Patent Application
20180238543
The present invention relates to industrial processes involving the use of activated carbon. More particularly, the present invention relates to methods and apparatus for incinerating activated carbon and scrubbing the exhaust gases.
Activated carbon is widely used in the chemical process industry to absorb hazardous materials. In gold mining, activated carbon is used to absorb gold out of a cyanide solution. During this process, gold-containing carbon fines are created and cannot be recycled into the process. It can be economically advantageous to recover the gold from the used carbon fines. In addition, the gold-process recovered carbon fines in many cases contain mercury, which presents environmental problems when attempting to extract the gold. Many gold mines simply stockpile the used carbon fines, which are later shipped to refineries to recover the gold. Transportation costs and environmental issues can make extraction of the gold uneconomical.
In addition, power plants are also turning to adsorption to process exhaust containing hazardous materials. Activated carbon and other adsorbents are being used to capture mercury in these industries. New regulations in the United States are forcing power plants to reduce mercury by 90%. The injection of activated carbon is the route many operators will take. This application of activated carbon requires an increasingly large volume of this material. According to some estimates, the market for activated carbon in the United States will effectively double. According to one source, annual industrial adsorption revenue in NAFTA is about $235 MM per year. In east Asia, the fastest growing region, it is about $430 MM/yr.
One of the problems with activated carbon is that it ends up being converted to fly ash, much of which is sold for use as a component in cement. The carbon, however, decreases cement strength. One solution for this has been the development of “cement friendly” activated carbon materials. If the activated carbon has been used to absorb mercury, a solution has to be found for removing the mercury before the carbon is incorporated into cement.
According to one aspect of the present invention, a downflow hearth furnace with a sacrificial or partially replaced bed of gas-permeable material on the hearth to facilitate removal of the final roasted product and/or to maintain the bed in optimal permeable condition even when it is partially degraded by the roasting reactions. The bed may be formed from silica sand or some other naturally occurring or man-made material that can withstand the temperature of the roasting reaction and will remain inert and not oxidized by the roaster gases. Such materials include, but are not limited to, various classes of naturally occurring rocks composed of silicates, aluminates, or alkaline earth oxides, and man-made materials such as ceramics or refractories. The bed of gas-permeable material is selected to possess filtering characteristics that prevent the movement of the fine ash (solids combustion products) from their initial location above the bed. The bed is supported by a layer of gas-permeable filter material that blocks passage of bed material particles. The bed and filter material are supported by a structure formed from a material that can withstand the combustion temperatures produced in the furnace. Negative pressure pulls gaseous combustion products through the bed and out of the furnace below the layer of filter material.
A body portion of the furnace includes the bed and filter material. A lid for the furnace is suspended above the body portion at a rest position. The body portion of the furnace may be moved laterally from a position clear of the outer periphery of the lid to a position directly under the lid. The body portion of the furnace is lifted vertically until it engages the lid, which is free to move upwardly to provide a seal with the body portion. An annular gasket is disposed at an upper surface of the furnace body.
An exhaust duct that is coupled to a source of negative pressure pulls the gaseous combustion materials out of the furnace body extends upwardly and terminates in a horizontally-oriented gasketed flange. As the furnace body is raised to mate with the lid, the gasketed flange engages a mating flange disposed at a fixed unmoveable vertical position. The vertical position at which the mating flange is disposed at a position that is selected to be higher than the rest position of the furnace lid so that weight of the lid has already engaged and sealed the lid to the furnace body at a vertical position below which the gasketed flange engages the mating flange. The vertical position of the mating flange is selected in conjunction with the height to which the furnace body is raised so that a seal sufficient to prevent leaks from the ambient atmosphere from decreasing the negative pressure that is pulling the gaseous combustion products through the system.
To operate the furnace, the furnace bed is moved laterally to a position where it will engage the furnace lid and is then lifted to engage the furnace lid and place the gasketed flange in contact with the mating flange. The carbon is introduced into the furnace and ignited. After ignition, the rate at which the carbon is introduced into the furnace is controlled to maintain a constant temperature of the region of the furnace where combustion is taking place. As the carbon enters the furnace it is evenly dispersed across the bed to promote even combustion and to prevent the formation of bypass regions to provide an even pressure differential over the entire area of the furnace between the combustion region above the bed and the region below the layer of filter material. The negative pressure pulling the gaseous combustion products through the system is monitored and the combustion is stopped by halting the feed of carbon when the pressure has increased to a preselected amount. The furnace body is then lowered and moved laterally away from the furnace lid.
According to one aspect of the invention, the furnace includes a refractory lined metal shell, which may be cylindrical or polygonal in shape, having an integral, non-permeable bottom and an open top and a removable refractory-lined or refractory lid having a set of holes designed to allow the entry of a flame generated by a burner inserted into the top of the lid, and to allow the entry of air to support reaction of the products. An insertable basket seals against the walls of the furnace to prevent excessive short circuiting of air around the basket, and the bottom of which consists of a screen or plenum with small holes to allow downward flow of gases. A replaceable layer of a permeable bed of sand or other material as elsewhere described, is located on the basket, to prevent the downward flow of solids. At least one outlet is formed through the side wall of the shell to allow for instrumentation and for the exhaust of the combustion gases from below the bed.
According to another aspect of the present invention, a device for the roasting of spent activated carbon, waste sludges, or other organic wastes includes at its head or feed end a furnace as just described, a first scrubber unit consisting of a closed tank of water, chemical solution, or other inorganic or organic liquid, with a venturi scrubber and a cyclone separator mounted on top of the tank such that the liquid effluents from the scrubber and separator will fall by gravity into the tank; and a connection from the furnace to the inlet of the venturi so that gases are sucked into the venturi then through the cyclone separator, and connections for introducing the scrubber (scrubbing) liquid and for removing excess liquid. The system includes a second scrubber unit essentially identical to the first scrubber unit, which will be operated under slightly different conditions or with a different chemical liquid to effectively remove objectionable impurities which may not have been removed in the first unit. The system may also include more absorber units as necessary to effect acceptable clean up of the gas stream for discharge to the atmosphere. An exhaust fan or blower sufficient to operate the system in an effective manner is employed to pull the gasses from the furnace and through the scrubber units.
The first unit may be operated with a moderate continuous flow of water or of process solution from elsewhere in the industrial complex, in order to lower the temperature of the process gas stream from the high temperature of combustion in the furnace to a temperature below about 350° C., and preferably below about 40° C. The second unit may be operated with a recycle stream of a cooling fluid such as cold brine which is cooled by an external chiller, such that the temperature of the process gas stream leaving this unit is low enough so that the vapor pressure of objectionable components is within the limits for atmospheric discharge of gases. The brine in the second unit is maintained at a lower temperature than the first unit, for example a temperature of between about −10° and about 0° C., with the object of controlling the vapor pressure of mercury in the gas stream. Where available “pregnant solution” (gold loaded cyanide solution) at a mine site can be used as a coolant.
A device according to the present invention may be employed for the control of mercury vapor discharge from any industrial process gas stream whereby the initial mercury level in the gas stream exceeds regulatory limits, which consists of an scrubber unit (venturi scrubber and cyclone separator mounted on a receiving tank), operating with a chilled brine stream at a temperature between about −30° and about +15° C., but preferably between about −10° and about +5° C., in such a manner that the brine is in intimate contact with the gas stream so that it simultaneously chills the gas stream and makes gas-liquid contact to remove mercury vapors to the regulatory limits. The brine may be a simple brine formulated to not freeze at the operating temperature, or it may be a more complex chemical brine which serves to chemically change and/or dissolve the mercury components for improved mercury removal.
The simple manually-operated furnace may be expanded to include multiple hearths such that the carbon can be fed continuously, for example, to center of the top hearth, slowly rabbled out, then back to center on a second hearth, then to the outside (to a discharge port) on a third hearth. Carbon will flow downward from hearth to hearth, gas will flow downward through the beds and out the bottom.
A removal mechanism for the manual furnace removes the ash layer with or without some of the permeable bed, using a scraper or a suction device. Bed material so removed can be cleaned and put back in the furnace, or sent with the ash for further processing. In one specific application, the purpose of ashing the carbon is to remove the carbonaceous components so that the carbon-free ash can be easily processed such as by leaching or smelting for recovery of its metal content, and the material in the permeable sand bed is silica sand or another material which is a normal component of the flux mixture used in the smelting of the ash.
The invention will be explained in more detail in the following with reference to embodiments and to the drawing in which are shown:
Persons of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons.
Referring first to
A removable refractory-lined or refractory lid 24 has a plurality of holes (two of which are designated with reference numeral 26) formed therein to allow entry of a flame generated by a burner 28 inserted into the top of the lid 24, and to allow the entry of air to support reaction of the products.
A sacrificial or partially replaced layer of gas-permeable bed material 30 is supported by a layer of gas-permeable filter material 32 that blocks passage of bed material particles. The bed may be formed from silica sand or some other naturally occurring or man-made material that can withstand the temperature of the roasting reaction and will remain inert and not oxidized by the roaster gases. Such materials include, but are not limited to, various classes of naturally occurring rocks composed of silicates, aluminates, or alkaline earth oxides, and man-made materials such as ceramics or refractories. In one illustrative embodiment of the present invention used to process activated carbon fines, the bed material may comprise layer of about 2 inches of #30 silica sand, although this thickness is not critical so long as it is large enough to contain the expected solid ash combustion products.
The bed and filter material are supported by a support structure 34 formed from a material that can withstand the combustion temperatures produced in the furnace. The bed material 30 has a granular size small enough to trap the ash solids that remain behind after combustion of the material being processed but larger than the interstitial spaces in the layer of gas-permeable filter material 32 to prevent the downward flow of solids through the layer of bed material 30. In
The refractory-lined or refractory lid 24 is suspended from supports 38 that are captured by support brackets 40 that are anchored to a support structure for the system (not shown). As previously noted, the lid 24 includes apertures 26 to conduct heat from burner 42 into the furnace 12 when it is closed as will be shown in
A loading chute 44 positioned in the center of the lid 24 communicates with furnace 12 through lid 24 through which material to combust 46 (usually in a granular form) is dispensed into furnace 12 from a loading reservoir 48 by a system such as conveyor belt 50. The conveyor belt 50 is driven from a motor 52 controlled by a conveyor motor controller 54. Conveyor motor controller 54 receives a signal from temperature sensor 56 communicating with furnace 12 through lid 24 that is used to control the speed of the conveyor 50, thus the controlling the rate of introduction of combustible material 46 into the furnace 12 to maintain the furnace 12 at a desired temperature. This feature of the invention is provided to control the combustion of the material 46 to prevent or control the extent of fusing of the bed material 30. For example where sand is used as the bed material 30, substances that are to later be extracted from the solid ash combustion product may be more easily extracted if they are not encapsulated in agglomerated fused bed material.
A disperser 58 inside furnace 12 under the loading chute 44 is rotated by a motor 60 coupled to disperser 58 by shaft 62 to evenly disperse the material to be combusted 46 over the bed material 30. According to one embodiment of the invention, the disperser 58 may be a cone shaped structure having a plurality of vertically oriented vanes or protrusions 64 on its outer surface to aid in dispersing the material 46.
A dispersal motor speed controller 66 varies the speed of motor 60 and thus the speed of disperser 58 to vary the distance that the entering combustible material 46 is thrown from the center of the surface of the bed material 30 to evenly disperse the material across the bed material 30 to achieve a uniform depth of the combustible material as it rests on the bed material 30. As will be readily understood by persons of ordinary skill in the art, the varying speed of the motor 60 will be determined in any particular case by the characteristics of the combustible material 46, including its mass and particle sizes.
As illustrated in
Negative operating pressure is applied to the plenum region 36 of the furnace body 12 through outlet duct 76. Outlet duct 76 terminates in a horizontally oriented flange 78. One or more annular gaskets 80 are disposed on the upper surface of flange 78 to seal the outlet duct 76 from the ambient atmosphere. A mating outlet duct 82 including a mating flange 84 that mates with flange 78 of output duct 76 is axially aligned with the vertical portion of output flange 78.
To operate the furnace 10, the furnace body 12 is laterally positioned under the furnace lid 24 and raised by jacks 68 using lift controller 88. Persons of ordinary skill in the art will appreciate that the configuration of lift controller will depend on the type of jacks 68 that are employed.
In some embodiments where the geometry of the furnace system does not permit even dispersion of the material to be combusted 46 to extend under disperser 58, a central spacer 92 is shown in
After the lid 24 is mated to the furnace body 12 and the flanges 78 and 84 have been mated and sealed, combustible material 46 (e.g., carbon fines) is conveyed to the furnace 12 and distributed evenly across the bed material 30, and burner 42 is lit to ignite the combustible material 46. Negative pressure pulls gaseous combustion products through the bed and out of the furnace below the layer of filter material 32.
Persons of ordinary skill in the art will readily appreciate that materials other than activated carbon may be processed in the downdraft furnace 10 according to the teachings of the present invention. If activated carbon having sufficient carbon content is being processed, it will be able to exhibit self-sustained combustion and burner 42 may be turned off after the material has been ignited, after which temperature control is achieved by material feed rate as described above. Persons of ordinary skill in the art will appreciate that burner 42 may need to remain on during the combustion process where other materials are involved as a function of the nature of the particular material being combusted.
In accordance with another aspect of the present invention, some downdraft sand bed furnace systems may employ more than one furnace body to increase system throughput. An example of such a system is shown in
In
By employing a downdraft sand bed furnace system having multiple furnace bottoms 12 such as the one shown in
Referring now to
Carbon scrubbing system 90 includes downdraft bed furnace 10, which may be configured as shown in
The cooled gasses are delivered by pipe 112 to a first tank 114. First tank 114 is filled with the water exiting the first venturi 108 to a level indicated by reference numeral 116. The cooled gasses are drawn through pipe 118 and introduced circumferentially into first cyclone 120, where centrifugal force forces the finely-divided water component to collide with the outer wall and condense. The water is drawn back into the first tank 114 through pipe 122, which terminates at a point below the water level in the first tank 114. Outlet pipe 124 maintains the water level in first tank 114. Any condensed or solidified waste such as mercury may be removed from the first tank 114 through pipe 126.
The separated and cooled gaseous component is drawn up into pipe 128 which extends down into first cyclone 120 below its top end, and pulled into second venturi 130 where it is mixed with chilled water from pipe 132 pumped by pump 134 from second tank 136 through pipe 138. Second tank 136 is filled with water to a level indicated by reference numeral 140. The design of second venturi 130 is similar to that of first venturi 108. The water in second tank 136 is chilled by heat exchange coils 142 coupled to chiller 144. Chiller 144 is thermostatically controlled to maintain the water in the second tank 136 at a temperature of between about 30° to about 32° F. or as cold as 28° F. or less. Where the temperatures in the second tank 136 are close to the freezing temperature of water, the second tank 136 may use a fluid such as a brine solution that is compatible with the materials encountered in the process and that will remain in a liquid state at such temperatures. For example, a heat exchange liquid such as ethylene glycol may be employed. The temperature drop in the second venturi 120 is about 48° to about 50° F., and is sufficient to condense the mercury or other vapor fractions drawn from the downdraft furnace 10.
The cooled gasses are delivered to the second tank 136. Second tank 136 is filled with the water exiting the second venturi 130 through pipe 146. The cooled gasses are drawn through pipe 148 and introduced circumferentially into second cyclone 150, where centrifugal force forces the finely-divided water component to collide with the outer wall and condense. The water is drawn back into the second tank 136 through pipe 152 which terminates at a point below the water level in the second tank 136. Any remaining condensed or solidified waste such as mercury may be removed from the second tank 136 through pipe 154.
The exhaust air from second tank 136 is virtually free of combustion components and is drawn up into pipe 156 which extends down into second cyclone 150 below its top end, and is pulled through pipe 158 by blower 104. The exhaust air is then pushed through pipe 160 through sulfur scrubber 162 to remove all residual mercury from the system. The exhaust air is then vented into the atmosphere by vent pipe 164. In a typical embodiment of the invention, the exhaust air may be at a temperature of about 32°.
The entire system may be built in an integrated modular fashion, and can be made small enough to be shipped as an integral unit using commercial shipping container or flatrack equipment.
Referring now to
The process begins at reference numeral 172. At reference numeral 174, the furnace is closed and charged with activated carbon or other material from which substances are to be removed. At reference numeral 176 the activated carbon or other material is ignited and air is pulled into the furnace at reference numeral 178. At reference numeral 180 the burner is extinguished and the temperature of the furnace is maintained by controlling the flow of material into the furnace. With some materials, the burner may have to be maintained on or periodically turned on to maintain the temperature in the furnace.
At reference numeral 182, the gaseous combustion products are pulled through the porous furnace bed. At reference numeral 184, the gaseous combustion products are mixed with water and pulled through a venturi to reduce their temperature. At reference numeral 186, the water and condensed combustion products are separated from the combustion products remaining in the gaseous phase. At reference numeral 188, a process which may be performed periodically, the condensed combustion products are removed from the system.
At reference numeral 190, the remaining gaseous combustion products are mixed with water and pulled through a second venturi to further reduce their temperature. At reference numeral 192, the water and condensed combustion products are separated. At reference numeral 194, a process which may be performed periodically, the final condensed combustion products are removed from the system.
At reference numeral 196, the exhaust air is pulled from the system and may be vented to the atmosphere or further filtered if necessary. Reference numeral 198 shows the final combustion products being passed through a sulfur scrubber for further processing. The negative pressure in the system is constantly monitored. When the negative pressure exceeds a target level, indicating that the bed material 30 has become clogged, at reference numeral 200, the carbon feed is stopped, and the remaining carbon is allowed to burn out. The furnace is then opened and the ash is removed from the furnace bed. The process ends at reference numeral 202. Persons of ordinary skill in the art will appreciate that the process 170 may be restarted if, as previously noted, a second furnace bed 12 can be moved into position after the initial furnace bed is moved out from under the furnace lid 24 as previously shown in
While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.
