TREATMENT OF WASTE

Abstract: A method of treating waste material which comprises cooling the material to a temperature below 0 DEG C in the presence of water, fragmenting the cooled waste and subjecting the fragmental waste to a wet oxidation process.

Description: TREATMENT OF WASTE This invention relates to the treatment of waste and, more particularly to the treatment and/or sterilisation of organic, medical, animal/human tissue and low level radioactive wastes. Large quantities of waste material are continually being generated, for example in the medical industry and in hospitals, which requires disposal by treatment, including incineration or vitrification involving the burning of such waste, or by storage in landfill sites with or without such treatment. None of the presently known methods for the treatment of waste is particularly satisfactory on environmental and other grounds. With increasing demands to meet more rigid environmentally acceptable targets, disposal of untreated waste, or even waste treated by known methods, is quite properly becoming increasingly difficult and therefore more expensive to achieve. There is therefore a clear need for a new method by which a more effective treatment of such material can be effected. In accordance with the invention, there is provided a method of treating waste material which comprises cooling the material to a temperature below 0°C in the presence of water, fragmenting the cooled waste and subjecting the fragmented waste to a wet oxidation process. The waste material may already be in an aqueus environmental in its original form but generally sufficient water or liquid waste should be added to the material, for example in a mixer or blender, prior to the cooling stage to dampen or to wet any dry components of the waste; alternatively a fine water spray may be used. The dampening/wetting step is essential as the water droplets are absorbed in to at least certain components of the waste, for example cloth, linen, gauze and other fabrics and fibrous substances, and therefore can ease the fragmentation step by rigidifying fibres not normally rigidified by simple cooling of the material. Preferably the overall temperature of the waste material is reduced during the cooling step to at least minus 20°C, and more preferably to at least minus 30°C. Advantageously the temperature is reduced to between minus 40°C and minus 100°C. If in particular the waste material comprises or contains plastics, for example nylon, PTFE, polyethylene, etc temperatures as low as minus 120°C or lower will generally be required. Although such temperatures may be employed, in alternative embodiments of the invention plastic material is separated from other waste prior to the cooling step and thereafter separately fragmented, for example at ambient temperatures. Any types of cooling process and/or apparatus can be employed to good effect. Preferably a liquid cryogen such as liquid nitrogen or liquid carbon dioxide is employed in heat exchange apparatus for example a rotary drum or freezing tunnel or by direct injection in to, or fine spray on to, the waste material. The fragmentation step can be effected by any suitable types of equipment, for example a heavy duty hammer, vertical disintegrator or a suitable grinding device. Preferably the fragmentation step is a multi-stage (two or more) process in which the waste is first broken in to coarse fragments and then fragmented in to finer particles, for example within the ranges of 0.1 to 1 mm in the subsequent stage or stages. Ideally the waste material particles from the first fragmentation process is from 3 to 10 mm, for example of the order of 6 mm, although longer stands of fibrous substances may also be present. The finer particles are then achieved in the subsequent stage(s). A cryogen may be used in the fragmentation stage(s) as appropriate to retain the desired low temperature. Cryogen, for example liquid nitrogen, consumption will generally be determined by the desired temperature and the specific grinding energy as expressed, for example, by KW/kg of material being processed. In the wet oxidation stage, the fragmented waste should be mixed with water (or other liquid waste) and subjected to wet oxidation, preferably at an elevated temperature or elevated pressure (or both), to effect oxidation of the waste, primarily to carbon-dioxide and water and a non-toxic end product. A temperature of up to 320°C is preferred. Most preferably the temperature is from 200 to 300°C, for example from 240 to 290°C. A pressure of up to 150 bar is preferred. Most preferably, the pressure is from 30 to 170 bar, for example from 50 to 160 bar. An oxygen-containing gas is necessary to support the wet oxidation step. Presumably the step is conducted with a gas containing at least 80%, more preferably 90%, oxygen. Advantageously the gas is substantially pure oxygen. Generally, the presence of a high percentage of oxygen reduces the volume of inert gases resulting from the wet oxidation step and therefore tends to minimise heat losses from the oxidation step and emissions of unreacted contaminants to the atmosphere. In advanced wet oxidation processes, certain catalysts may be employed and, separately or simultaneously, the waste may be rendered acidic through the addition of, for example, sulphuric acid. For certain wastes, for example those containing medications or low level radioactive substances, the wet oxidation stage can be or can be followed by a supercritical water oxidation in order to break down more resistant, for example refractory, substances. Supercritical water oxidation employs the high temperatures required for supercritical water to increase the rate of oxidation of the organic waste. As the water is heated above its critical temperature of 374°C and critical pressure of about 22.1 MPa, its density drops considerably - typical operating densities are 0.15 to 0.2 g/cm 3 - and the dielectric constant substantially decreases also. With these changes to the physical properties of the waste, organic waste is highly soluble but ionic waste is not. The organic waste is dissolved in an oxygen-containing environment and oxidation occurs rapidly in the high temperatures employed. If necessary, the basic waste material may be separated to remove metal or plastic or other potentially undesirable substances prior to the first cooling step. Again if necessary, additions of organic matter, for example sewage sludge, may be made to the waste prior to the wet oxidation step in order to improve the organic to inorganic material ratio to be fed in to the wet oxidation system for the purpose of an additional waste stream process and/or to improve and potentially enhance the performance of the wet oxidation system. The wet oxidation step may be carried out using an oxygen/ozone injection if required. As an example of the carrying out of the method of the invention, soiled waste material comprising primarily hospital waste including cottons, gauzes, bandages and other substances was subjected to a moisturising step by spraying with water and thereafter to a cooling step in a rotary drum in to which a spray of liquid nitrogen cryogen was injected to reduce the temperature of the wetted waste of the order of minus 40°C. cryogen was injected to reduce the temperature of the wetted waste of the order of minus 40°C. The cooled waste was then subjected to a first fragmentation stage in a vertical disintegration apparatus employing a heavy duty hammer to reduce the waste to particles of about 6 mm in size but including some longer fibres. In a second fragmentation stage, an industrial grinding machine was employed to reduce the waste down to a particle size of the order of 0.5 mm (and some longer fibres). The waste was then placed in a wet oxidation system in which, in a semi-continuous process in which the waste was acidified and fed through a chamber at a temperature of 275 °C and at a pressure of 100 bar, the waste was oxidised primarily to carbon dioxide and water leaving a sterile waste as the end product which could be safely disposed. The wet oxidation system may advantageously include a tall cylindrical chamber in which the waste is placed with an oxygen/ozone injection towards the bottom of the chamber so that an increased pressure is achieved from the weight of the waste above the oxidation zone of the chamber. Means may also be employed continually to remove oxidised material from the base of the chamber at a rate corresponding to the rate of production of oxidation products therein.

Claims: CLAIMS 1. A method of treating waste material which comprises cooling the material to a temperature below 0°C in the presence of water, fragmenting the cooled waste and subjecting the fragmental waste to a wet oxidation process. 2. A method according to Claim 1 in which the material is cooled to a temperature of minus 20°C or lower. 3. A method according to Claim 1 or Claim 2 in which the material is cooled to a temperature of minus 30° or lower. 4. A method according to any preceding claim in which the material is cooled to a temperature of between minus 40°C and minus 100°C. 5. A method according to any preceding claim in which the fragmentation step is a multi-stage process. 6. A method according to Claim 5 in which the particles form the first fragmentation process are from 3 to 10 mm. 7 A method according to Claim 5 or Claim 6 in which the particles for a subsequent fragmentation process are from 0.1 to 1 mm. 8. A method according to any preceding claim in which a temperature of up to 320°C is employed: the wet oxidation step. 9. A method according to any preceding claim in which a pressure of up to 180 bar is employed in the wet oxidation step. 10. A method according to any preceding claim in which the wet oxidation step is a superintical water oxidation process.

Inventors: FERGUSON IAN GEOFFREY, FERGUSON, IAN, GEOFFREY
Applicants: BOC GROUP PLC, FERGUSON IAN GEOFFREY, FERGUSON, IAN, GEOFFREY, THE BOC GROUP PLC
Application year: 1997
Main classification: B09B3
IPC codes: A61L11, B09B3, C02F11, G21F9