Use of a treatment vessel for destroying sodium-containing waste

Abstract: Waste materials including organic and inorganic by-products and combined sodium in the amount of at least about 0.1%, and most often at least about 5%, by weight are destroyed by combustion at a temperature above about 700°C in a vessel lined with a refractory comprising at least about 85% by weight combined aluminum calculated as aluminum oxide. The refractory may be of sintered or fused-cast construction; if the former, it also comprises an aluminosilicate binder, preferably mullite, and contains at least about 6.5% combined silicon calculated as silicon dioxide. Such refractories have improved resistance to the corrosive effects of the sodium in the waste stream.

Description: This invention relates to the destruction of waste materials, and more particularly to the combustion of waste streams which contain organic compounds and which are high in sodium. A major cost element in chemical processes carried out on a commercial scale is the disposal of non-recyclable by-products and wastes. These may include organic and inorganic compounds, many of which are toxic or otherwise hazardous. Increased attention to environmental hazards has made it imperative to develop improved methods for disposing of such waste products. In particular, many of such products must be destroyed so that they do not pass into the environment and cause harm. Among the industries demanding effective waste disposal is the plastics industry. Millions of pounds of plastic are produced per year, and almost every production process also creates large amounts of by-products and wastes. A single example of a plastics production process which produces wastes is the process of making polyimides, and particularly polyetherimides. Many steps are involved, only a few of which need be described here to indicate the complexity of the waste materials which must be dealt with. In a typical polyetherimide preparation method, an early step is the nitration of a phthalimide with nitric acid, which produces as wastes excess nitric acid and various nitro-substituted phthalic acid derivatives. The nitrophthalimide is slurried with water and the slurry is contacted with an organic solvent, whereupon the nitrophthalimide dissolves in the organic phase and leaves behind an aqueous waste containing traces of various compounds, organic and inorganic. The nitrophthalimide in the organic solution undergoes a displacement reaction with the disodium salt of a dihydroxyaromatic compound such as 2,2-bis(4-hydroxyphenyl)propane, commonly designated bisphenol A, with the formation of a bisimide and sodium nitrite as a by-product. This displacement reaction ordinarily takes place in the presence of a phase transfer catalyst, traces of which may also be found in the waste stream. Numerous other by-products, as well as further quantities of those already mentioned, are removed by further extraction of the organic reactant solutions with aqueous sodium hydroxide. The bisimide subsequently undergoes further reactions to form the polyetherimide. The complexity of the combined aqueous waste stream formed in this production process will be apparent from the above description. It contains large quantities of water, typically 60-80% and usually on the order of 70% by weight. About 15-30%, usually about 20%, is ordinarily nitrite ion (in the form of sodium nitrite) and about 5-10%, usually about 7-8%, various organic compounds, and the remainder includes sodium hydroxide. The sodium ion content of the aqueous waste stream is substantial, often on the order of 5-15% and usually about 7-8% by weight. pH values may be in the range of about 2-12 but are usually on the alkaline side, typically about 10-12. A typical waste disposal method for such by-products involves feeding the aqueous waste stream into a multi-stage destruction apparatus including oxidation/combustion and reduction stages. In such an apparatus, organic compounds are combusted and nitrogen compounds converted to nitrogen oxides in one or more stages conducted in an oxygen-rich atmosphere. At least one other stage may be conducted in a reducing atmosphere created by the injection of natural gas, the effect of which is to convert nitrogen compounds to elemental nitrogen. All of these stages are carried out at very high temperature, typically on the order of 1000-1300°C. DE-A-3004186 discloses an apparatus for destroying waste materials, having its inner surface lined with a refractory material. However DE-A-3004186 does not recognise or even suggest that it is advantageous that the refractory material contains a high percentage of aluminium. The effect of sodium compounds under such conditions is profound and adverse. Even in concentrations as low as 0.1% by weight, sodium can corrode metal vessels so fast that such vessels, if unlined, are utterly useless as containers. A typical way to minimize such corrosion is to line the vessel with a refractory material. It has been found, however, that sodium is also profoundly destructive of most refractories. A majority of the sodium is converted to sodium oxide which subsequently reacts with carbon dioxide to form sodium carbonate. Any remaining sodium, however, reacts with the refractory material to produce a slag which spalls off the inside surface of the chamber in varying quantities. The result is ultimate destruction of the refractory, exposing the bare metal to the action of the sodium and ultimately rupturing the vessel. As a side effect, large quantities of slag are produced and they complicate disposal of the treated waste stream. It has been estimated that as much as 90% of the deterioration of refractories in these environments is the result of corrosion by sodium compounds. Various types of refractories have been evaluated for use in a process such as that described above. They are typically in the form of brick containing various proportions of combined aluminum, generally in combination with combined silicon and other combined metals and non-metals. For the most part, however, such brick has been found to have a very short life when exposed to a waste stream high in sodium. US 4237103 discloses a method for disposal of sodium containing waste material whereby deterioration of refractory materials in the incinerator by reaction with free sodium is reduced comprising simultaneously introducing sodium containing waste material and silica bearing material into an incinerator lined with a refractory material which tends to react with free sodium and deteriorate, the ratio by weight of the sodium and silica in the thus introduced materials being in the range of 0.4 to 0.8; and combusting the sodium containing waste material and silica bearing material to provide a combustion product including a sodium silicate compound. During combustion, free sodium produced by the sodium containing waste material has a affinity for silica, and since the silica bearing material is readily available for reaction, it is believed that the sodium in the sodium containing waste material will preferentially react with the silica in the silica bearing material, thus reducing the degree to which the silica and/or the alumina in refractory walls reacts with the free sodium. The present invention is based on the discovery of certain types of refractory materials high in aluminum which are particularly resistant to the corrosive effects of sodium compounds. Combustion chambers lined with these refractory materials are capable of undergoing hundreds of hours of contact at high temperatures without failing. Moreover, the slag produced when such refractories are used is small in volume and relatively easy to remove and dispose of. Accordingly, the invention provides the use of a treatment vessel having a refractory lining formed of a refractory material of sintered construction having a bulk density of at least 2.9g/cm 3 and comprising at least 90% by weight combined aluminium calculated as aluminium oxide for destroying organic waste products containing at least 0.1% by weight combined sodium, wherein said waste products are fed as a stream to the vessel, the waste products subjected to temperatures in the range of 1000 - 1300°C to effect combustion thereof, and by-products including sodium compounds removed from said vessel. The drawing illustrates a typical combustion apparatus for wastes which may be employed in accordance with the invention. The present invention is capable of employment to destroy a wide variety of organic and inorganic waste products. For the most part, it is advantageous in connection with waste streams containing organic chemicals and a relatively high concentration of sodium compounds, the latter being at least about 0.1% by weight and frequently at least about 5% by weight. The vessel into which the waste stream is fed may be made of any non-porous material which is capable of withstanding the combustion temperatures of about 1000-1300°C attained during the process. Metals such as steel are typically used. It is necessary, however, that the interior surface which contacts the waste products be a suitable refractory material in order to prevent the rapid corrosion of metal which occurs upon contact with sodium compounds at the prevailing combustion temperatures. Accordingly, the nature of the refractory lining of the vessel is a key feature of the invention. It should comprise at least about 90% combined aluminum by weight calculated as aluminum oxide (alumina). The fact that such materials effectively resist attack by sodium compounds is extremely unexpected, particularly in view of observations made by Caprio and Wolfe of Harbison-Walker Refractories in a paper entitled "Refractories for Hazardous Waste Incineration―An Overview", in which it is observed that alkalies attack high-alumina refractories and the rate of attack is severe as the alumina content is increased to values on the order of 70%. In sintered refractories, the presence and nature of a binder material are also critical. Said binder should comprise a major proportion of aluminosilicate, and should be present in sufficient amount to provide at least about 6.5% combined silicon, calculated as silicon dioxide (silica), in the refractory. Mullite is a particularly effective form of aluminosilicate for use as a binder. It is also generally important that the refractory be free from heavy metals, which may be environmentally harmful if present in the slag. For example, some refractories contain chromium which remains in the slag in the hexavalent state, known to be an environmental hazard. Such materials are unsatisfactory for the purposes of the invention. The bulk density of the refractory used according to the present invention is generally at least about 2.9 g./cm. 3 . It is most often above 3.0 g./cm. 3 . Various commercially available brick compositions have been identified which are particularly valuable for use according to the present invention. They include "Korundal XD" manufactured by Harbison-Walker Refractories, and "GreenAl-90" manufactured by A.P. Green Industries, Inc. The essential properties of these materials are given in the following table. Korundal XD GreenAl-90 Type S S Percent by weight: Al 2 O 3 90 90 SiO 2 9 9 MgO 0.1 - - Fe 2 O 3 0.1 0.2 TiO 2 0.1 0.1 CaO 0.1 - - Na 2 O - - 0.2 P 2 O 5 - - 1.5 "GreenA1 - 90" contains 1.5% phosphorus as phosphorus pentoxide. In actual plant installations, "GreenAl-90" has been found slightly superior to "Korundal XD" in wear characteristics. Accordingly, in a preferred embodiment of the invention the refractory also contains at least about 1% combined phosphorus as P 2 O 5 . The slag produced in the method of this invention may vary in nature from a uniformly granular material to one which is thick and flowable near the refractory surface and more granular at a distance from said surface. The formation of a thick, flowable slag is frequently advantageous in that less shearing of the pebbles from the refractory surface may occur than with a granular slag. The organic constituents of the waste stream are combusted in the aforementioned vessel, at temperatures in the range of about 1000-1300°C. The precise nature of the combustion step is not critical for the purposes of this invention. Most often, however, it comprises at least one oxidizing and at least one reducing step, conducted under fuel-lean and fuel-rich conditions, respectively. The fuel is normally natural gas and is employed in admixture with an oxygen-containing gas, typically air. A typical combustion operation according to the invention utilizes the apparatus shown in the drawing. The waste stream is charged at 2 to the top zone 4 of a three-zone combustion stack 1, where it is contacted with air and natural gas entering at 6 and 8, respectively. The stream then passes downward via passage 10 into second zone 12, and finally through passage 16 into third zone 18. Additional natural gas and air are introduced via nozzles 14 and 20, producing fuel-rich or fuel-lean zones as desired to result in reduction of the nitrogen oxides to elemental nitrogen or oxidation of nitrogen compounds to nitrogen oxides, respectively. From zone 18, the vapors of the waste stream are passed through downcomer 22 where they are quenched and removed by contact with water introduced through water jet 24. Volatile components are removed via at least one gas conduit, with two being shown at 26 and 28, and may be passed to scrubbers for further treatment. Liquid components, predominantly water, are removed from quench tank 30 through liquid conduit 32. In terms of construction, combustion stack 1 includes an external metal shell 34, typically of steel, and an inner refractory lining 36, typically of brick as previously described. The thickness of lining 36 will vary with the conditions but is generally on the order of 15-30 cm., with about 20-25 cm. often being preferred. In the course of the waste treatment process in stack 1, attack by sodium compounds on lining 36 often causes spalling of said lining in the form of pieces of various sizes, ranging from small particles to large chunks. Most of the larger pieces are caught by grate 38, with smaller particles passing through said grate and being caught in funnel 40. Prior to the present invention, some pieces dislodged by spalling were so large that they could penetrate the wall of downcomer 22 and fall elsewhere in quench tank 30. They were known on occasion to be so hot that they caused vaporization of the liquid in the quench tank, occasionally resulting in rupture of safety pressure disks in the system. The present invention has materially reduced the amount of spalling and alleviated this problem. An apparatus, for destruction of waste materials is illustrated in the drawing. Said apparatus comprises a plurality of intercommunicating vertically disposed combustion stacks, means for feeding waste and fuel materials to the uppermost of said stacks, and a downcomer from the lowermost of said stacks. Each of said stacks, and each passage connecting them, has its inner surface lined with the above-described refractory. Said apparatus optionally further comprises fuel-lean and fuel-rich zones and means for supplying oxygen-containing gas and fuel, respectively, to said zones, as described hereinabove. Various advantageous results of the present invention will be apparent to those skilled in the art. For example, the treatment system can be operated for substantially longer periods without deterioration of the interior of the treatment vessel. Further, the essential elimination of spalled refractory causes a substantial decrease or even elimination of the operational and cost burdens of conveying such material to a landfill. The overall result, with particular reference to the substitution of a 23-cm. thickness of "GreenAl-90" for a similar thickness of a refractory substantially lower in aluminum and higher in silicon, has been to decrease down time of the unit and increase waste stream capacity by 90% and 36%, respectively. A waste destruction unit according to the invention can operate without down time for periods of at least 128 days.

Claims: The use of a treatment vessel having a refractory lining formed of a refractory material of sintered construction comprising alumina grains and an alumino silicate binder and containing at least 6.5% silicon dioxide, said refractory material having a bulk density of at least 2.9g/cm 3 and comprising at least 90% by weight combined aluminium calculated as aluminium oxide for destroying organic waste products containing at least 0.1% by weight combined sodium, wherein said waste products are fed as a stream to the vessel, the waste products subjected to temperatures in the range of 1000 - 1300°C to effect combustion thereof, and by-products including sodium compounds removed from said vessel. Use of a treatment vessel according to claim 1, wherein the refractory is free from heavy metals. Use of a treatment vessel according to claim 1 or claim 2, wherein the waste products include nitride ion in the amount of about 15-30% by weight. Use of a treatment vessel according to any preceding claim wherein the concentration of combined sodium in the waste products is at least about 5% by weight. Use of a treatment vessel according to claim 1, wherein the aluminosilicate in the binder is mullite. Use of a treatment vessel according to claim 1, wherein the binder contains at least about 1% combined phosphorus as P 2 O 5 . Use of a treatment vessel according to any preceding claim, wherein the combustion operation comprises at least one oxidizing and at least one reducing step, conducted under fuel-lean and fuel-rich conditions, respectively.

Inventors: ALLEN TIMOTHY MARK, GUGGENHEIM THOMAS LINK, PETERS HERBERT CLARK, SPIRO CLIFFORD LAWRENCE, ALLEN, TIMOTHY MARK, GUGGENHEIM, THOMAS LINK, PETERS, HERBERT CLARK, SPIRO, CLIFFORD LAWRENCE
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1993
Main classification: F23G5
IPC codes: B09B3, F23M5, F23G5, F23G7