Abstract:
A process for cooling sulfur-bearing slag while inhibiting the formation of hydrogen sulfide. An oxidant is mixed with the cooling water and the slag is brought into contact therewith, the solution cooling the slag to permit handling thereof while the oxidant inhibits the release of hydrogen sulfide.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The cooling of sulfur-bearing slag such as blast furnace slag is conventionally accomplished by the use of water, for example by pouring it onto the slag in pits, by pouring molten slag into a reservoir of water, or dampening the ground on which thin layers of molten slag are poured. The slag is cooled by the water in order to permit its handling within a reasonable time after pouring. However, this cooling has heretofore been accompanied by the release of hydrogen sulfide which is malodorous, noxious and therefore considered an undesirable air pollutant. In fact, recently enacted anti-pollution codes in some localities prohibit the water quenching of slag except under conditions which prevent the discharge of all hydrogen sulfide or other air contaminants into the open air. 
     2. Description of the Prior Art 
     No process is presently known which inhibits the formation of objectionable hydrogen sulfide during the water cooling of hot sulfur-bearing slag. The Battelle Memorial Institute, Columbus, Ohio, in a 1968 paper by F. H. Woehlbier and G. W. P. Rengstorff entitled &#34;Preliminary Study of Gas Formation During Blast-Furnace Slag Granulation with Water,&#34; discusses the possible reasons for the creation of hydrogen sulfide and suggests two approaches for suppressing its formation. One is to add oxidants to the slag prior to granulation, that is, while the slag is in its molten state. This suggestion is believed unfeasible because of the inability to stir an oxidant into molten slag while in a large ladle. The second suggestion is to conduct the granulation in such a manner as to destroy any possible protective gaseous layers which prevent combustion of the hydrogen sulfide, for example by a strongly agitated water bath or in a waterair jet. However, this process would create very fine, friable weak particles of insufficient strength or size to be used for example as an aggregate road base. 
     Knuppel U.S. Pat. No. 2,829,959 is a process for producing slag wool in which the slag is desulfurized by subjecting molten slag to an oxidizing treatment. Aside from being an entirely different process than that of the present invention, the Knuppel method would result in sulfur dioxide, an undesirable contaminant, being released into the atmosphere. Other U.S. patents such as Elbers U.S. Pat. No. 278,002, Wuth U.S. Pat. No. 643,856, Parsons U.S. Pat. No. 1,551,616, and Smyers U.S. Pat. No. 3,249,402 shows various methods for the removal or recovery of sulfur from blast furnace slag, but none of these are related to the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the invention, a readily reactive water soluble oxidant such as sodium hypochlorite (NaOCl) is mixed with the cooling water, and the sulfur-bearing slag is brought into contact with this solution so as to cool it sufficiently for digging or other handling. It has been found that during this cooling operation the oxidant inhibits the release of hydrogen sulfide from the slag to a major degree satisfying anti-pollution requirements. There is also no release of other contaminants such as sulfur dioxide. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One manner of carrying out the process of this invention is in connection with air cooled slag, that is, slag which is poured while in its molten state into pits in the earth where it cools in air and solidifies. Normally, the great amount of heat in the mass of slag prevents it from cooling in air within a reasonable time to permit digging. The rate of cooling is therefore accelerated by the sprinkling or pouring of water onto the slag. As the outer slag layers are cooled, they crack, permitting the water to penetrate to the next layers which, when they crack upon cooling, will allow still deeper water penetration. 
     The hydrogen sulfide which is released into the air by this conventional cooling method is not only objectionable because of its repugnant odor, but is also highly noxious. Ten parts per million by volume of hydrogen sulfide will produce slight toxic symptoms, whereas 600 parts per million will kill most animals in a short time. 
     According to the invention, an oxidant is dissolved into the cooling water before it is brought into contact with the slag to be cooled. The oxidizing agent used must be soluble in water, should be substantially more readily reactive than the oxygen inherently entrained in the cooling water, and must have no harmful side or polluting effects. The oxidizing agent should also be economical, and its reaction products must not be a hazard to the slag product or the environment. Suitable oxidizing agents are an oxidizing metal hypochlorite such as sodium hypochlorite, chlorine, an oxidizing metal permanganate such as potassium permanganate, a peroxide type oxidizing material such as sodium peroxide or hydrogen peroxide, and fluorine. The strength of solution will depend upon various factors, such as the particular oxidant used, the economy and effectiveness desired, and the amount of sulfur in the slag. The more sulfur found, the more concentrated must be the solution. For use with blast furnace slag, approximately 200 to 400 parts per million by weight of sodium hypochlorite has been found suitable. 
     In use with air cooled slag poured into pits, the solution of oxidant in cooling water would be poured onto the slag through the conventional piping systems, such as swiveled nozzles or fixed piping networks. The oxidant could be metered into the cooling water as it flows toward the slag pit. 
     In using the process for light weight slag, which is conventionally poured in single layers onto slightly damp ground so that the water steams through the slag, the ground would be initially dampened by water to which the soluble oxidant has been added. The steaming water would thus carry the oxidant with it through the slag layer. 
     Another manner of carrying out the invention is in connection with the pouring of molten slag into a reservoir containing a large excess of water which rapidly cools and breaks up the slag. According to the invention, the soluble oxidant would be first added to the water in the reservoir and the molten slag then poured in. As the slag cools and breaks up, the oxidant will inhibit the release of hydrogen sulfide therefrom. 
     One advantage of the process of this invention is that it creates intimate exposure to oxidizing conditions at all areas of incipient hydrogen sulfide creation. The exact nature of the reactions which occur to inhibit the formation of hydrogen sulfide are uncertain. However, the aforementioned Battelle Memorial Institute report indicates that the amount of hydrogen sulfide formed by water quenching of slag is directly proportional to the amount of hydrogen formed. It is believed that the process of the present invention serves to neutralize this hydrogen by the following formula (assuming sodium hypochlorite as the oxidant): 
     
         NaOCl + H.sub.2 → Na Cl + H.sub.2 O. 
    
     where hydrogen sulfide might have been formed, for example by the combination of calcium sulfide in the slag and water, it can be oxidized by the reaction: 
     
         NaOCl + H.sub.2 s→  Na Cl + H.sub.2 O + S.