Field of the Invention

The present invention relates to the remediation of spent pot liners. In particular it relates to plasma field remediation of spent pot liners where toxic and hazardous compounds are rendered safe, and remediation yields repurposed materials and value-add product.

Background

A spent pot liner (SPL) is waste generated from the graphite and refractory liner used inside an electrolytic aluminum smelting pot.

A smelting pot frame is first lined with a refractory material such as a high-temperature alumina oxide-based refractory, into which is placed a pre-formed graphite liner. The electrolytic aluminum smelting pots are then turned on and run continuously for several months until the graphite pot liner becomes too thin for safe operation. At this point, the electrolytic smelting pot is drained and turned off for maintenance, the SPL is removed and is re-lined with a new refractory and a new graphite liner.

The SPL is most commonly removed from the smelting pot frame with a hydraulic impact hammer mounted on an excavator.

Each electrolytic smelting pot (also referred to as ‘cell’) produces multiple tons of SPL that must be disposed of. Historically, SPLs were disposed of in landfill waste sites, buried elsewhere, or put into the ocean. However, as SPL contains toxic gases, compounds and heavy metals, SPL was classified as a hazardous material in the late 1980s and its disposal made subject to legislation and related regulations that, among other measures, ban such forms of disposal.

As a consequence, aluminum producers store SPL pending safe and environmentally sustainable means of disposal. Worldwide millions of tons of SPL still require remediation to prevent environmental pollution and toxic exposure to humans and wildlife. SPL contains numerous toxic gases, compounds and heavy metals, in particular fluorides, arsenic metalloid, organic cyanides, and various compounds. Remediation of SPLs using traditional methods (such as wet chemical neutralization and incineration) is thus challenging, as many give rise to adverse reactions with the chemical compounds trapped inside the SPL: for example, soluble fluoride salts and trace toxic cyanides, and generation of inflammable, toxic and explosive gases (such as hydrogen sulfide, arsenic pentafluoride, and methane).

For decades, the aluminum industry and independent researchers have conducted research into how to resolve the problems associated with the disposal, destruction, and/or repurposing of SPLs. Several processes have been trialed but have generally fallen short of a solution due to the production of additional toxic waste streams that must be managed and ultimately addressed. For example, potential solutions have comprised incineration in cement kilns by cofiring the SPL with coal, and wet chemical oxidation with strong oxidizers such as potassium hydroxide. However, Incineration, even at temperatures around 1 ,200 C does not fully destroy the toxic compounds, but rather forms different types of toxic chemical compounds and does not address the hazardous fluoride salt or the produced fluorine off gasses from heating.

A method and system for remediation of spent pot liners is required which resolves these problems.

Summary of the Invention

According to an aspect the present invention comprises a method and system for plasma field remediation of spent pot liners (SPLs). There may be a system for remediation of SPLs comprising a primary plasma arc furnace to receive and decompose the SPLs to produce a raw syngas. There may be a secondary plasma arc furnace to receive and decompose the raw syngas to produce a refined syngas. There may be a controller to monitor and control the remediation.

The controller may configured to maintain a temperature in a range of between 3500° and 12000° centigrade in a primary plasma field into which the SPLs are fed in the primary plasma arc furnace. The controller may be configured to maintain a temperature in a range of between 3500° and 8000° centigrade in a secondary plasma field into which the raw syngas is fed in the secondary plasma arc furnace.

The primary plasma field and or the secondary plasma field may be used in the remediation of the SPLs. Toxic and hazardous compounds in the SPLs may be rendered safe or relatively safe or less toxic or less hazardous. Toxic and hazardous compounds may be rendered into the lowest basic element forms like hydrogen H2, carbon monoxide CO, and carbon dioxide CO2.

Remediation may yield repurposed materials and value-added product. They may be value added products for the aluminum industry. The repurposed materials may be able to be safely used downstream, including for example syngas, electricity, metals and ‘prills’. Remediation may be appropriate to the chemical composition of the SPL components, for example, on both the graphite and refractory components of the SPL.

According to an aspect there is a method for remediation of Spent Pot Liners (SPLs) by operating the system disclosed wherein each SPLs is broken up and separated into one or more stockpiles. There may be a first stockpile comprising mostly graphite. There may be a second stockpile comprising mostly alumina oxide. The method may include blending the first and second stockpile together into a feedstock. The feedstock may comprise approximately fifty percent graphite and fifty percent alumina oxide by weight. The method may include feeding the feedstock into the primary plasma arc furnace.

In the method and system to remediate the SPLs, a crusher may be used, for example a jaw crusher, to crush the SPL into debris. The debris of crushed SPLs may have a particle size of 40 mm or less. The debris may be fed into a hopper. There may be a crusher and feed apparatus to crush SPLs into debris and feed the debris into primary plasma arc furnace under the monitoring and control by controller.

The crushed SPL is stored, for example in a feeding hopper that is vented to atmosphere through a high-efficiency carbon filter to prevent potential off gas generation from escaping into the working environment. The filter will be changed on a regular basis and fed into a primary plasma arc furnace for safe disposal and conversion into raw syngas. The crusher and feed apparatus may comprise the carbon filter in a vent to atmosphere to filter out off-gas and dust from crushed SPLs. The carbon filter may disposable to be fed into the primary plasma arc furnace. Toxic compounds caught in the filter may be remediated and rendered non-hazardous.

Crushing the SPLs into debris may produce powder and dust in addition to the debris of crushed SPLs. The system to remediate the SPLs may comprise a powder collection apparatus to collect the powder. The powder collection apparatus may be fitted to the crusher. There may be conduit from the crusher to the powder collection apparatus. A dust collection apparatus to collect the dust may also be fitted to the crusher. There may be a conduit from the crusher to dust collection apparatus.

The crusher and feed apparatus may comprise the powder and/or dust collection apparatus to collect powder and/or dust produced by crushing the SPLs and to feed the powder and/or dust into the primary plasma arc furnace under the monitoring and control by the controller.

The SPL may be crushed with a low rpm jaw crusher to avoid generation of excess dust. The crushed debris, powder, and/or dust may be supplied to the feeding hopper on a just in time basis to minimize the generation of off-gassing due to size reduction that increases surface area exposed to the atmosphere.

The system for plasma field remediation may comprise a plasma arc furnace which may comprise electrodes inside a furnace body. The electrodes may be configured to generate a plasma field inside the furnace body. The electrodes may comprise mainly graphite. The electrode may extend down through or from a top lid of the furnace body into the furnace body.

There may be nitrogen injectors to bring nitrogen gas through the furnace body into where electrodes are located to stabilize the plasma field with nitrogen. The method of remediation may require temperatures within the plasma field of between 3,500° and 12,000° centigrade. The system may comprise a primary plasma arc furnace of this type. The primary plasma arc furnace may comprise a nitrogen injector under supervision of the controller to direct the plasma stabilizing gas that is nitrogen to a region where electrodes produce an electrical arc to generate the primary plasma field.

An electrical arc may be struck between the graphite electrodes. The arc may produce a stable plasma field by the injection of the nitrogen gas from the tips of the graphite electrodes. The electrodes may be pipes through which the nitrogen flows to the tips. The gap between the two electrodes may be adjustable to control the size of the plasma field, which controls the power input into the furnace. The wider the gap, the larger the plasma field and higher the temperature inside the furnace.

The primary plasma arc furnace is preferably of narrow and tall dimensions (for example 1 .5 m x 3 m). It may be lined with water cooled high-temperature refractory. In region below the ultra-hot plasma field molten slag collects and floats above molten metal. In this region the primary plasma arc furnace operates at temperatures between 1 ,700°-2,800° centigrade for most applications.

The primary plasma arc furnace may be mounted on load cells to regulate the in-feed of crushed SPL and the out feed of raw syngas, molten metal, and molten slag. There may at least one load cell having an input to the controller monitor weight of material in the primary plasma arc furnace. The secondary plasma arc furnace may also comprise a load cell.

The system may also comprise the secondary plasma arc furnace or secondary electrodes inside the furnace body to generate a secondary plasma field.

The debris, dust, and powder of crushed SPLs may be fed from the crusher, hopper, powder collection apparatus, and/or dust collection apparatus into the primary plasma arc furnace. A portion of the debris, dust, and/or powder may also be fed into the secondary plasma arc furnace or field. The screw feeder may feed them.

The debris, dust, and powder may be fed directly or indirectly from the crusher, powder collection apparatus, and/or dust collection apparatus into the plasma field.

The ultra-high temperature plasma field of between 3,500° and 12,000° centigrade phase changes the organic solid compounds into raw syngas. Preferably the temperature is at least 5000° centigrade. The ultra-high temperatures may also melt and/or drive chemical reaction which the inorganic refractory components of the debris, powder, and dust into molten slag and metal. The toxic and hazardous compounds the debris, powder, and dust are transformed into short chain hydrocarbons.

As the crushed SPL debris, powder, and dust enters the furnace, it is exposed to the temperature of the plasma field between the electrodes. This phase changes the organic components to raw syngas which mostly comprises of short chain hydrocarbons and CO. The hydrocarbons do not have the opportunity to combust due to the injection of nitrogen gas providing an oxygen free environment in the furnace.

During this process, all the organic chemical compounds are partially or completely broken down into a syngas comprising only or mostly short chain hydrocarbons, thereby neutralizing the hazardous organic compounds present in the SPL. The produced raw syngas from the SPL may be further refined in a second stage plasma inductor of a secondary plasma arc furnace to ensure full destruction of the hazardous compounds.

The syngas or a portion of it may be drawn off the primary plasma arc furnace and processed through a secondary plasma arc furnace. The secondary plasma arc furnace may comprise a steam driven plasma inductor. Temperatures within the plasma field of the secondary plasma arc furnace may be between for example 3,500° and 8,000° centigrade.

The steam driven plasma inductor of the secondary plasma arc furnace may use steam as its working gas to conduct the electrical arc between the anode and cathode producing the plasma field. The secondary plasma arc furnace may comprise a steam injector under supervision of the controller to direct a plasma stabilizing gas that is steam to a region where electrodes produce an electrical arc to generate the secondary plasma field. This supplies abundant free radicals of hydrogen H2 and oxygen O2 that combine with the incoming raw syngas supplied by the primary plasma arc furnace. Consequently, high-quality syngas is formed comprising mainly hydrogen H2 and carbon monoxide CO. Preferably the plasma inductor uses at least a 5,000° centigrade steam driven plasma field to transform the raw syngas into refined high- quality syngas comprising mainly H2 and CO.

There may be electrodes in the primary and/or secondary plasma arc furnace that each comprise at least one pipe to inject a plasma stabilizing gas into region where the electrodes produce an arc to generate a primary and/or secondary plasma field.

There may be a refined gas cooling apparatus which operates under supervision of the controller to receive the refined gas from the secondary plasma arc furnace and rapidly cool or quench the refined gas to prevent formation of dioxins and/or furoins. There may be a dioxin and/or furoin detector with an input to the controller to monitor the refined gas from the secondary plasma arc furnace. The refined high-grade syngas may exit the plasma inductor around 1 ,200° centigrade. It may be immediately water quenched to prevent the formation of dioxins and furoins. The produced gas may also be scrubbed to remove any acidic residue and ultra-fine particles.

The filters used in the scrubbing process may be fed back into the primary plasma arc furnace for reprocessing, eliminating all toxic and hazardous wastes. There may be a fuel cell and/or generator to receive the refined syngas to generate electricity under supervision of the controller. The clean high-grade syngas may be then fed directly to the fuel cell or an internal combustion engine or a turbine to generate electrical power or to a storage vessel to be used later. The fuel cell and/or generator may be electrically connected to the primary and/or secondary plasma arc furnace for their power under supervision of the controller. The high-grade syngas produced from the carbon/graphite portion of the SPL has a high CV value. It may provide sufficient energy to run the complete remediation process as a stand-alone, off grid operation.

A controller may control the plasma inductor may be controlled to control the plasma field temperature. The inflow of syngas from the primary plasma arc furnace into the secondary plasma arc furnace may be controlled by the controller. By controlling these factors, the proportion of hydrogen H2 and carbon monoxide CO and the temperature of the refined high-grade syngas which exits the plasma inductor of secondary plasma arc furnace may controlled.

The syngas may be cooled and filtered to remove any particulates. The syngas may be used as fuel and there will be less particulate pollution in exhaust.

There may be a gas connection connecting the primary plasma arc furnace to the secondary plasma arc furnace to transfer the raw syngas to the secondary plasma arc furnace.

The system for plasma field remediation may comprise a fluid connection from the secondary plasma arc furnace or from a particulate filter. The fluid connection may transport the syngas or a portion of it to a heater which burns the syngas to warm a building or factory. The heat may be used to heat the primary and/or secondary plasma arc furnace to make it operate more efficiently. The heat may be used to dry products produced by the method and system for plasma field remediation. The system for plasma field remediation may comprise a fluid connection directly from the secondary plasma arc furnace or from a particulate filter to transport the syngas directly to a fuel cell and/or to an internal combustion engine or gas turbine, or to a storage vessel to be used later.

The syngas may be used in the fuel cell to generate electricity and to generate heat. The internal combustion engine or gas turbine may drive a generator to produce electricity and to generate heat. The electricity may be used to provide current to the plasma generator in the primary and/or the secondary plasma arc furnace and/or to warm a building or factory.

The system and method for plasma field remediation provides energy efficient beneficiation of the SPLs.

The non-organic components of the SPL mainly comprise alumina oxide, silica, aluminum metal, and other trace oxides and metals. These materials may be melted into region of the furnace body underneath the plasma field of the primary plasma art furnace. Some of these molten materials encounter CO produced from the phase changing graphite and smelt (reduce) into metal.

The produced metal from this process has a higher specific gravity than the slag and thus migrates to the bottom of the primary plasma arc furnace. The produced metal ingots mainly comprise aluminum metal that can be further recycled, producing aluminum and other metal concentrates for downstream industry.

Molten metal generated in the primary plasma arc furnace may be tapped. There may be a channel from the primary plasma arc furnace to ingot molds where the tapped metal may solidify into ingots. There may be a first tap through the furnace body. The first tap may be located at the bottom of the primary plasma arc furnace to the channel and/or molds. The primary plasma arc furnace may comprise the first tap located at or near its bottom to draw out molten metal below slag under supervision of the controller. The ingots provide further beneficiation and repurposing into raw metal feed stock.

Slag may be mostly melted alumina oxide and silica. These materials may represent the majority of the molten material in the plasma arc furnace. In normal furnacing operations, calcium carbonate or calcium oxide may be added as a fluxing agent to lower the melting temperature of the alumina oxide and silica (slag). This may be done to keep the slag liquid at the lower molten metal temperatures, making it easier to separate from the molten metal and lowering the energy input to produce metals and alloys.

The primary plasma arc furnace has the capability to operate above the melting temperature of the alumina oxide and silica, thereby keeping it liquid and free of flux additives. This provides the opportunity to produce a high-quality alumina-based refractory out of the slag material that would normally be a waste product. The molten slag mainly comprises alumina oxide and silica making it a suitable refractory material for the foundry and casting industry

Molten slag generated in the primary plasma arc furnace may be tapped through the same first tap. The molten slag may be tapped through a second tap through the furnace body located above the first tap. The second tap may be located at a lower side of furnace body, though above the first tap. The molten slag may be tapped from the lower side of the furnace body into an atomizer. The atomizer may be connected to the second tap into the primary plasma arc furnace to draw molten slag and then atomize a first stream of the molten slag into a spray of droplets of the molten slag.

The atomizer may comprises a venturi and a slag feed line to supply molten slag to the venturi to produce the first stream of molten slag. The molten slag feed line may have a diameter between 5 mm and 20 mm.

The atomizer may comprise at least one ultra-high temperature material. The atomizer may comprise components and some or all of the components may comprise at least one ultra-high temperature material. The ultra-high temperature materials include tungsten, refractory material, ceramic, graphite, silicon carbide, mineral wool, or other material with a melting temperature above 2000° centigrade. A heat exchanger to heat air supplied to the atomizer may also comprise the ultra-high temperature materials.

The atomizer may comprise an electrical heater to heat the atomizer to at least 2000° centigrade. The atomizer and/or particular components of the atomizer may be heated to a temperature of between 2000° centigrade. to 3000° centigrade to keep the slag molten as is passes through. Particular components which may be heated to this temperature range may include an atomizing spray nozzle, a feeding reservoir, a venturi, a venturi housing, a molten slag feed line, an inductive heating element and/or a resistive heating element.

The atomizer may comprise the atomizing spray nozzle to mix the first stream of molten slag with a second stream of air to produce the spray. The atomizer may comprise a component which comprises the atomizing spray nozzle. The atomizing spray nozzle atomizes the molten slag into droplets.

Preferably the atomizing spray nozzle is induction heated. It may be heated by a resistive heating element. The atomizing spray nozzle may be heated to 2,200° centigrade or above to keep slag molten as is passes though.

The atomizer may comprise a component which comprises a feeding reservoir which supplies molten slag to a molten slag feed line inside a venturi housing. The molten slag feed line may be between 5 mm and 20 mm in diameter, for example it may be 10 mm, 12 mm, 15 mm in diameter. The molten slag feed line may connect to the bottom of the feeding reservoir. The molten slag feed line may deliver molten slag through the heated venturi housing and into the spray nozzle. The molten slag feed line may comprise an electrical conductor, for example tungsten, to be induction heated by an induction coil around and/or proximate the venturi housing.

The molten slag feed line may enter the upper portion of the venturi housing and make a sweeping turn of for example 90°, to run parallel with the venturi housing. The feed line may pass through the middle of the hollow venturi housing and stop approximately level with or level with the end of the venturi housing.

The feeding reservoir may comprise a chamber which may be cylinder-shaped for example of 200 mm diameter that reduces to a cone toward a lower portion. The feeding reservoir may be manufactured from tungsten. It may be clad with ultra-high temperature refractory to prevent heat loss. The feeding reservoir may be induction heated by an induction coil that wraps around the outside of the reservoir and may be protected with mineral wool and refractory cladding.

The atomizer may comprise the venturi and it may comprise the venturi housing. It may hold the molten slag feeding line as well as the feeding reservoir. The venturi housing may form a body of the atomizer. The venturi housing may provide compressed air pre-heated to the external-mix spray nozzle. The housing may be induction heated and clad with ultra-high temperature refractory to prevent heat loss.

Components of the atomizer may be housed inside a cast refractory to maintain the internal heat and protect water-cooled induction coils that heat the venturi housing and/or spray nozzle. The induction coils may be wrapped in mineral wool and may be clad with a removable high temperature shroud that protects the assembly from accidental exposure to molten material. A stand-a-loan induction power supply and water-cooling system may be used to power the induction coils around the venturi spray assembly.

Pre-heated air may pass through the venturi housing into the atomizing spray nozzle to atomize the molten slag into fine droplets. The pressure of the air may be at least five bar, at least fifteen bar or at least twenty bar. The pressure may be at most twenty- five bar, at most thirty bar, or at most forty bar. The atomizing spray nozzle may form the hot, high-pressure air into a cone shaped discharge that encompasses the molten slag discharge.

Compressed air may be supplied by a two-stage screw air compressor. An air drier may be included to make a compressor-drier unit. Instrument grade, dry air is provided at a pressure in a range between the least and most pressure to drive the induction heated tungsten atomizer. There may be an air pressure regulator to regulate the pressure of the air to have a total pressure of at least 5, 10, 20, or 30 bar.

There may be a heat exchanger upstream of the nozzle to heat air in the second stream to at least 2000o centigrade. The heat exchanger may configured to heat the compressed air. The heat exchanger may be located downstream of the compressor and the air drier unit and upstream of the atomizer. The dry high-pressure air may flow through the heat exchanger. The heat exchanger may comprise tungsten and/or a refractory material. The heat exchanger may be inside a heat exchanger housing. The heat exchanger housing may comprise an ultra-high temperature material which may comprise tungsten and/or a refractory material. The heat exchanger housing may be surrounded by an induction coil or a resistive heating element that heats the heat exchanger. Operative temperature of the heating element may be for example 2,000 - 3,000° centigrade. The compressed air may be heated to, for example 2,000 - 3,000° centigrade by the heat exchanger. The heat exchanger preheats the compressed air before it enters the atomizer to maintain the working temperature inside the atomizer.

The heat exchanger is upstream of the atomizer and supplies heated air to the venturi to produce the first stream of the molten slag.

The atomizing spray nozzle may comprise vanes to induce a vortex of rotating motion in the second stream of air which intercepts the first stream. The vanes may be angled to direct the rotating motion of the second stream at angle of between 30° and 60° to the direction of motion the first stream of molten slag.

The atomizing spray nozzle may be an external-mix spray nozzle which threads onto the end of the venturi housing. It may comprise a coarse pitch Acme thread.

The cone of hot, high-pressure air may pass through the vanes which may be in the external-mix nozzle to induce a vortex or rotating motion of an air stream. The highspeed rotating air stream may contact the molten stream of slag at an angle causing the molten slag to atomize into fine spray droplets. The vanes may induce the vortex so that the angle is between 30° to 60°, or between 40° to 50°.

The droplets may be accelerated forward in the rotating vortex that causes the individual droplets to rotate at high-speed, providing a stabile vector and shaping them into mini spheres as they solidify in the air. The expanding cone spray pattern produced by the nozzle prevents the forming spheres from encounter each other as they solidify.

The system for plasma field remediation may comprise a tank with an open top. The tank may hold a quenching liquid. The quenching liquid may comprise water or it may be pure water. The atomizing spray nozzle may direct a spray of the droplets to land in the tank of quenching liquid. The tank may be in a cooling chamber. The cooling chamber may be configured to capture steam produced by quenching of the droplets.

The atomized droplets of molten slag may be sprayed into the cooling chamber which is of adequate size to contain the full spray pattern from the atomizer. The atomizer may be oriented to direct the spray of droplets into the cooling chamber. This allows the atomized droplets to expand out into the air and then drop. The atomizer may spray the molten slag into the air as fine spherical droplets, which solidify before falling into the quenching liquid in the tank. The atomizer may the fine droplets into the steam.

The droplets are shock-cooled by the quenching liquid into a spinal crystalline structure (referred to here as ‘prills’). The ‘prills’ may be spherical having acquired that shape being sprayed out of the atomizing nozzle. The prills may be collected, dried, screened into specific size ranges, and packaged as a product for the foundry and casting industry.

There may be vapor from the quenching liquid in the chamber. The vapor may be the steam. The droplets of atomized slag may form into spheres on entering the highspeed vortex generated by the external-mix nozzle. As the mini rotating droplets pass through the vapor inside the chamber, their outer surface contacts the vapor and begins to solidify. This solidification holds the spherical shape as the prills drop into the cooling water for quenching.

The rapid cooling of the quenching liquid forms an internal spinel crystal structure inside the prills making them extremely hard and resilient. The quenching of the hot prills generates steam that is maintained inside the cooling chamber to assist in the forming process of the prills. Steam may also be piped into the chamber, or the quenching liquid may be heated to boiling to be produce steam in the chamber.

The lower portion of the cooling-water tank may be angled, for example to the side and back end of the tank, causing the cooled prills to collect at a collection point. A conveyor may extract the prills out of the cooling tank. The conveyor may comprise a rotating screw, a rake, a trowel, or other suitable collector and conveyance of sediment. The conveyer may discharge the prills onto a small sonic dewatering screen. There may be a drier to dry the prills adapted to be heated by exhaust from an electrical generator or fuel cell reactions powered by the refined syngas. Hot exhaust (for example 520° centigrade or above, preferably at least 620° centigrade) produced from the combustion or fuel cell reactions of the refined syngas in the electrical generation system may be used as a thermal medium to dry the prills. For example they may be dried to a surface moisture of 5% or less and preferably 1 % or less, before size screening and packing for shipment. The drying system may be supplemented with IR dryers.

This atomizing system produces high-quality spherical refractory prills, that have a high refractory temperature, making them suitable for precision casting-sand applications in the foundry industry. Among other advantages, the prills are hard and durable, do not generate dust and are screened to specific size ranges for multiple, respective applications. For example, they are used for precision mold printing and numerous mold and core applications in the foundry industry. The prills may be screened into different size fractions suitable for specific applications.

SPL material preparation may entail an approximate 50/50 ratio to ensure sufficient production of raw syngas to power the primary and/or secondary plasma generators. That is, when an SPL is removed from a smelting pot frame, it is typically separated into two stockpiles referred to as first cut and second cut. The first cut material comprises mainly graphite, and the second cut material comprises mainly refractory comprising mostly of alumina oxide. The first and second cut materials may be blended into an approximate 50/50 ratio by weight so that sufficient raw syngas is produced from the graphite in the primary plasma arc furnace. This will provide feed stock for the plasma inductor to produce high-grade syngas to power the generators and/or fuel cells providing electricity for the plant.

The system for plasma field remediation may comprise a controller. It may be an electronic controller which may be programmable. The controller may have input terminals to read in data from temperature sensors, weight sensors, and other sensors and devices. It may have output terminals to send command signals to the input feeder and other actuators.

The controller may be in electronic communication with temperature sensor inside the primary and/or secondary plasma art furnace. The controller may read in measurements of the incoming flow rate by weight in the in-feed material as compared to the outgoing materials. In particular the controller may read in weight measurements from the load cells mounted on the primary plasma arc furnace and/or the secondary plasma arc furnace and/or the hopper and/or the tank of quenching liquid. The controller may read in measurements by temperature sensers which measure temperatures in the primary and/or secondary plasma arc furnaces. The temperature sensor may measure temperature in the plasma field and/or in the molten slag and/or molten metal. The controller may command the screw feed conveyor that feeds the furnace to speed up or slow down to maintain the input flow rate of SPL debris, powder, and dust as about equal to the output flow rate by weight of molten metal and slag.

The invention will now be described, by way of example only, with reference to the accompanying figures in which:

Brief Description of the Figures

Figure 1 is a process flow diagram of a method of remediation of spent pot liners; and

Figure 2 is a schematic diagram of system of remediation of spent pot liners.

Detailed Description of the Invention

Referring to the Figures, there are shown a method and system for remediation of spent pot liners (SPLs).

Specifically Figure 1 shows a process flow diagram 100 of the method. The first steps in the method are to remove the SPLs from a frame of each pot. The SPLs may be broken out the frame in pieces because in the following steps the SPLs will be decomposed. The pieces of SPLs are then crushed in a crushing step 115 into debris. Crushing produces dust which is collected by a dust collector in a dust collection step The next steps involve feeding the debris and dust into a primary plasma field arc furnace to undergo primary plasma field gasification 130. These steps include a step loading the debris into a feed hopper 120 and then operating a screw conveyer 125 to convey the debris from the feed hopper into the primary arc furnace. There is also a step of blowing, sweeping, or pouring 119 the dust into the primary plasma arc furnace.

Figure 2 shows in a schematic diagram how the first steps are carried out by the system of remediation 200. The pieces of the SPLs which are broken out of the frame are separated into a first stockpile 205 a second stockpile. Pieces selected for the first stockpile 205 comprise mostly graphite. Pieces selected for the second stockpile 210 comprise mostly alumina oxide. The pieces of graphite and alumina oxide are then recombined into a feedstock 215 which is loaded into a feed hopper 220. The bend of graphite and alumina oxide in the feedstock 215 is approximately fifty percent each by weight. The feedstock 215 is conveyed by a conveyer 225 into the primary plasma arc furnace 230 where primary plasma field gasification occurs in a primary plasma field 233.

As Figure 1 shows the step of primary plasma field gasification 130 is central to the method for remediation of SPLs 100.

As Figure 2 shows the feedstock 215 reacts and decomposes in the primary plasma field 233. The feedstock 215 decomposes partially into raw syngas 260 which is removed from the primary plasma arc furnace 230 through a duct 255. Figure 1 also shows the raw syngas removal step 160.

Figure 2 shows that in the primary plasma field 233 the feedstock 215 also partially decomposes into molten metal 245 which settles at the bottom of the primary plasma arc furnace 230. The molten metal is recovered through a first tap 247 into an ingot mold 250. Figure 1 shows the step of metal recovery 150 in the process flow diagram 100.

Figure 2 shows that in the primary plasma field 233 the feedstock 215 also partially decomposes into molten slag 230 which floats above molten metal 245. The molten slag is tapped out of the primary plasma arc furnace 230 through a second tap 295.

The second tap leads to an atomizer 310 in a cooling chamber 305. The molten slag is blown out of the atomizer 310 as a spray of droplets 315. The molten slag is blown with compressed air which is heated to range 2000° to 3000° centigrade.

In the cooling chamber 305 is a tank 320 holding pool 325 of quenching liquid. The atomizer 310 launches the spray of droplets 315 into the cooling chamber 305 above the pool 325 and the spray of droplets 315 falls into the quenching liquid. The droplets are cooled suddenly in the quenching liquid into solid spherical particles called ‘thermaprills’ or ‘prills’ for short. A pile 330 of the prills builds up in the pool 325 of quenching liquid.

The prills are useful in several industries and so the pile 330 of prills is recovered from the pool 325. In Figure 1 a step of recovery of thermaprills 155 is shown.

A step of recovery of cryolite salt 160 at a product of the decomposition of the feedstock of crushed SPLs 115 is also shown in Figure 1.

Both Figure 1 and Figure 2 show that decomposition of the feedstock of crushed SPLs 115 in the primary plasma field 233 causes gasification. In particular raw syngas is produced. There is a step of transferring the syngas 160 from the primary plasma art furnace 230 to the secondary plasma arc furnace 270. In the secondary plasma arc furnace the raw syngas decomposes in step of secondary plasma field gasification 170 in a secondary plasma field 267.

The raw syngas flows out of the primary plasma arc furnace 230 through a duct 255. The duct 255 has an entrance that the raw syngas flow into near top of the primary plasma arc furnace 230 because the raw syngas 260 is contained above the molten slag 240. The duct 255 has an exit into the secondary plasma arc furnace 275. The exit directs the raw syngas 260 into the secondary plasma field 267. The secondary plasma field is produced at the tips two electrodes in the secondary plasma arc furnace 270.

The step of secondary plasma field gasification 170 in shown in Figure 1. This step produces refined syngas 272 which Figure 2 shows in the secondary plasma arc furnace 270.

As Figure 2 shows there is a conduit 272 connected to the secondary plasma arc furnace 270 and gas turbine electricity generator 285. The electricity generator could also be an internal combustion generator or a fuel cell.

The refined syngas flows 272 out of the secondary plasma arc furnace through the duct 275 and into the electricity generator 285. The refined syngas is fuel for the electricity generator.

There is an electricity power cable 290 which is electrically connected to the electrodes in the first plasma arc furnace 230 and in the second plasma arc furnace 270. Electricity produced by the electrical generator 285 supplies the current required to produce the primary plasma field 233 and the secondary plasma field. As Figure 1 shows the electrical generation step 185 provides power by electrical power transfers 191 , 192 to the primary and secondary plasma arc furnaces.

Hot exhaust 287 from the electricity generator 285 may be used to dry the prills 330 after the prills are removed from the pool 325 of quenching liquid.

The invention has been described by way of examples only. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the claims.