Abstract:
A process and apparatus for producing calcium sulfide is provided comprising a container housing one or more drums, each drum having a substantially horizontal longitudinal center axis and containing one or more screw conveyors therein. Multiple drums are connected by passageways, and passages are provided for passing material into and out of the container and the drum(s). A heater is provided for supplying heat to the material within the drum(s). The process comprises passing the reactants, gypsum and charcoal, into the container and drum(s), heating the reactants within the drum(s) thereby initiating a chemical reaction converting reactants to products (including calcium sulfide), moving the reactants and products substantially horizontally in the drum(s) through the use of the screw conveyor(s) contained therein, and passing the products out of the drum(s) and the container.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to the art of producing an inorganic sulfide, and more specifically to an improved process and apparatus for producing calcium sulfide and the calcium sulfide produced by the process. 
     A variety of laboratory processes exist for producing calcium sulfide. U.S. Pat. No. 4,348,299 discloses a laboratory process for producing calcium sulfide which involves heating a mixture of an oxidic material, such as carbonate, oxide, or an oxalate of calcium, with a reactive sulfurizing flux in a covered crucible in air at temperatures between 800° and 1200° C. for a time between 0.2 and 72 hours. U.S. Pat. No. 4,629,509 involves the addition of dry lime and an aqueous solution of an inorganic salt supplying sulfide (the combination resulting in calcium sulfide) to a solid residue in order to immobilize the residue. 
     Although the prior art devices may be suitable for their intended purposes, there is much room for improvement in the art of producing calcium sulfide. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an improved process and apparatus for producing calcium sulfide. 
     It is another object of this invention to provide calcium sulfide produced by the improved process and apparatus according to this invention. 
     It is another object of this invention to provide an improved process and apparatus that produces calcium sulfide economically. 
     It is yet another object of this invention to provide an improved process and apparatus that produces calcium sulfide in commercial quantities. 
     It is yet another object of this invention to provide an improved process and apparatus for producing calcium sulfide that can be set up and operated without difficulty. 
     These and other objects of the invention are achieved by a container housing at least one drum, the drum(s) having a substantially horizontal longitudinal center axis, wherein the drum(s) contain at least one screw conveyor substantially therein, the screw conveyor(s) being generally parallel to the horizontal center axis, an inlet passage for passing material into the container and drum(s), an outlet passage for passing material out of the drum(s) and container, and a heater for heating material within the drum(s); whereby reactants can be passed into the container and drum(s), wherein the reactants can be heated up within the drum(s) thereby substantially initiating a chemical reaction, the chemical reaction substantially converting reactants into products, wherein the reactants and products can be caused to move substantially horizontally in the drum(s) by the turning action of the screw conveyor(s), and wherein the products can be passed out of the drum(s) and container. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view in partial cutaway illustrating a preferred embodiment of the apparatus used in producing calcium sulfide according to the present invention. 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG. 1 illustrating a preferred embodiment of the apparatus used in producing calcium sulfide according to the present invention. 
     FIG. 3 is an elevational view illustrating a preferred embodiment of the apparatus used in producing calcium sulfide according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     According to this invention, it has been found that a process and apparatus for economically producing calcium sulfide in commercial amounts can be utilized. This and other features of the invention will become apparent from the description that follows with reference to the various figures of drawing. 
     Although calcium sulfide has been produced in the past, the prior processes have been limited to a laboratory scale. One major reason for this is the high cost involved in producing this compound utilizing the prior art devices. Thus, there have been no processes for producing calcium sulfide which will operate on a commercial scale. Consequently, companies which use calcium sulfide to stabilize heavy metals in soil and for other purposes have traditionally had trouble obtaining calcium sulfide in the amounts they require. Also, while the prior art may be concerned with producing relatively pure calcium sulfide, uses exist for calcium sulfide in concentrations of 45-50%. 
     Referring to the drawings, FIG. 1 illustrates the preferred embodiment of the apparatus used in producing calcium sulfide according to the present invention. Apparatus  10  comprises container  12 , which houses a first drum  14  and a second drum  16  used in transforming the reactants of gypsum and charcoal into products including calcium sulfide. The calcium sulfide produced is typically in powder form. Although two drums are preferred, utilizing one central drum is acceptable. Additionally, coal may be used in place of charcoal; however, better results have been obtained using charcoal. Carbon may also be used in place of charcoal. The use of carbon is advantageous because carbon can often be obtained cheaply and easily. Also, the heating of carbon produces significantly less gaseous emissions than does the heating of charcoal. 
     It is believed that the gypsum, or anhydrous calcium sulfate, is reduced to calcium sulfide when it is heated with charcoal. Several byproducts may also be formed, including calcium oxide and water. The basic chemical reaction thought to be occurring during this transformation of reactants to products is shown below. 
     
       
         2(CaSO 4 *2H 2 O)+4C→CaS+4CO 2 +3H 2 O+CaO+H 2 S  
       
     
     The amounts of calcium sulfide and calcium oxide produced are roughly equal. While this is believed to be the reaction by which the above-stated reactants are being transformed to the above-stated products, other mechanisms may be present. Calcium phosphate and calcium carbonate can be mixed with the calcium sulfide upon its removal from container  12  for uses such as heavy metal stabilization. If desired, the calcium oxide can be mixed with these compounds as well. 
     The present invention produces calcium sulfide in concentrations of about 45-50%, sufficient for uses such as the stabilization of heavy metals in soil. The process and apparatus of the present invention can easily produce 7000 tons of calcium sulfide per year, and does so economically. As discussed above, this has not been possible with the prior art devices. 
     FIGS. 1 and 3 illustrate a preferred embodiment of the apparatus used in producing calcium sulfide according to the present invention. As shown in the figures, apparatus  10  comprises container  12 . Housed within container  12  are a first drum  14  and a second drum  16 , both of which have a generally horizontal longitudinal center axis. An inlet passage  18  extends into container  12  and connects with first drum  14 . Inlet passage  18  is used for passing reactants from a conveyor into first drum  14 . An outlet passage  19 , connected to second drum  16 , extends out of container  12  and is used for passing products out of container  12 . A passageway  15  connects first drum  14  and second drum  16 , thus allowing material to flow between them. First drum  14 , second drum  16 , and passageway  15  are preferably made from stainless steel. 
     Heaters  26  are provided to heat up the material within drums  14  and  16 , which is usually a combination of reactants and products depending on the location. Heaters  26  can be heating elements, such as heating coils or burners, proximate drums  14  and  16  which can be activated to supply heat. Heaters  26  can also be passages supplying a heating medium, such as steam or pressurized gas, to the inside of container  12  proximate drums  14  and  16 . In a preferred embodiment of the present invention, five burners are provided below second drum  16  and four burners are provided below first drum  14  (as illustrated in FIG.  2 ). Heat shields  27  are attached to the bottom of drums  14  and  16  directly above the burners. Heat shields  27  are used to extend the useful life of the drums by preventing the flame from being in direct contact with the drums. 
     Insulation  28  is provided to cover the interior walls of containers  12 . A piece of insulation  28  runs generally horizontally through the center of container  12  between drums  14  and  16  for dividing the container into two distinct sections. Insulation  28  serves to maintain heat within container  12  and proximate drums  14  and  16 . Consequently, less heat from heaters  26  is required to carry out the chemical reaction. Insulation  28  and the upper burners rest on supports  29 , which are fixedly attached to at least two walls of container  12 . 
     The heat supplied by heaters  26  comes into contact with the exterior of drums  14  and  16 . The heat is transferred through the walls of drums  14  and  16  to the material inside. For best results, heaters  26  should supply heat in an amount to obtain a temperature of 1800-2000° F. in container  12 . As discussed above, it is believed that this heating of the reactants causes the chemical reaction which substantially converts reactants to products. The pressure in drums  14  and  16  is about two or three psig. 
     First drum  14  and second drum  16  contain screw conveyors  22  and  24 , respectively, housed substantially therein. Screw conveyors  22  and  24  run generally parallel to the horizontal longitudinal center axis of drums  14  and  16 . In the preferred embodiment, screw conveyors  22  and  24  are located in the center of drums  14  and  16  and have a mechanical connection extending out of drums  14  and  16  and container  12 . Using one central screw conveyor in each drum is preferred; however, using two or more strategically placed screw conveyors in each drum is acceptable. The turning action of screw conveyors  22  and  24  impart a driving force to move material substantially horizontally down drums  14  and  16 . The turning of screw conveyors  22  and  24  is controlled by pump  30 , which can any suitable conventional pump. As the material is being moved horizontally, it is also being rotated circularly around screw conveyors  22  and  24 , thereby mixing the material. It is believed that this mixing also aids in the conversion of reactants to products. Screw conveyors  22  and  24  are preferably made from stainless steel. 
     For purposes of understanding scale, it is preferred that container  12 , drums  14  and  16 , and screw conveyors  22  and  24  are all approximately 30 feet long. Thus, apparatus  10  is large enough to produce significant volumes of calcium sulfide, yet small enough to be set up in any location of choice. 
     Box exhaust line  31  communicates with drum  14  and is provided to allow for the escape of pressurized gases, such as water vapor and black smoke resulting from the burning of charcoal, to the surroundings. Box exhaust line  31  is opened for short periods of time by automatic means (not-shown) for allowing the pressurized gases to escape. Box exhaust line  31  can be connected to one or two cyclone-type separators. Such separators serve to “clean” gaseous streams, thereby preventing undesirable emissions from being vented to the atmosphere. As mentioned earlier, if carbon is employed as a reactant instead of charcoal, gaseous emissions will be low and separators will not be necessary. 
     Pressure discharge line  32  communicates with first drum  14  and ensures that the pressure in drum  14  does not reach such a level at which an explosion may occur. Pressure discharge line opens automatically when the pressure within first drum  14  reaches a certain level, and closes when the pressure within first drum  14  returns to normal. The pressure within first drum  14  can be monitored by an exterior pressure gauge (not-shown) connected thereto. 
     In the preferred embodiment of the process and apparatus for producing calcium sulfide according to this invention, gypsum and charcoal (the reactants) are fed from a conveyor into inlet passage  18 . The reactants are preferably premixed and in the form of powder being less than 100 mesh. Upon entering inlet passage  18 , the reactants fall into first drum  14 . Heaters  26  provides heat in such an amount as to obtain a temperature of 1800-2000° F. in container  12 , to which the reactants are immediately exposed upon entering first drum  14 . Once the reactants have entered first drum  14 , they are moved substantially horizontally down first drum  14  by the turning action of screw conveyor  22 . During this time, the reactants are continuously being heated and converted to calcium sulfide and other byproducts as discussed above. As also discussed above, pressurized gases are allowed to escape first drum  14  via box exhaust line  31 , and pressure discharge line  32  ensures that the pressure in drum  14  does not reach such a level at which an explosion may occur. 
     Once the material approaches the end of first drum  14 , it is transferred to second drum  16  by gravity via passageway  15 . Screw conveyor  24 , located substantially within second drum  16 , moves the material substantially horizontally down second drum  16  in the opposite direction as it was being moved in first drum  14 . Screw conveyor  24  is substantially identical in structure and function to screw conveyor  22 , with the above-stated exception of rotating in the opposite direction. While in second drum  16 , the material is still being heated by heaters  26 , and the reactants continue to undergo transformation to products. The products and any remaining reactants are allowed to exit second drum  16  and container  12  through outlet passage  19 . The point of connection between container  12  and outlet passage  19  is opened periodically for allowing products to exit container  12 . The products are then preferably pumped by a screw (not-shown) up inclined pipe  20 , and dropped into small, cone-shaped tank  21 . From cone-shaped tank  21 , the products can be dropped out and collected in containers suitable for shipping or storage. As mentioned earlier, the calcium sulfide and calcium oxide can be mixed with other compounds such as calcium phosphate and calcium carbonate upon removal from apparatus  10 . 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG.  1 . As shown in FIG. 2, the amount of raw materials added to first drum  14  should be enough to where both first drum  14  and second drum  16  are filled about one-half way (to level  36 ) during steady state operation. This level is beneficial because it allows room for expansion within drums  14  and  16 . Expansion of the material can occur during the chemical reaction described earlier, and as much as one foot of expansion has been observed during operation. Furthermore, this level inherently provides safer operating conditions, i.e., providing more space for pressurized gases to occupy. 
     It is thus seen that an improved process and apparatus for producing calcium sulfide can be utilized. It is also seen that the improved process and apparatus for producing calcium sulfide according to this invention economically produces calcium sulfide in commercial quantities. It is also seen that the improved process and apparatus for producing calcium sulfide according to this invention can be set up and operated without difficulty. 
     It is understood that many variations will become apparent to one of ordinary skill in the art upon reading the specification. Such variations are within the spirit and scope of this invention as defined by the following appended claims.