thus, a wide range of ingredients is envisioned within the metes and bounds of this invention.