WO1997006387A2

WO1997006387A2 - Destruction of hazardous wastes using a molten oxidizing alkali bath

Info

Publication number: WO1997006387A2
Application number: PCT/US1996/012386
Authority: WO
Inventors: Shawn Ray Mcgillivray; Michael John Runow
Original assignee: Terra Environmental
Priority date: 1995-07-28
Filing date: 1996-07-26
Publication date: 1997-02-20
Also published as: WO1997006387A3

Abstract

This invention is a process useful for the destruction of waste materials that contain organic materials or hazardous materials, comprising introducing the waste material into a molten alkali metal hydroxide bath that contains an oxidizing agent under conditions such that the waste material is oxidized. Silica is finally combined with the treated waste and the temperature is increased which results in the glassification of the residues.

Description

DESCRIPTION

DESTRUCTION OF HAZARDOUS WASTES USING

A MOLTEN OXIDIZING ALKALI BATH

BACKGROUND OF THE INVENTION

This application claims priority on provisional patent application Serial Nos. 60/001,674, filed July 28, 1995 and 60/003,271, filed August 23, 1995, both entitled "Destruction of Hazardous Wastes Using a Molten Oxidizing Alkali Bath," by inventors Shawn McGillivray and Michael Runow.

This invention relates to the destruction of waste materials containing organic or other hazardous wastes such as radioactive wastes. More particularly, this invention relates to the destruction of waste materials containing organic or hazardous wastes using a molten alkali metal salt bath containing an oxidizing agent.

It is relatively common to dispose of organic waste materials such as polyethylene by depositing these materials in a landfill. Alternatively, the organic materials are also sent to an incinerator where the materials are burned. However, if the waste is mixed and is contaminated with wastes which do not combust or which decompose into toxic substances upon incineration, the contaminated mixed waste material cannot be incinerated. This is especially the case for mixed wastes that are contaminated with radioactive substances. Furthermore, to send such contaminated waste materials to a landfill may be prohibited. Cost effective treatment methods for such wastes have not yet been identified satisfactorily by the generators ofthe waste and many alternatives are under consideration.

The present inventors have identified that a need exists to develop an inexpensive and effective method for treating wastes that are contaminated with organic or other hazardous materials. More particularly, it can be seen that a problem exists in how to dispose of mixed waste materials which cannot be sent to a landfill or incinerated. A solution to this problem would be highly desirable and is provided in the present invention.

SUMMARY OF THE INVENTION

The present invention provides an inexpensive solution to one or more problems discussed above by advantageously providing a method for the destruction of organic materials while simultaneously destroying and/or transforming the inorganic materials and preparing them for encapsulation in a solid. This invention operates at temperatures much lower than those required by incineration. Further, this invention enables the contaminant to be isolated and trapped within a glassy composition.

In one broad aspect, the present invention is a process useful for the destruction of waste materials that contain organic materials or hazardous materials, comprising introducing the waste material into a molten alkali metal hydroxide bath that contains an oxidizing agent under conditions such that the waste material is oxidized.

In one embodiment, the present invention is a process useful for the destruction of hazardous and/or non-hazardous waste, comprising: introducing the hazardous and/or non-hazardous waste into a molten alkali metal hydroxide bath that contains an oxidizing agent under conditions effective to convert at least a portion ofthe organic material to carbon dioxide and water vapor. The process may serve to convert any undesirous or unstable waste components into more stable forms. Organic materials employed in practicing the present invention include, but are not limited to, organic solvents, halogenated solvents, polymers, organic salts, aqueous solutions, hydrocarbons, and gasses. It is envisioned that this invention will be particularly useful for treating organic contaminated mixed wastes, such as highly alkaline sodium nitrate sludges contaminated with solvents, polyethylene, polypropylene or combinations thereof. It is also contemplated the invention can be used to destroy a wide variety of hazardous wastes such as acid gases (e.g., H₂S), sodium nitrate and uranium hexafluoride.

These materials can be introduced into the salt bath by adding them to the surface ofthe liquid phase or, preferably, beneath the surface ofthe liquid phase where mixing and retention time are improved. Under certain conditions depending on the rate which the waste is added and the type of waste, whether volatile or not, it may be desirable to pre-heat the feed waste material to roughly the temperature ofthe bath prior to addition and to add the pre-heated waste while mixing.

The hazardous waste may contain many materials including, but not limited to, organic materials, organic and metallic salts, metals, vitreous materials, hydrocarbons, oxidizers, reducing agents, radioactive materials, gases, and mixed wastes.

As used herein the phrase "molten alkali metal hydroxide bath" refers to a composition consisting of an alkali metal (e.g. lithium, sodium, potassium, rubidium and cesium) hydroxide which has been heated to its melting point to convert it into a liquid. Anhydrous hydroxides of sodium and potassium are preferred. The temperature ofthe bath depends upon the hydroxide employed and should be maintained above the hydroxide's melting point to prevent solidification. For example, temperatures between about 600 and about 1000°F are preferred, with from about 600 to about 800°F being more preferred. After waste materials have been added the temperature may have to be increased to prevent solidification because the introduction of various materials, particularly silica, may increase the melting point ofthe bath. A substantial amount ofthe hydrocarbons are converted into sodium carbonate, which may accelerate the need for increasing the bath temperature. The higher temperatures seem to promote carbonization ofthe organic materials, as indicated by the formation of a black precipitate in the solution. However, after prolonged reaction, the carbon deposits disappear.

Soda ash may be removed from the system using a soda ash/lime precipitation circuit. The soda ash precipitate is drawn off from the reaction vessel and dissolved to water. A slaked lime slurry is added to the soda ash solution and the resultant chemical reaction produces sodium hydroxide and calcium carbonate. The sodium hydroxide will remain in solution and the calcium carbonate, being of low solubility in water, will precipitate. Depending on the composition ofthe waste material, many other inorganic anions, such as phosphate and fluoride will also precipitate. The sodium hydroxide solution may be recycled back to the reactor vessel and the precipitated salt mixture will be a waste product.

Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO^

Various waste streams may be treated by molten alkali hydroxide baths under oxidizing conditions. Novel ion removal circuits are envisioned that contribute to recycling and waste reduction. For example, when wastes containing high percentages of sodium are introduced into the system, other contaminating alkali metal ions tend to precipitate and are removed as residuals. Also, carbonates, chlorides and fluorides as well as many other potential sodium salts are expected to precipitate and may be removed from the bath. However, in the case of sodium nitrate, production of excess sodium hydroxide may be a problem because the reactor vessel is at a fixed volume. To remedy this, excess production of alkali metal hydroxides can be removed from the system as a liquid through normal liquid-solid separation techniques. Contaminants ofthe bath with cesium ions (which may be radioactive) can be addressed in two ways. If the bath is operated at temperatures in excess of 800°K, CsOH will evolve from the bath as a gas and will be recovered and separated as part ofthe off-gas treatment system. At lower temperatures, the CsOH will form a solution and remain in the bath. This can be removed by having a bath recycle/separation circuit that will take of a portion ofthe bath material and dissolve it in water. Any cesium present can then be separated by ion exchange, and the resultant, purified hydroxide recycled back to the bath. Excess sodium or potassium hydroxide remaining can be stored for resale in the chemical market after being so removed, or it may be converted into soda ash by adding CO₂ into the molten alkali hydroxide removal circuit. Soda ash or potash have less rigorous storage conditions than the respective hydroxides, and they may be the desirable form to produce and store until irradiated sodium or potassium have had sufficient time to decay for safe use ofthe finished product into the marketplace.

It may be desirable to separate phosphates, iron oxide, aluminum oxide and other metal oxides and hydroxides that have commercial value prior to encapsulation in silicates.

The heating may be accomplished by standard methods, such as with ohmic heat or microwave energy. It is envisioned that the use of microwave energy will be preferable because it allows for efficient temperature control. The operating temperature is surprisingly low, roughly half the temperature in degrees Fahrenheit, as compared to known techniques which employ sodium carbonate.

It was recognized that other previously developed waste treatment methods which utilized other salts as the reaction medium had many associated problems. Most of these problems (such as corrosion and volatilization) were associated either directly or indirectly with the high operating temperature. It was decided that an examination of lower melting salts might avoid many of these problems. Initially, salts with even lower melting points than hydroxides were tried, however, the practice ofthe present invention was incompatible with salts other than hydroxides. Potassium bisulfate and potassium pyrosulfate were tried but they proved to be difficult to control under oxidizing conditions, probably due to t heir acidic nature. Although the hydroxides have higher melting points, it was believed that the highly alkaline environment produced by these chemicals would be ideal for this process. Hydroxides are also good for stabilizing peroxides, which is important for maintaining a highly oxidizing environment. It is envisioned that other salts in conjunction with different oxidizing agents may work.

These molten materials are very corrosive. Therefore, reaction containers should be composed of resistant, inert materials. Examples include, nickel, platinum, zirconium dioxide, or oxide coated resistant materials like zirconium dioxide coated nickel. Stainless steel was also employed in the present invention. Although the invention was operable, the molten bath developed a bright green color, due to the degradation ofthe stainless steel container and the dissolution into the bath. Therefore, certain catalysts, such as metal ions of chromium, nickel chromium or iron, may also be important in the practice ofthe present invention under special conditions.

The molten alkali bath should also contain an oxidizing agent. The present invention should work with most oxidizing agents that are compatible with molten anhydrous alkali hydroxides. Oxidizing agents that provide an oxygen-rich environment in the alkali bath are preferred, with calcium and magnesium peroxide being particularly preferred initially and with sodium nitrate, potassium nitrate and/or oxygen to sustain the oxygen-rich environment. The added oxidizing material may also be a component ofthe waste material. Acidic oxidizing agents, such as nitric acid or hydrogen peroxide, are incompatible with the present invention. Although the oxidizing agent can be added in any way to the alkali hydroxide, it is generally preferred to heat the alkali hydroxide to form a liquid and then to add the oxidizing agent. It is also possible to heat the alkali hydroxide and add the organic material prior to addition ofthe oxidant.

In a preferred embodiment of this invention, sodium hydroxide is heated to about 800°F until it forms a liquid then a mixture of calcium and magnesium peroxide is added. The addition ofthe oxidizing agent causes an immediate reaction and the formation of bubbles that become trapped in the molten salt bath, similar in consistency to a "meringue". After introduction ofthe waste contaminated organic material, additional sources of oxygen are required. These sources of oxygen may be in the form of an oxidizing agent, nitrate salts, molecular oxygen or ozone.

As used herein the phrase "conditions effective to convert the organic material to carbon dioxide and water vapor" is used to refer to conditions under which the decomposition of organic materials to carbon dioxide and water vapor is substantially complete. These conditions will vary with different organic materials employed. Generally, elevated temperatures, usually the temperature ofthe molten bath, are required but elevated pressures are not necessary although the use of pressure may significantly improve the rate of decomposition. It may be necessary to use a pressure reaction vessel and recycle any gasses or vapors that are emitted back to the molten bath to insure that all the waste is treated and ultimately transformed into an inorganic form.

In another embodiment, the invention further comprises combining silica with the resultant products ofthe bath. The silica being employed in an amount such that the amount of sodium or potassium in a resulting composition is less than about 30 percent by weight; and allowing the resulting mixture to form a solid. Once the waste material has been so treated, the temperature should be raised up to about 2400°F after transferring the material to a ceramic crucible to remove the remaining water and vitrify or glassify the waste. Calcium, magnesium and/or aluminum oxides may also need to be added to improve the quality ofthe final glass material. DETAILED DESCRIPTION OF THE TNVENTTON

The organic materials that may be treated in accordance with this invention may vary widely. The waste materials containing organic or other hazardous wastes may be in the form of either a gas, a liquid or a solid at standard temperature and pressure. Representative examples of such waste materials include polymeric materials (e.g., polyethylene and polypropylene), solvents, halogenated hydrocarbons, vitreous materials, acids, bases, metal salts, alkali metal salts, oxidizers, oxides, gases, radioactive materials or any combination ofthe foregoing.

The waste materials may be contaminated organic materials or contain a wide variety of contaminants including radioactive materials. Such contaminants may be present in the waste materials at a wide variety of concentrations. These contaminants include organic materials, organic and metallic salts, metals, hydrocarbons, oxidizers, reducing agents, radioactive materials, gases, and mixed wastes.

The following examples are illustrative ofthe invention are not to be construed as limiting the scope of this invention or claims thereof. Unless otherwise indicated, all percentages are by weight.

EXAMPLE I

Potassium hydroxide (15.9g) was added to a stainless steel cup with a capacity of approximately two ounces and heated on a conventional hot plate to just above the melting point (716°F). High density polyethylene ("HDPE", 1.20g) was then introduced in one single piece. After a few minutes, the polyethylene melted and floated on the surface. MgO₂ (commercially available, 25% MgO₂ by weight in Mg(OH)₂, 5.25g) was then added. The addition of magnesium peroxide caused a reaction to occur which resulted in the production of a foam like materials with the consistency of "meringue." This also resulted in approximately a doubling ofthe reactant volume.

After maintaining the materials at elevated temperature for about four hours, the organic material had lost approximately 16% by weight. This weight loss was calculated by the following procedure: a portion ofthe mixture containing the molten alkali hydroxide and polymer was diluted with water (4g of sample to 28g ofwater); sufficient hydrochloric acid was added to acidify the solution resulting in a less than IN solution; and then the organic material was isolated, dried and weighed.

EXAMPLE II

Sodium hydroxide (15.2g) was added in place of potassium hydroxide in the above Example and HDPE (1.17g) was introduced as well as CaO₂ (1.50g). The resulting mixture was allowed to react for two hours, and the organic material was separated as described above and weighed. The weight loss of HDPE was 98.6%.

EXAMPLE III

Sodium hydroxide (15.2g) was used as in Example II, MgO₂ (2. OOg) as in

Example I and HDPE (1.3 lg). The reaction time was two hours. The weight loss of HDPE after separation and drying was 72.2%.

Claims

1. A process useful for the destruction of waste materials that contain organic materials or hazardous materials, comprising:

introducing the waste material into a molten alkali metal hydroxide bath that contains an oxidizing agent under conditions such that the waste material is oxidized.

2. The process of claim 1, fiirther comprising combining silica with the resultant molten bath product subsequent to oxidation ofthe waste material, the silica being combined in an amount such that the amount of sodium in a resulting composition is less than about 30 percent by weight and thereafter forming the resulting composition in a solid form.

3. The process of claim 1 wherein the organic material is a polymer.

4. The process of claim 1 wherein the organic material is polyethylene or polypropylene.

5. The process of claim 1 wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

6. The process of claim 1 wherein the oxidizing agent is magnesium peroxide, calcium peroxide or a mixture of magnesium peroxide and calcium peroxide.

7. The process of claim 1 , wherein the bath further contains sodium nitrate, potassium nitrate, oxygen or ozone.

8. The process of claim 1 wherein the temperature ofthe bath is from about 600 to about 1000°F.

9. The process of claim 2 wherein the temperature is from about 600 to about

800°F.