Field of the Invention

The present invention relates to a process to thermally beneficiate spent pot liners (SPLs) into inert slag, crude iron (‘pig iron’) and synthetic gas (‘syngas’). The SPLs may be spent from use in aluminum smelters. The syngas may be combusted in a generator providing supplemental power for the beneficiation process.

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

Multiple tons of SPL are produced by a single electrolytic smelting pot (also referred to as ‘cell’) all of which 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, prohibit such forms of disposal.

Consequently, 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, cyanides and various compounds. Remediation of SPLs using traditional methods (such as wet chemical neutralization and incineration) is thus challenging, as many such methods 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).

Proposed solutions to the problems associated with the disposal, destruction, and/or repurposing of SPLs have generally failed. One reason is due to the inherent production of additional toxic waste streams that must also be managed and ultimately addressed. For example, known attempts 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 chemicals, but rather forms different types of toxic chemicals that are still hazardous.

The present invention resolves these problems. It builds on the remediation method and system described in PCT/IB2022/056395 ‘Method and System for Remediation of Spent Pot Liners’.

Summary of the Invention

According to a first aspect of the invention there is a system for Spent Pot Liners (SPL’s) beneficiation comprising: an Inducto-smelt Reduction Furnace (IRF) comprising a plasma heater and an induction heater to provide a plasma and an alternating magnetic field in a container in which to heat and decompose the SPLs into raw syngas, slag, and molten metal; and a secondary plasma arc furnace to receive and decompose the raw syngas to produce a refined syngas.

According to a second aspect of the invention there is a method for Spent Pot Liner (SPL) beneficiation by operating the system described herein wherein each SPL is broken up and separated into a first stockpile comprising mostly graphite and a second stockpile comprising mostly refractory material, blending iron oxide, calcium oxide, and the first and second stockpiles together to form a feed material, feeding the feed material into the IRF, smelting the feed material to produce the raw syngas, slag, and molten metal, and generating electricity powering the plasma heater and/or induction heater by providing the raw syngas or refined syngas as fuel to a generator or fuel cell. The first and second stockpiles may be blended in approximately equal proportions by weight.

Preferably the Inducto-smelt Reduction Furnace (IRF) is a hybrid, induction smelting system as described in PCTIB2022/055499 ‘Improved hybrid smelting system’ (referred to herein as RF).

The system may comprise a crusher to crush the SPLs to provide crushed SPLs to a mill. The mill may comprise a ball mill. The mill may be airtight and/or comprise a dust collector.

The system may comprise the mill which may be configured to reduce a feed material comprising the SPLs into particles having a preselected maximum dimension, or a range of the maximum dimension. The maximum dimension may be less than 1200pm, or 600pm, or 300pm, or 150um, or 75pm, or 50pm. The maximum dimension may be at least 10pm, or 20pm, or 40pm.

The first stockpile of SPL, containing mainly spent refractory and the second stockpile, comprising mainly carbon in the form of graphite, may be jaw crushed, and fed into the sealed, airtight ball mill for batch or continuous milling.

Calcium oxide (burnt lime) and an iron oxide may be weighed out according to the amount of SPL and added to the mill along with the SPL.

The SPL, calcium oxide and iron oxide may be milled together in a mill until all the material is less than the preselected maximum dimension or within a range of the maximum dimension at least as noted above to less than as noted above. For example, the range of the maximum size may be between 1200pm and 10pm, or for example less than 150pm.

The system may comprise a pelletizer configured to receive the feed material from the mill and transform the feed material into pellets. The pelletizer may comprise a rotary pelletizing die. The milled mix may be discharged into the rotary pelletizing die and pelletized into small pellets.

There may be one or more hoppers to receive the jaw crushed stockpiles and/or feed material from the ball mills for temporary storage. There may be a hopper to receive the feed material that is transformed into pellets. The pelletized material may be stored in a hopper and discharged on demand into the IRF for thermal processing including smelting. A hopper may comprise a dust collection system, for example the hopper that stores the jaw crushed stockpiles may comprise dust collection system.

The container may comprise a refractory lining comprising zirconium oxide, wolfram oxide, magnesite oxide, or a combination thereof.

The IRF may comprise a container to receive feed material; a feed system comprising a feed entry opening to introduce feed material into the container. The container may have a lower molten metal zone and an upper slag zone in the container. The IRF comprises coils of the induction heater to transmit energy into the molten metal. To keep the slag at the same or similar temperature as the molten metal (avoiding furnace “freeze”) the IRF comprises the plasma heater above the upper slag zone configured to transmit energy into the slag. The lower molten metal zone is situated under the upper slag zone radiant heat from the molten metal to the slag. The slag stays molten and in a low viscosity liquid state due to the radiant heat from the molten metal below the upper slag zone and energy from the plasma heater above the upper slag zone.

The IRF system combines heating the feed materials within the furnace, the feed materials being heated directly via electromagnetic induction and subsequent joule heating (e.g., for conductive contents), and radiant heat transfer from both the molten pool of metal and the plasma energy source (e.g., for non-conductive contents). Among other advantages: the IRF provides means to monitor and analyze materials constitution; means to monitor and control in real time process parameters (including injection rate, temperature, height and power level of the plasma field, rate of smelting concentrate injection, and rate of molten metal discharge; and the optimized reduction zone is evenly heated from the radiant thermal energy of the overhead plasma field and the radiant thermal energy from the induction heated molten pig iron. This dual heating method keeps the slag molten and evenly heated across the entire surface providing an optimal reduction environment for the pelletized SPL to rapidly smelt in. It also provides the consistent high temperature molten slag interface that quickly and efficiently converts the fluorine gas released by the graphite into calcium fluoride, thereby preventing it from exiting the IRF as a hazardous off gas. In addition, the IRF enables a continuous smelting operation (as opposed to a traditional batching method) continuously decanting molten slag through a side “tea pot” style spout and molten metal from the bottom of the furnace to maintain a set level in the IRF for continuous smelt, thereby reducing down-time and loss of thermal inertia.

Preferably heating in the IRF is started with an ingot of pig iron that magnetically couples with the induction field of the induction furnace and melts into a molten pool of iron that provides a heal smelting environment.

The pelletized SPL containing calcium oxide and iron oxide is fed into the top of the IRF and lands on top of the molten pig iron. The thermal energy of the IRF’s plasma hot topper positioned on the top of the IRF and the thermal energy of the molten pig iron rapidly smelts the pelletized SPL into additional pig iron, molten slag and Co rich off-gas.

The IRF may be configured to operate at a smelting temperature of at least 1 ,450° C. The induction heater and plasma heater may be configured to heat the molten metal in the lower molten metal zone to at least 1 ,450° C. The induction heater and plasma heater may be configured to heat the slag in the upper slag zone to at least 1 ,450° C. The hazardous organic cyanide compounds present in the SPL are destroyed above 900° C as the pellets rapidly increase to a smelting temperature of 1 ,450° C in the IRF.

Preferably the plasma heater is configured to maintain a temperature range of between 3500° C and 12000° C in a primary plasma field into which the feed material is fed in the IRF. The primary plasma field may be situated above the upper slag zone.

The carbon (graphite) portion of the pelletized SPL reduces the iron oxide into pig iron and in the process, phase changes from a solid to a gas, mainly consisting of carbon monoxide. This phase change releases the stored arsenic, fluorine, and other minor compounds into the molten slag head.

The molten slag head is generated by the refractory component of the SPL, comprising alumina, silica, and magnesium. The calcium oxide is present in the feed material to lower the melting temperature of the slag by up to 700° C from the otherwise melting temperature of the refractory component, depending on the amount added. Thus, the calcium oxide helps keep the slag molten with less energy from the induction heater and plasma heater.

The calcium oxide in the molten slag also combines with the fluorine to form calcium fluoride, which in turn is combined with the alumina, silica, arsenic metalloid, and magnesium. This molten glass matrix forms the body of the slag head.

It is an advantage that an inert amorphous glass slag aggregate is produced which provides a permanent and safe storage solution for the arsenic and fluorine compounds. It can then be safely landfilled or used as aggregate in the construction industry.

The molten metal may be molten iron. It may be a molten mixture of metals as a molten alloy. The molten alloy may comprise mostly molten iron.

It is an advantage that molten pig iron can be decanted into ingots for commercial sale.

It is an advantage that the carbon monoxide rich off-gas can be combusted on its own to generate electricity.

It is an advantage that the carbon monoxide rich off-gas can be processed in a plasma inductor with steam to generate high-grade syngas. Syngas is a combination of Co and H2 that has a relatively high calorific value with products of combustion that are much lower than most fuels. The syngas can be used to provide supplemental power for the beneficiation process, or used as process heat for drying applications, or bottled and used as a liquified gas source for heating, running LP powered vehicles, and other LP gas applications.

The IRF may be configured to allow the raw syngas, the slag, and the molten metal to be withdrawn from the container separately from each other. There may be a first exit through the container to egress the raw syngas separately. The first exit may be fluidically connected to the secondary plasma arc furnace. In this way the secondary plasma arc furnace may be configured to receive and decompose the raw syngas to produce a refined syngas.

There may be a second exit through the container below the first exit to egress the inert glass slag separately. The second exit may be below the first exit because the second exit is from the upper slag zone which is below a space in the container where the primary plasma field operates and in which the raw syngas collects.

There may be a third exit through the container below the second exit to egress the molten metal(s). The third exit may be below the second exit because the third exit is from the lower molten metal zone which is below the upper slag zone.

The secondary plasma arc furnace may comprise a steam injector to direct plasma stabilizing steam to a region where an electrical arc generates a secondary plasma field in the secondary plasma arc furnace. The secondary plasma arc furnace may be configured to maintain a temperature in a range of between 3500° C and 8000° C in a secondary plasma field into which the raw syngas is fed in the secondary plasma arc furnace.

The system for SPLs beneficiation may comprise a fuel cell and/or generator configured to receive the raw syngas or refined syngas to generate electricity. The generator may comprise a gas turbine or internal combustion engine that turns an electromagnetic device. There may be a first fluid conduit from the first exit to the fuel cell or generator for provide raw syngas as fuel. There may be a second fluid conduit from the secondary plasma arc furnace to the fuel cell or generator to provide refined syngas as fuel.

The fuel cell and/or generator may be configured to provide electricity to the IRF and/or secondary plasma arc furnace via an electrical connection. The induction heater and/or the plasma heater in the IRF may be powered by the electricity from the fuel cell or generator. The secondary plasma arc furnace may comprise a secondary plasma heater to transform the raw syngas to refined syngas that is powered by the electricity from the fuel cell or generator.

As the fuel cell and/or generator may produce heat in addition electricity, the fuel cell and/or generator may be configured to provide heat to the IRF and/or secondary plasma arc furnace to via a thermal connection. Exhaust from the fuel cell and/or generator may be configured to provide the plasma stabilizing steam or a portion thereof to the IRF and/or secondary plasma arc furnace via a fluid connection.

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 shows a process diagram for Thermal SPL beneficiation.

Detailed Description of Embodiments

Referring to Figure 1 , a process diagram 500 shows general steps for thermal SPL beneficiation.

In a first step of the process, a first stockpile of Spent Pot Liner (‘SPL’) containing mainly spent refractory 5 and a second stockpile of SPL containing mainly carbon in the form of graphite 10 are jaw crushed. The crushed first and second stockpiles 15 are discharged into a first hopper 20 comprising a dust collection system 25. The first hopper is at ground level.

In a second step the crushed first and second cuts of SPL are fed from the first hopper 20 into a ball milling system 40. It comprises a sealed, airtight ball mill. Either batch milling or continuous process milling is possible. Calcium oxide (burnt lime) 30 and iron oxide 35 are weighed out according to the amount of first and second stockpiles 5, 10 of SPL and added to the ball milling system 40. The crushed first and second stockpiles of SPL 5, 10, calcium oxide 30 and iron oxide 35 are milled together as a milled mixture in the airtight ball mill 40 until the particles in the milled mixture are small, for example less than 150pm. The milled mixture is then discharged into a second hopper 45 which is enclosed.

In a third step the milled mixture is fed from the second hopper 45 into a rotary pelletizer 50 comprising a rotary pelletizing die. In the rotary pelletizing die, the milled mixture is pelletized into a pelletized material comprising pellets. The pelletized material is then discharged into a third hopper 60 where it may be stored temporarily. The third hopper is an enclosed hopper.

The chemical composition of the pellets comprises for example the milled calcium oxide, iron oxide, first and second cut of SPL graphite and refractory. The pellets may include, for example, zirconium oxide, wolfram oxide, magnesite oxide, or a combination thereof.

In a fourth step, the pelletized material is a feed material for a furnace 65 that is fed on demand from the third hopper into the furnace for thermal processing.

The IRF 65 comprises a container 70 to receive feed material which is pelletized. The container 70 of the IRF 65 is equipped with a high temperature refractory lining 85 suitable for the chemical composition of the pellets. The refractory lining 85 comprises zirconium oxide, wolfram oxide, magnesite oxide, or a combination thereof.

The container 70 of the IRF 65 comprises has a feed system comprising a feed entry inlet 90 to introduce the feed material into the IRF.

The feed material smelts inside the container 70 to form a lower molten metal zone 95 and an upper slag zone 100. Induction coils 80 below and/or beside the container 70 transmit energy into the molten metal in the lower molten metal zone 95.

To keep the slag at the same or similar temperature as the molten metal (avoiding furnace “freeze”) the IRF 65 also comprises a plasma heater 75 above the upper slag zone 100 that transmits energy into the slag. The IRF 65 provides dual heating because the induction coils provide heating and the plasma energy source also provides heating.

The slag in the upper slag zone 100 is also heated through radiant heat transfer to the slag from the molten metal in the lower molten metal zone 95 keep the slag in a safe, molten, and low viscosity liquid state.

To recount, the feed materials from the third hopper 60 are heated within the container 70 of the IRF 65. The feed materials are heated directly via electromagnetic induction coils 80 and subsequent joule heating (e.g., for conductive contents), and radiant heat transfer from both the molten pool of metal and the plasma heater 75 (e.g., for non-conductive contents).

There is a first exit 105 through the container 70 to exit raw syngas. The first exit 105 is above the upper slag zone 100. The first exit 105 is fluidically connected to a secondary plasma arc furnace 120 to deliver the raw syngas from the IRF 65.

There is a second exit 1 10 through the container from the upper slag zone 100 to egress the molten slag. The second exit 1 10 is below the first exit 105 because the second exit 1 10 is from the upper slag zone which is below a space in the container 70 where the primary plasma heater 75 operates and in which the raw syngas collects.

There is a third exit 1 15 through the container below the second exit 110 to egress the molten metal(s) from the lower molten metal zone 95 which is below the upper slag zone 100.

The IRF 65 comprises means to monitor and analyze materials constitution; means to monitor and control in real time process parameters, temperature, height, and power level of the plasma field of the plasma heater 75, rate of injection of smelting concentrate (pelletized feed material), and rate of molten metal discharge. An optimized reduction zone is evenly heated from the radiant thermal energy of the overhead plasma field and the radiant thermal energy from the induction heated molten metal which is mainly molten iron or iron alloy.

The dual heating provided in the IRF 65 keeps the slag molten and evenly heated across the surface providing an optimal reduction environment for the pelletized feed material to rapidly smelt in. It also provides a consistent high temperature molten slag interface that quickly and efficiently converts the fluorine gas released by the graphite into calcium fluoride, thereby preventing it from exiting the IRF 65 as a hazardous off gas. In addition, the IRF 65 enables a continuous smelting operation (as opposed to a traditional batching method) continuously decanting molten slag through the second exit 1 10 side “tea pot” style spout and molten metal from the third exit 1 15 through the bottom of the furnace to maintain a set level of molten metal and molten slag in the IRF 65 for continuous smelt, thereby reducing down-time and loss of thermal inertia.

The process is started in the IRF 65 by placing a conductive metal or alloy ingot 125, typically comprising pig iron, into the IRF 65 lower molten metal zone 95. The ingot magnetically couples with the induction field of the induction heater 80 and melts into a molten metal pool that provides a heal smelting environment.

The pelletized SPL containing calcium oxide and iron oxide feed material is fed into the container 70 through the inlet 90 near the top of the container 70 of the IRF 65 and lands on top of the molten metal pool that provides a heal smelting environment.

The plasma heater 75 is disposed inside the container 70 in a space above the feed material and transmits energy into the slag formed as the feed material is smelted. The thermal energy of the IRF’s plasma heater 75 and the thermal energy of the molten metal pool rapidly smelts the pelletized SPL into additional pig iron, molten slag, and carbon monoxide CO rich off-gas.

Hazardous organic cyanide compounds present in the pelletized SPL are destroyed above 900° C as the pellets rapidly increase to a smelting temperature of 1 ,450° C in the IRF 65.

The carbon (graphite) portion of the pelletized SPL reduces the iron oxide into pig iron which settles into the lower molten metal zone 95 and in the process, phase changes from a solid to a gas, mainly consisting of carbon monoxide. This phase change releases arsenic, fluorine, and other minor elements and compounds stored in the pelletized SPL into the molten slag head in the upper slag zone 100.

The molten slag head in the upper slag zone 100 is generated by the refractory component of the SPL, comprising mostly alumina, silica, and magnesium. These refractory components of the SPL are melted by the dual heating into a molten slag. The calcium oxide (burnt lime) in the pellets aids melting because it lowers the melting temperature of the slag by about 700° C, depending on the amount added.

The calcium oxide in the molten slag also combines with the fluorine to form calcium fluoride, which in turn is combined with the alumina, silica, arsenic metalloid, and magnesium to produce a molten glass matrix that forms the body of the slag head. The slag head is poured or otherwise removed out of the IRF 65 molten slag zone 100 through the second exit 110 as a molten inert amorphous glass slag. When cooled and solidified the slag solidifies into a solid amorphous glass slag aggregate that can be safely landfilled or used as aggregate in the construction industry. The aggregate is a permanent and safe storage solution for arsenic and fluorine compounds.

Molten pig iron that is the IRF 65 lower molten metal zone 95 under the molten inert amorphous glass slag can be decanted through the third exit 1 15 into ingots for commercial sale.

The off-gas the raw syngas is rich in carbon monoxide. The off-gas can be used as a fuel to generate heat and electricity. There is a first exit 105 through the container 70 to exit raw syngas to the secondary plasma arc furnace 120.

The carbon monoxide rich off-gas can be processed in the plasma secondary plasma arc furnace with steam to generate high-grade syngas. Syngas is a combination of carbon monoxide CO and hydrogen H2 that has a relatively high calorific value with products of combustion that are much lower than most fuels. The syngas can be used to provide supplemental power for the beneficiation process by electrical generation 125 by electrical generator or fuel cell. The syngas can be used to provide process heat for drying applications or bottled and used as a liquified gas source for heating, running LP powered vehicles, and other LP gas applications.

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.