WO2012017200A1

WO2012017200A1 - Hci production method

Info

Publication number: WO2012017200A1
Application number: PCT/GB2011/001160
Authority: WO
Inventors: David Deegan; Fan Zhang
Original assignee: Tetronics Limited
Priority date: 2010-08-02
Filing date: 2011-08-02
Publication date: 2012-02-09
Also published as: MY157559A; CA2807280A1; BR112013002473A2; CN103153846B; AU2011287422A1; AU2011287422B2; EP2601133A1; CN103153846A; GB2482485A; GB201012985D0

Abstract

The present invention provides a process for the production of aqueous HCI from a material that contains chlorine atoms, the process comprising: (i) plasma treating the material in a plasma treatment unit to produce an off-gas containing at least some of the chlorine atoms; and (ii) passing at least some of the off-gas to a condensing unit to recover HCI in an aqueous form.

Description

HCI Production Method

The invention relates to a process for the production of aqueous HCI from a material that contains chlorine atoms. In particular the invention relates to a method and apparatus for the remediation of Air Pollution Control (APC) residue to obtain a product.

Air Pollution Control (APC) residues are a mixture of fly ash, organic pollutants (including dioxins and furans), carbon and alkaline salts in powder form. APC residues are classified as hazardous waste and are captured by the off-gas system and environmental pollution abatement systems of thermal plants. For example, they are generated from treatment processes associated with the operation of Municipal Solid Waste (MSW) incinerators, biomass combustion power production plants and other thermal and/or pyrometallurgical processes. Current practice for handling these APC residues involves transporting them significant distances to high-cost, hazardous waste landfills, and other land- based disposal sites including salt mine disposal, that have finite disposal capacity. Here they are disposed of after suitable pre-treatment and compliance acceptance testing. Typically, the APC residues are neutralised with acidic waste, or solidified with cementitious materials, before disposal. Due to rising levels of cost/taxation, tightening regulatory pressures and limited capacity this practice is increasingly undesirable and costly. Alternative sustainable treatment methods are urgently required. The pre-treatment methods which are currently used to treat dispose of APC residues are fairly rudimentary, e.g. mixing and washing. These methods represent simple packaging, dilution and dispersion and only serve to displace the problem presented by the waste. Alternative disposal methods include use of 'specialist' cement stabilisation products followed by storage underground. However, all these non-recovery based solutions ultimately rely on disposal/storage (at landfill or foul sewer) and have poor overall environmental performance that is also subject to escalating taxation and regulation. Waste treatment using plasma technology is known (see, for example, US

4,509,434 A). EP 1896774 discloses a waste treatment method comprising a gasification step followed by a plasma treatment step. This results in

improvement of the energy efficiency of the process as well as the production of a cleaner syngas. However, this process itself produces APC residues when used to treat raw municipal waste. Known plasma treatment systems for wastes focus on the abatement of undesirable off-gases and have not appreciated the potentially recoverable side products of these processes.

Accordingly, there is a need for improved and more robust methods for

sustainable hazardous waste management. These methods should exhibit reduced management costs and improved environmental performance. In particular, there is a need for a method of waste treatment in which a high proportion of the waste is transformed into a commercially useful product(s) as opposed to alternative compliant effluent streams for disposal.

The object of the present invention is to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto. In a first aspect, the present invention provides a process for the production of aqueous HCI from a material that contains chlorine atoms, the process

comprising:

(i) plasma treating the material in a plasma treatment unit to produce an off-gas containing at least some of the chlorine atoms; and

(ii) passing at least some of the off-gas to a condensing unit to recover HCI in an aqueous form. The present inventors have discovered that under the high temperatures produced by a plasma device (such as a plasma torch) in a plasma treatment unit it is possible to remediate a material containing chlorine atoms so as to produce a useful product; namely, aqueous HCI or hydrochloric acid. This useful by-product was surprising since the inventors expected the thermodynamic conditions at the typical operating temperature of a plasma treatment unit to lead to calcium chloride production. It is speculated that the high temperatures volatilise the chlorine before it can contact the calcium in the flux or any calcium in the waste and become trapped in a vitrified material.

The method of the present invention is particularly suitable for treatment of a waste material that contains chlorine atoms, more particularly to a hazardous waste that contains chlorine atoms. It is noted that the chlorine atoms are most likely part of larger molecules such as inorganic compounds. The method allows for the remediation of this waste and, in comparison to conventional treatment methods, produces a useful product form an otherwise less useful waste stream. The recovery of aqueous HCI adds value which may, at least, offset the cost of the waste treatment, and also eliminates a large component of the effluent stream.

It has been found that the use of the high temperature plasma treatment provides a large number of advantages. Firstly, any remaining organic or volatile components in the material are vaporised and removed, reducing the amount of waste product produced. Furthermore, the waste product takes the form of a solid vitrified material which may be used in, or as, secondary products (such as building materials) and, in any event, has a reduced size. When dealing with toxic or hazardous materials this solid form allows for easier handling and disposal. The method is especially suitable for the treatment of Air Pollution Control residues (APC residues). UK APC residues typically consist of 15 - 25 wt% chlorine. APC residues typically consist of 4 - 25 wt% and more preferably from 5 to 20 wt% chlorine. This means that if chlorine is recovered as a product, a large proportion of the waste mass becomes a product and, thus, the secondary wastes and environmental impacts are minimised. As a consequence of the method of the present invention, secondary APCs (those produced by treatment of the original APCs) are minimised.

Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The term "off-gas" as used herein refers to the gaseous product that leaves the plasma treatment unit when carrying out plasma treatment of a material.

The term "slag" refers to the vitreous residue produced in the plasma furnace of the plasma treatment unit. It is formed as a result of the plasma treatment of the chlorine-containing material. The term "molten slag" used herein refers to a slag that is solid at room temperature but molten at the operating temperature of the plasma treatment unit.

The term "plasma treating" used herein refers to a method of applying plasma to a material. A plasma is an electrically neutral, highly ionised gas composed of ions, electrons and neutral particles and is distinct from other forms of matter. The term "plasma treatment unit" refers to any unit in which plasma is applied to a material, such as a plasma furnace. In a plasma furnace, electricity is passed between two or more electrodes spaced apart creating an electrical arc. The plasma may preferably be produced in a plasma torch which allows for targeted plasma treatment. Gases, typically inert gases, under high pressure are passed through the arc and are turned into plasma. Plasma is a clean, functional heat source with strong environmental characteristics. It is also very efficient in destroying Persistent Organic Pollutants (POPs). The plasma treatment unit is preferably a plasma furnace.

The term "aqueous HCI" as used herein refers to a solution of hydrogen ions and chlorine ions in a solvent comprising water. Preferably the aqueous HCI is a solution of aqueous HCI.

The term "condensing unit" used herein refers to an apparatus capable of extracting HCI from the gas phase into either the solid, liquid or aqueous phases. Condensing apparatus are well known in other fields of technology. The produced HCI is cooled in the condenser, and absorbed into water to form aqueous HCI. The process may comprise a further step of adjusting the pH of the aqueous HCI as required. In the condensing unit, HCI is selectively and preferentially dissolved in water to produce a HCI solution. The HCI solution may have a concentration of 1-38 %w/w (kg HCI/kg), preferably, 3-30 %w/w, even more preferably 10-20 w/w %, and still even more preferably 12-18 w/w %. In one embodiment the solution may be recycled through the condenser to increase the HCI concentration (i.e. typically until it has an HCI concentration of 12 % w/w or more). This is preferred as this is an efficient approach to obtaining a more concentrated product.

Preferably the condensing unit is maintained under acidic conditions so as to preferentially recover HCI. The HCI solution is acidic, i.e. it has a pH less than 7. Preferably, the aqueous HCI product has a pH of less than 2, more preferably less than 1, even more preferably less than 0 and still even more preferably less than -0.5. Advantageously, these highly acidic pH values have been found to limit the solubility of other gases such as sulphur oxides and hydrogen fluoride, which are rejected ensuring a low level of acid cross contamination. The concentrations of other gases dissolved in the aqueous HCI is preferably less than 25 %, even more preferably less than 5 %, still even more preferably less than 1 % and still even more preferably less than 0.5 %. The skilled person will understand that HCI can be dissolved in a solvent comprising water and an organic component, for example an aqueous alcohol such as aqueous methanol. Preferably the process further comprises:

(a) pre-treating the off-gas used in step (ii) in a thermal oxidiser before performing step (ii); and/or

(b) pre-filtering the off-gas used in step (ii) before performing step (ii), preferably maintaining the off-gas at a temperature of at least 180 °C during filtration.

Contacting at least some of the off-gas produced in step (i) with a thermal oxidiser before performing step (ii) results in oxidation of any residual flammable gases and metallic/elemental species contained in the off-gas, e.g. carbon monoxide, lead, zinc, cadmium and hydrogen. This reduces the hazards associated with handling flammable gases and also reduces the number of contaminants contained in the aqueous HCI produced by the method of the present invention. Preferably the off-gas is cooled after being contacted with the thermal oxidiser. Preferably the cooling is carried out with the use of water injection and

evaporative cooling in a quench column. In this case the latent heat of

evaporation of the water may be used to cool the off-gas stream efficiently when compared to other intermixing methods, e.g. cooling air addition. This has the advantage of reducing the resulting off-gas volume flow rate and the associated scale of the off-gas abatement plant. In addition, the same water may be used subsequently in the condenser to form the aqueous HCI solution and therefore advantageously becomes part of the product stream. Preferably the method involves filtering at least some of the off-gas produced in step (i) before performing step (ii), which results in the removal of particulates from the off-gas, which could contaminate the aqueous HCI solution produced. In addition, such particulates may be recovered as secondary APC residues. Such filtering may comprise the dosing of Activated Carbon (AC) as a physical sorbent for the capture of mercury and other volatile metals. Alternatively, or in addition, such filtering comprises on-line cleaning facilities e.g. the use of a reverse pulse jet for the removal of particulates. Advantageously, secondary APC residues, which are usually disposed of, may be recovered from the off-gas as a result of the filtering. Preferably the off-gas is maintained at a temperature above 180 °C during filtration. This avoids blockage of the filter elements and corrosion due to condensation of water vapour and soluble gases contained in the off-gas.

Preferably the process further comprises treating the waste off-gas produced in step (ii) once the HCI has been recovered in a caustic wet scrubber. The term "wet scrubber" used herein refers to a device that removes pollutants from a gas stream. In a wet scrubber, the polluted gas stream is brought into contact with a scrubbing liquid, so as to remove the pollutants from the gas stream by physical and/or chemical means. The term "caustic wet scrubber" used herein refers to a wet scrubber in which the liquid contains an alkaline or caustic scrubbing agent, such as lime milk, sodium bicarbonate or sodium hydroxide. The use of a caustic scrubbing agent makes the scrubber particularly effective for the removal of acidic gases from a gas stream. The wet scrubber removes the residual gaseous contaminants, such as residual acid gases, contained in the off-gas. The removal of such gaseous contaminants means that a reduced amount of environmental pollutants are released when the off-gas is subsequently discharged to the atmosphere through a stack and assessed for regulatory compliance. It should be noted that the caustic wet scrubber is distinct from the condensing unit. Preferably, the off-gas passes to a continuous emission monitoring system (CEMS), after the caustic wet scrubber, and prior to discharge through a stack. This ensures that measurements are taken to confirm compliance with a set of Emission Limiting Values (ELVs) is achieved, such as those defined in the Waste incineration Directive (WID) or included within the plant's associated environmental permit to operate.

In a typical process of the present invention, the steps are typically carried out in the following order: plasma treating the material in a plasma treatment unit to produce an off-gas containing at lest some of the chlorine atoms (step (i)), passing the off-gas to a thermal oxidiser, filtering the off-gas (for example contacting the off-gas with a physical sorbent or physical membrane), passing at least some of the off-gas to a condensing unit to recover HCI in aqueous form (step (ii)), passing the off-gas to a caustic wet scrubber, passing the off-gas to a CEMS, discharging the off-gas to the atmosphere. However, the skilled person will understand that these steps can be carried out in any other suitable order. Alternatively, one or more of the steps may be carried out concurrently.

Preferably the plasma treatment is carried out in the presence of a plasma stabilising gas. Preferably the plasma stabilising gas is selected from one or more of nitrogen, argon, helium and steam.

Typically the step of plasma treatment further produces a molten slag. Any nonvolatile hazardous inorganic materials contained in the chlorine-containing material, such as heavy metals or compounds thereof, are typically incorporated into the molten slag, producing an inert vitreous or semi-crystalline product. The process can be tweaked so that the inert vitreous or semi-crystalline product conforms to local product qualifications.

Preferably the material is maintained at a temperature of 1400 - 1600 °C during the plasma treatment step. This ensures that any molten slag produced during the plasma treatment remains in the liquid state. Accordingly, any such molten slag can be more easily removed from the plasma treatment unit if desired. The plasma treatment process can be carried out between 1000 and 3200°C.

However, the preferred lower limit for efficient HCI recovery is at least 1200°C. The use of higher temperatures requires increased energy for the treatment process. Accordingly, a preferred balance of yield and energy requirements is from 1200°C to 2000°C, more preferably 1400 - 1800 °C and most preferably from 1400 - 1600 °C.

Preferably the molten slag is continuously removed from the plasma treatment unit. Typically, the molten slag is continuously removed at a dedicated channel. This encourages positive plug flow movement of molten slag without the associated build up of process gases. A build up of process gases within the plasma treatment unit could be hazardous. Preferably, the removed molten slag is cooled to form a solid vitrified material. This results in inorganic materials contained in the slag being trapped within the glass matrix. Typically, the solid vitrified material exhibits a composition and leachability of metals below the inert waste landfill WAC (Waste Acceptance Criteria) leaching limits. This means that the solid vitrified material can be disposed of in landfill or more preferably qualified as a product in line with the requirements of the Waste Framework Directive.

Preferably the process further comprises adding one or more fluxing agents, if required, to the chlorine-containing material either before or during the plasma treatment. Typical APC residues may be self-fluxing. Lime-based APCs, for example, contain CaOH.xH₂0 which on heating provides a source of Calcium oxide and water. However, flux ensures that a low melting point, low viscosity molten stable slag is produced from any inorganic, non-combustible materials that are present in the chlorine-containing material. In addition, use of a fluxing agent results in environmental immobilisation of any heavy metals, such as lead, zinc and cadmium, or any compounds thereof, contained in the chlorine- containing material. Typical fluxing agents are comprised of one or more of lime, alumina and silica. One or more network stabilising agents may also be used alone or in combination with the fluxing agents. Network stabilising agents are known in the art.

Preferably the one or more fluxing agents comprise incinerator bottom ash (IBA) or alternative waste. Incinerator bottom ash (IBA) is a form of ash produced in incineration facilities, and is currently classified as non-hazardous waste in the UK. This material is discharged from the grate of municipal solid waste

incinerators. Following combustion the ash typically has a small amount of ferrous metals contained within it. IBA is typically comprised of a mixture of two or more compounds such as Si0₂, CaO, AI2O3, Fe203, MgO, K₂0, P₂0₅ and S. The use of IBA to replace virgin flux materials such as S1O2 and AI2O3 reduces the cost of flux materials accordingly and avoids the need to dispose of IBA in landfill.

Preferably, the IBA is pre-treated before being used as a flux material. Such pre- treating may include the removal of oversized (e.g. with a diameter greater than or equal to 10 mm) material and/or drying. This results in the plasma treatment unit being more stable and efficient. The use of 'dried' IBA avoids rapid pressure increase within the plasma treatment unit due to the production of large amounts of steam. The removal of oversized material means that the heat transferred from the plasma arc is better able to contact the chlorine-containing material under the intended steady state conditions. Both of these effects help to stabilise the voltage of the plasma, which is directly related to the plasma power.

Where a flux is included, it can be mixed with the material containing chlorine atoms either before or during plasma treatment. The flux, preferably IBA, can form from 0 to 50wt% of the treated material, preferably from 15 to 35wt% and most preferably about 25wt%. This allows for a molten slag product with predictable characteristics while minimising the amount of extra heating required. The present inventors have discovered that the moisture content of the material containing chlorine atoms treated can affect the recovery of the HCI. With increasing water content the recovery rate increases. For example, a five-fold increase in the moisture content can lead to a tripling of the recovery rate of HCI(g). However, the increased presence of moisture also increases the energy consumption of the process. Accordingly, the moisture content of the material containing chlorine atoms before treatment is preferably from 0.5 to 15wt%, more preferably from 1 to 5wt% and most typically from 2 to 3wt%.

Preferably, the process further comprises a step of producing gaseous HCI from the aqueous HCI. This may be performed by techniques well known in the art. This may be stored as a gas for sale and dispatch.

Preferably the material containing chlorine atoms is a waste material, preferably comprising inorganic material. Typically, the waste material may also comprise organic material. The material is preferably a hazardous waste and more preferably APC residue. Preferably the material has a chlorine content of 5 - 40 wt%, more preferably 10 - 35 wt%, even more preferably 15 - 30 wt%, and still even more preferably 20 - 25 wt%. Preferably the chlorine-containing material is an Air Pollution Control (APC) residue, sometimes referred to as "fly ash".

Typically, the APC residue is produced from high temperature incineration or other thermal waste management, manufacturing or power production processes. It is not necessary for APC residues to undergo any pre-treatment, such as washing, before being used in the method of the present invention.

While a range of CI content is preferably as indicated above, the present inventors have discovered that a minimum chlorine content required to viably recover HCI from the treated material is about 2.5wt%. Obviously, this threshold is dependent on waste chemistry. Below this limit the majority of the CI is lost in the off-gas system as KCI(g) and NaCI(g), where species may be formed. As the waste's chlorine content increases, the amount of CI reporting to the vitrified product increase and plateaus at a low and reproducible level. This is surprisingly compared to thermodynamic predictions. Accordingly, the upper limitation for the technical recovery rate for HCI from the off-gas of the process is determined by the chemistry of the system, phase partitioning and availability of both

components, namely hydrogen and chlorine within the system. The technical recovery rates of HCI have been observed to be higher than thermodynamically predicted and this is considered to be due to the combined plasma effects.

According to a second aspect of the present invention there is provided an HCI recovery apparatus for performing the process of the present invention, the apparatus comprising:

a plasma treatment unit for treating a material containing chlorine atoms to produce an off-gas containing at least some of the chlorine atoms; and

a condensing unit configured to receive the off-gas and to recover aqueous HCI therefrom. A typical plasma treatment unit for use in the present invention comprises a furnace and a graphite electrode system comprising one or more graphite electrodes to generate in use a plasma arc inside the furnace. During operation a chlorine-containing material is inserted into the furnace, typically through an inlet port. A plasma arc then transfers from the tip of the graphite electrode to the chlorine-containing material. Typically the return electrical path is via an electrically conductive path built into a furnace sidewall or hearth. Preferably this will be via conductive refractories and/or interconnecting metal-encased bricks. Periodically this will need to be replenished to ensure good hearth electrical contact as it can become depleted through slow consumption process like reaction with chlorine.

In a typical use of the apparatus, the chlorine-containing material is fed into the furnace of the plasma treatment unit at a controlled rate and the plasma power is modulated to maintain the melt at a suitable liquid temperature, typically in the range 1400 - 1600 °C. Power is modulated in accordance with the feed rate of the material, of known bulk chemistry, undergoing treatment. It is preferred that the chlorine-containing material and any additional materials are fed into the plasma treatment unit under gravity. Preferably the feed material falls under gravity past the plasma device, whereby the plasma heat warms and volatilises the material, allowing kinetic reaction of the chlorine content before the mixture enters the bulk melt pool. In this way, the yield of HCI is surprisingly higher than predicted under the thermodynamic conditions of the melt pool.

Preferably the apparatus further comprises one or more of:

a thermal oxidiser;

a filter; and

a caustic wet scrubber.

The filter may comprise one or more of an activated carbon dosing system and a particulate filter.

The apparatus of the present invention is typically arranged so that the off-gas produced in the plasma treatment unit enters each component in the following order: (i) the thermal oxidiser, (ii) the filter, (iii) the condensing unit and (iv) the caustic wet scrubber. However, the skilled person will understand that the apparatus can be arranged in any other suitable arrangement.

Preferably the plasma treatment unit comprises an overflow spout for the removal of molten slag from the plasma treatment unit. Preferably the overflow spout comprises heating means, preferably a plasma torch. This avoids possible solidification of the molten slag, which would hinder its removal from the plasma treatment unit. Preferably the plasma torch is distinct from the heating component of the plasma treatment unit. Preferably the condensing unit comprises a graphite-lined heat exchanger. The use of a graphite lining avoids corrosion to the condensing unit by the aqueous HCI produced. Alternative linings such as chloro/fluoropolymer and/or enamels could be employed. Suitable lining systems exhibit chemical

resistance/compatibility with HCI and thermal conductivity. In a third aspect, the present invention provides a waste treatment plant for treating chlorine-containing material, the plant comprising:

the apparatus of the second aspect; and

an incinerator capable of producing incinerator bottom ash (IBA) and/or Air Pollution Control residues.

Preferably, the incinerator produces both the IBA and APC residues and this leads to quick efficient remediation of the dangerous materials on site without delay. In addition, co-location of such plants provides benefits in the efficiency of the associated infrastructure.

The incinerator may be a plasma treatment unit.

The present invention is described by way of example in relation to the following figures.

Figure 1 is a schematic of an example of an apparatus according to the second aspect of the present invention.

Figure 2 is a perspective view of an example of a waste treatment plant containing an example of an apparatus according to the second aspect of the present invention.

Figure 3 shows graphs of the plasma characteristics of the plasma treatment unit used during Example 1 (top) and Example 2 (bottom). In each diagram the dashed rectangle indicates the feeding period. In the top graph, during the total 3.5 hour feeding period, average current = 756 A, average volts = 194 V, average power = 145 kW (vs PFD 127.4 kW), total blended feed = 185 kg and feed rate = 52.6 kg/hr (vs PFD 50.0 kg/hr). In the bottom graph, during the total 1.98 hour feeding period, average current = 727.2 A, average volts = 235.4 V, average power = 164.1 kW (vs PFD 127.4 kW), total blended feed = 114.0 kg and feed rate = 57.7 kg/hr (vs PFD 50.0 kg/hr).

Figure 1 shows a flowchart of the components and method steps used in the apparatus 100 and method of the present invention. The apparatus 100 comprises a plasma treatment unit 101.

Flux materials 102 can be supplied to the hopper 103 where they pass to the blending system 105 for blending with APC residues 104. The blended APC residues and flux materials are then supplied to the plasma furnace 107 via the feeder 106. Plasma is then supplied to the plasma furnace 107 from the plasma source 108 (not shown).

The plasma source 108 comprises a cooling system 109, a pump 111 and a furnace manifold 112. The pump 111 pumps cooling water 110. The furnace manifold 112 is, in use, supplied with plasma gas 113. In use, the APC residues undergo plasma treatment in the plasma furnace 107 to produce an off-gas and a molten slag.

The molten slag is passed to a molten slag handling system 114 to be stored in a cold slag reservoir 115.

The off-gas then passes to the thermal oxidiser 116, where air 117 is supplied to oxidise any flammable gases contained in the off-gas. The off-gas then passes to the gas cooling system 118, where water 119 is injected so as to cool the off-gas by evaporative cooling or equivalent means. The off-gas then passes to the filter 20, where activated carbon 21 is used in order to capture any mercury, and other volatile species, contained in the off-gas. Secondary APC residue 122 is recovered from the filter 120.

The off-gas then passes to the condensing unit 123 in order to recover aqueous HC1 124. The off-gas then passes to the caustic wet scrubber 125, where a solution of alkaline scrubbing reagent, such as sodium bicarbonate, 126 is added in order to fix other acidic gases from the off-gas and to produce soluble salts such as Na₂S03 which are dissolved in solution 127. The off-gas then passes through an ID fan 128, which controls the pressure in the plasma furnace 107, and is then monitored by an emission monitoring system 129 to ensure

compliance with a set of emission limits. The off-gas is then passed to the stack 130 for release into the atmosphere. The outline mass and energy balance is indicated at each stage. Figure 2 shows a waste treatment plant containing a device according to the second aspect of the present invention. The plant comprises air blast cooler 201 , a control room 202, a plasma power supply 203, a solid vitrified material reservoir 204, a plasma furnace 205, a flux material storage reservoir 206, APC residue storage reservoirs 207, a secondary fly ash storage reservoir 208, an off-gas system 209 and a stack 210. Such a waste treatment plant is designed to fit inside a standard industrial unit.

Examples The present invention will now be described by reference to the following non- limiting examples. In particular, it is noted that the recovery yields exhibited are lower than expected due to limitations of scale inherent in the pilot plant. On a larger scale affording longer residence times in the condensing unit, the yields are expected to be greater.

• Example 1 The process of the first aspect of the present invention was carried out using IBA as a flux material. The IBA was obtained from a municipal solid waste (MSW) incinerator and had the composition according to Table 1 :

Table 2. Composition of feed

The APC residues used comprised -16.33 wt% elemental chlorine. After being blended with the flux materials (IBA plus convention flux materials) the chlorine was diluted to 1 .43 wt%. The total mass of input material was 204 kg, including 187.0 kg of blended feed (containing 130.9 kg APC residue, 37.4 kg silica flux, 18.7 kg IBA), 0.0 kg of pig iron and 17.0 kg of remaining slag/metals. The overall input mass of chlorine was 21.59 kg. Before being combined, the IBA was graded to <10 mm and then naturally (under ambient conditions) dried for more than three days. During the drying process, the colour of the IBA changed from dark grey to light grey.

The feed was then fed into a plasma treatment furnace at a rate of 52.6 kg/h and a plasma was applied to produce an off-gas and a molten slag. The average plasma power during feeding was 145 kW and the furnace pressure was maintained at 50 + 20 Pa. Throughout the trial, the voltage of the plasma was quite stable (200 ± 50 volts). The off-gas was then passed to a thermal oxidiser where flammable gases were oxidised, and then passed to a baghouse filter where secondary APC (SAPC) residues were collected. Around 4.0 kg of SAPC residues was collected from the thermal oxidiser and the baghouse filter. This does not include SAPC residue contained on the inner wall of the off-gas ducts. The off-gas was then passed to a graphite-lined heat exchanger, where aqueous HCI was recovered. The off-gas was then passed to a caustic wet scrubber to remove other acidic gases, before being monitored by a continuous emission monitoring system and then discharged to the atmosphere.

Two bottles of recovered HCI solution were collected during the operating period from the open end of the graphite lined, water-cooled, heat exchanger. The HCI concentrations were calculated using a titration method and the results are shown in table 3 below. The low concentration HCI (0.38 %wt) was collected when feeding occasionally during the warm up period, whereas the medium concentration HCI (8.63 %wt) was collected during the feeding period. A large portion of recovered HCI solution was condensed before it reached the graphite lined heat exchanger and therefore was not collected. This was due to the presence of an upstream multistage indirect water heat exchanger (the main heat exchanger). A large quantity of HCI solution was formed in the main heat exchanger and the baghouse filter. The level of HCI evolution had to be

estimated (see Table 3). It is to be noted that the graphite heat exchanger is physically small and designed to demonstrate a principle, i.e. the recovery of a concentrated HCI solution, as opposed to recover HCI solution at high technical material efficiencies. Table 3 lists the input and output mass data of the process. The mass of off-gas was calculated based on the assumption that no metal was tapped out (i.e. the collected molten slag was "pure" without any iron).

Input (kg) Weight All CI wt% CI

(kg) elements element

(kg) (kg)

204.0 21.59

Blended materials, where: 187.0 1 1.43 21.37

APC residues 130.9

Silica (Si0₂) 37.4

IBA 18.7

Slag remaining in furnace 9.0 2.39 0.22 from previous use

Metal from previous use 8.0 0.00 0.00

Pig irons pre-loaded 0.0 0.00 0.00

Water steam 0.0 0.00 0.00

Oxygen 0.0 0.00 0.00

All CI wt% CI element

Output (kg) elements (kg)

(kg)

206.0 1 1.96

Slag collected (included 167.0 2.26 3.77 fritted slag)

Slag remaining in the 4.0 2.26 0.09 furnace

Metal remaining in the 5.0 0.00 0.00 furnace

SAPC (Secondary APC 4.0 38.93 1.56 residues)

Off-gas from furnace 26 25.14 6.537 excluding argon

HCI in the off-gas was

distributed into:

Table 3. Mass balance calcu ation resu ts (results n ta cs are est mates

• Example 2

The process of the first aspect of the present invention was carried out in the same manner as in Example 1 but with the following differences:

• IBA that had not been pre-treated (i.e. containing 5-10 wt% moisture) was used as a flux material.

• The feeding rate was slightly higher (57.7 kg/h).

• The average plasma power during feeding was slightly higher (164 kW).

• The thermal oxidiser was not fully running and, therefore, temperatures in the off-gas ducts (downstream of the thermal oxidiser) were lower.

In comparison with Example 1 , the following differences were observed:

• The plasma voltage was less stable (240 + 120 volts)

• The HCI solution obtained was less clear • The HCI concentration of the HCI solution obtained was 48% lower.

These results indicate that the use of pre-treated IBA improves the stability of the plasma voltage. This is illustrated in Figure 3. This stability presumably occurs because the use of dried IBA avoids a rapid pressure increase due to the production of steam. In addition, because oversized particles had been removed from the IBA, the plasma arc was able to contact the APC residue resulting in a more immediate heat affliction. Based on the foregoing examples and considering a typical APC formulation, the inventors calculated the thermodynamically predicted effects of chlorine content (table 4), temperature (table 5), and moisture content (table 6) on the HCI production rate. However, further experimentation concluded that significantly more HCI was recoverable than had been predicted.

Table 4 - Effect on Chlorine content

The inventors have discovered that when the CI in APCR is less than 2.58% (wt/wt), an undesirable amount of CI will be dispersed into off-gas as KCI(g) and NaCI(g). In general terms, these results show that on heating the volatile metal chlorides form first.

The inventors have also found that once the CI in APCR is more than 2.58% (wt/wt), the ratio of CI dispersed into off-gas as HCI(g) increases with the increase of CI contents in APCR, until CI content reaches around 11.09% (wt/wt). Above a theoretical value of about 11.09% (wt/wt), more CI will be vitrified into slag predicted to form as CaC^, rather than dispersed into off-gas, although the absolute value of HCI(g) still increases. This is undesirable as one aim of the invention is to minimise the amount of Chlorine retained in the solid process waste. In practice, the upper limit is higher since the CI does not partition as

CaCl2 to the extent expected.

Table 5 - Effect of temperature change

The inventors have found that one of the main reasons that the use of plasma technology is particularly suited to the recovery of HCI(g) from APCR is the high temperature generated. For example, for a typical APCR (e.g. CI = 18.9% wt/wt, H20 = 2.3% wt/wt), if APCR is fed at 4029 kg/h, flux Si02 at 1136 kg/h, then HCI(g) produced at 1100 °C, 1600 °C and 2100 °C will be 6.0, 46.5 and 86.8 kg/h respectively. However, to offset this, the energy costs of achieving these higher temperature is a further consideration. Table 6 - Effect of water content on chlorine partitioning at 1600 °C

The inventors have found that the H₂0 content in raw APCR can have an effect on HCI(g) production. Higher H2O content will leads higher production rate of HCI(g), and more electricity will be consumed accordingly.

As noted, the actual recovery rates in practice proved to be even higher than those calculated in tables 1 , 2 and 3. That is, the chlorine did not partition as CaCI₂ as thermodynamically predicted, i.e. more HCI was formed that predicted in the gas phase. Without wishing to be bound by theory, it is speculated that the increased recovery rate of HCI can be attributed to the use of a plasma furnace reactor. Specifically:

1. The high plasma arc temperatures break down CaCI₂ in the feed;

2. Rapid reaction of hydrogen and chlorine when activated in the plasma discharge;

3. Close proximity of the feed to the plasma arc allowing for super-heating;

4. Rapid gas phase removal of the formed gaseous products (this inhibits CaCI₂ reformation through mass transport restrictions) as part of the continuous process.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents. All percentages are by weight unless indicated otherwise.

Claims

Claims:

1. A process for the production of aqueous HCI from a material that contains chlorine atoms, the process comprising:

(ii) passing at least some of the off-gas to a condensing unit to recover HCI in an aqueous form.

2. The process of claim 1 , wherein the condensing unit is maintained under acidic conditions so as to preferentially recover HCI.

3. The process of claim 1 or claim 2, wherein the process further comprises:

4. The process of any of the preceding claims, wherein the process further comprises treating the waste off-gas produced in step (ii) once the HCI has been recovered in a caustic wet scrubber.

5. The process of any of the preceding claims, wherein the material is maintained at a temperature of 1400 - 1600 °C during the plasma treatment step.

6. The process of any of the preceding claims, wherein the step of plasma treatment further produces a molten slag which is continuously removed from the plasma treatment unit.

7. The process of any of the preceding claims, wherein the process further comprises adding one or more fluxing agents to the material containing chlorine atoms either before or during the plasma treatment.

8. The process of claim 7, wherein the one or more fluxing agents comprises incinerator bottom ash (IBA).

9. The process of any of the preceding claims, wherein the process further comprises a step of (iii) treating the aqueous HCI to produce gaseous HCI.

10. The process of any of the preceding claims, wherein the material containing chlorine atoms is an inorganic waste material, preferably an air pollution control (APC) residue.

11. The process of any preceding claim, wherein the material containing chlorine atoms has a chlorine content of from 5 to 40 wt%.

12. An HCI recovery apparatus for performing the process of any of claims 1 to 11 , the apparatus comprising:

a condensing unit configured to receive the off-gas and to recover aqueous HCI therefrom.

13. The apparatus of claim 12, further comprising one or more of:

a thermal oxidiser;

a filter; and

a caustic wet scrubber.

14. The apparatus of claim 12 or claim 13, wherein the plasma treatment unit comprises an overflow spout for the removal of molten slag from the plasma treatment unit, the overflow spout optionally comprising a heating means to ensure the slag remains molten when leaving the plasma treatment unit.

15. The apparatus of any of claims 12 to 14, wherein the condensing unit comprises a graphite-lined heat exchanger. 6. A waste treatment plant for treating a material comprising chlorine atoms, the plant comprising:

the apparatus of any one of claims 12 to 15; and

an incinerator capable of producing incinerator bottom ash (IBA) and/or Air

Pollution Control residues.