GB2210867A

GB2210867A - Destruction of environmentally hazardous halogenated hydrocarbons, with lead

Info

Publication number: GB2210867A
Application number: GB8809012A
Authority: GB
Inventors: Edward Charles Mantle
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-10-12
Filing date: 1988-04-15
Publication date: 1989-06-21
Anticipated expiration: 2008-04-15
Also published as: GB2210867B; GB8809012D0; GB8723889D0

Abstract

Chlorinated hydrocarbons, especially PCBs, are destroyed safely by reacting them with molten lead. The molten lead extracts the chlorine values from the chlorinated hydrocarbons to give lead chloride from which the lead and/or chlorine values can be recovered.

Description

Destructtan of nvironenta1ly zardous Halogented Hydrocarbons such as Polychlortnated Bphenyls This invention relates to the safe destruction of polychlorinated hydrocarbons such as polychlorinated biphenyls commonly known as PGB's.

Its field of application includes liquid chlorinated hydrocarbons, chlorinated hydrocarbons in the form of fine powder and solids contaminated with chlorinated hydrocarbons.

The disposal of hazardous chlorinated hydrocarbons of which PCB's are important examples is attended with considerable difficulties Mostly these compounds are not biodegradable and they persist in the environment. They can enter the food chain of aquatic organists and creatures feeding on them with serious consequences. It is, therefore, necessary to destroy these compounds as part of the disposal process.

At present, such compounds are usually subjected to incineration, but complete destruction requires an adequate exposure to a high temperature which is not readily achieved. Partial destruction can result in the formation of noxious breakdown products such as dioxines which are even more hazardous than the original chlorinated hydrocarbons. The problems are compounded in the case of solids contaminated with PCB's and the like as these do not readily lend themselves to incineration.

This invention provides a safe method for the destruction of these chlorinated hydrocarbons in which the chlorine component of the compounds is removed as a harmless salt by reaction with molten lead leaving residual non-toxic material The process may be applied to liquid chlorinated hydrocarbons; to contaminated solids, in which case the chlorinated hydrocarbon is first extracted with a suitable hydrocarbon solvent; or even to solid chlorinated hydrocarbons in the form of fine powders.

The decomposition of chlorinated hydrocarbons by molten metal is not novel, since such compounds have been used for many years for the removal of hydrogen and magnesium from molten aluminium. The most common compound for this purpose is hexachlorethane which is decomposed by the molten aluminium to produce chlorine to react with the hydrogen or magnesium. In this case, the chlorinated hydrocarbon is used as a convenient way of introducing chlorine into the melt, but there is no reason why such a reaction should not be used for the deliberate destruction of chlorinated hydrocarbons. However, aluminium has some practical drawbacs.The volatility of the chloride, which sublimes at 0 172 C and readily hydrolyses, introduces difficulties in its separation from the waste gases.Further, molten aluminium is reactive to metals resulting in complications in the construction of the plant. Sodium or magnesium are among other possible metals for effecting the destruction of chlorinated hydrocarbons, but their chlorides have high melting points and they are not attractive on economic grounds.

Lead has the advantage of a low melting point. The molten metal is inert towards iron and steel and so can be easily handled. It may, for example, readily be converted into molten droplets as in the production of lead shot or atoinised as in the production of powder, so increasing the surface area available for reaction with the chlorinated hydrocarbon. The reaction product with the chlorinated hydrocarbon, lead dichloride, separates as a molten slag at the relatively low temperature of 500 C and lead may be recovered from this for recycling by conventional smelting processes or by electrolytic refining.

In describing the manner in which the invention may be performed, it will be assumed that the invention is being applied to the destruction of a liquid polychlorinated biphenyl (PCB) or of solids contaminated with PGB, although the invention can equally be applied to other chlorinated hydrocarbons. Three examples of carrying out the invention will be described.

EXAMPLE 1 In its simplest form, the invention consists of vaporising the liquid PCB ( or an extract of the PCB in a suitable solvent such as kerosene), preferably in an inert atmosphere such as nitrogen, and injecting the vapour near to the bottom of a closed reactor vessel containing a deep bath of molten lead held at a temperature above the melting point of lead dichloride (about 500G). The molten lead strips the PCB of its chloride content as the bubbles of vapour rise to the surface producing lead dichloride which separates as a molten slag on the surface of the lead, the lead being gradually consumed. Residual vapours are piped to a condenser and the recovered condensate if it comprises the PCB solvent may be returned to the process.

In the preferred method of carrying out Example 1, the vaporised PCB carried in a stream of inert gas is introduced into the reaction vessel via a rotating lance attached to the bottom of which is an impeller. The function of the impeller is to disrupt the emerging bubbles of vapour to produce a shower of small bubbles, thus increasing the surface area of contact with the molten lead to facilitate the reaction between the molten lead and the FOB. In fig 1, the method is illustrated with a single rotating lance, but several such lances could be incorporated within a single reaction vessel.

In this figure, a pressure vessel 1 contains the FOB. It is capable of being heated to vaporise the PCB and is supplied with a stream of nitrogen via the pipe 2 to flush air from the system. It is instrumented to monitor temperature and pressure. The PCB vapour enters a rotating lance 6 through a rotary gas valve 3. The lance is rotated by a belt drive 4 from a motor and passes into the reactor 5 through a gas-tight bearing 7. The reactor is supplied with molten lead at a temperature in e excess of 500it through the side well 8. The PCB vapour emerges from the bottom of the lance and is dispersed as numerous small bubbles by the impeller 9. As they rise to the surface, the bubbles react with lead and the lead chloride slag resulting floats on the surface of the lead.

Waste gases such as the decomposition products of the PCB or of solvents used to extract FOB from contaminated solids pass to the condenser 10.

Slag passes over the weir 11 into a collecting vessel 12. The molten slag may be refined electrolytically to recycle the lead and recover the chlorine or the lead may be recovered by conventional smelting processes.

EWEPLE 2 This involves effecting the reaction between vaporised PCB (or an extract in a suitable solvent) and droplets of molten lead. The objective is to increase the surface area of contact between the PGB vapour and the lead and so increase the rate at which the destruction of PCB can take place.

Refering to figure 2, the PCB is contained in the storage tank 1 from which it flows into an evaporator 2. The evaporator and down-stream parts of the plant may be flushed with nitrogen or other inert gas through the inlet 3. PCB vapour from the evaporator enters the reactor tower 5 from the inlet pipe 4 through multiple ports arranged round the circumference of the reactor. Lead is melted and super-heated to above 5000C in the melting unit 6 and enters the top of the reactor tower where it passes through a filter bed 7 which splits the molten lead into numerous streams of droplets. Reaction then takes place between the rising PCB vapours and the falling droplets of lead. Lead chloride slag and unreacted lead collect in the sump 8 in the base of the reactor tower and flow into the slag separator 10 under the weir 9. Lead from the bottom of the slag separator is returned via pump 11 to the lead melting unit while the slag is skived off for recovery of the lead and chlorine by electrolysis or reserved for smelting. Vaste gases from the reactor tower are led to the condenser 12. The condensate may be returned to the circuit or re-used for the extraction of PCB's from contaminated solids.

EXAMPLE 3 In an elaboration of Example 2, a more intimate mixing of the PCB and molten lead is obtained by using the PCB vapour or liquid PGB in the form of a 'mist' carried in a stream oi nitrogen (or other inert gas) to 'atomise' the molten lead in a manner analogous to the production of lead powder. Vith lead this 'atolising' may be achieved at the comparatively low pressure of 3 or 4 bars.The method can also be used for the destruction of solid chlorinated hydrocarbons carried in a finely'divided form in a stream of nitrogen which is used to 'atomise' the lead.

The reactor and lead/slag handling system is similar to that in Example 2, but the filter bed is dispensed with and instead molten lead and PCB enter the top of the reactor through an 'atomising' or metal spraying head. This portion of the plant is shown schematically in Figure 3. The storage tank 1 for the PCB is equipped with a heater to raise the temperature of the PCB so as to reduce its viscosity to make it suitable for spraying. The PCB is pumped 2 to the atomising head 7 at the top of the reactor. One design for this atomising head is shown enlarged.

In this, the PCB under pressure is converted into a 'mist' at the nozzle 3. Nitrogen is admitted through the feed 4 with the dual purpose of initially purging the system of air and subsequently diluting the PCB mist if necessary to control the rate of reaction. The PCB mist is vaporised in chamber 5 which is surrounded by a jacket of molten lead fed from the melting unit via pipe 6. The PCB vapour and the molten lead meet in the atomising head 7 where the lead is blown into small droplets providing a large surface area for the reaction with the PCB vapour during its decent to the sump at the base of the reactor tower.

Claims

1. A method of destroying a chlorinated hydrocarbon comprising bringing the chlorinated hydrocarbon into contact with molten lead whereby the molten lead and the chlorinated hydrocarbon react to form lead chloride.

2. A method according to claim 1, wherein the molten lead is maintained at a temperature above the melting point of lead chloride.

3. A method according to either claim 1 or claim 2, wherein the chlorinated hydrocarbon is contacted with the molten lead in the absence of oxygen.

4. A method according to any one of claims 1 to 3, wherein the chlorinated hydrocarbon is introduced into a body of molten lead.

5. A method according to claim 4, wherein the chlorinated hydrocarbon in the form of a vapour is bubbled into the body of molten lead.

6. A method according to either claim 4 or claim 5, wherein the chlorinated 'hydrocarbon is carried in a stream of inert gas.

7. A method according to either claim 5 or claim 6, wherein the chlorinated hydrocarbon is introduced into the body of molten lead via a rotating lance having an impeller attached to the bottom thereof whereby the impeller, as the lance rotates, disperses bubbles containing chlorinated hydrocarbon in the melt.

8. A method according to any one of claims 1 to 3, wherein the chlorinated hydrocarbon is brought into contact with molten lead that has been atomised into a spray of very fine droplets.

9. A method according to claim 8, wherein the chlorinated hydrocarbon is in the' form of a vapour optionally diluted in a stream of inert gas.

10. A method according to claim 8, wherein the chlorinated hydrocarbon is liquid and is provided in the form of a mist of fine liquid droplets: carried in a stream of inert gas.

11. A method according to claim 8, wherein the chlorinated hydrocarbon is solid and is provided in a finely divided form carried in a stream of inert gas.

12. A method according to any one of claims 8 to 11, wherein a stream of chlorinated hydrocarbon containing inert gas is introduced into a atomizer head together with the molten lead.

13. A method according to any one of claims 1 to 3, wherein chlorinated hydrocarbon - containing vapour is passed into and through a stream of falling droplets of molten lead.

14. A method according to claim 13, wherein the chlorinated hydrocarbon vapour is carried in a stream of inert gas.

15. A method according to any one of claims 1 to 14, wherein the chlorinated hydrocarbon comprises polychlorinated biphenyls.

16. A method according to any one of claims 1 to 14 including the further steps of collecting the lead chloride formed by the reaction between the molten lead and the chlorinated hydrocarbon and recovering lead and/or chlorine values from the lead chloride.