Calcium carbide desulfurization

Foundries that generate furnace dusts that are EP-Toxic due to lead and cadmium have found that by carefully mixing the dusts with water treated calcium carbide desulfurization slag (which contains calcium hydroxide), they may be rendered non-EP-Toxic. Great care must be taken with this method since at high pH levels the lead may leach out. In addition to this problem, the effect of other hazardous materials in the dust and in the slag may still render this waste as hazardous (Stephens 1988). 

Ductile Iron Production/Melting - To reduce the sulfur content of iron, some foundries use calcium carbide desulfurization in the production of ductile iron. The calcium carbide desulfurization slag generated by this process may exhibit the characteristic of reactivity. 

This paper will discuss how some foundries have evaluated the application of these waste management options for calcium carbide desulfurization slag and melt emission control residuals. 

Calcium carbide desulfurization slag has a distinctive odor. Since pure acetylene is odorless, the odor must be produced by other trace constituents in the off-gases. A calcium carbide desulfurization slag sample from one ductile foundry was treated with water at a 1 1 solid-to-liquid ratio, and the gas was collected in a Tedlar bag for analysis by GC-MS. Several trace gases were identified, including arsine, divinyl sulfide (CHj-CH S, ethanethiol (ethyl mercaptan), methane, phosphine, and carbon monoxide. 

For example, if a higher sulfur content is acceptable, the foundry will not have to go to the far end of what the calcium carbide desulfurization treatment process will produce. Instead of having to add a 20 or 30 percent excess calcium carbide to the molten metal for complete desulfurization, a lower dose could be used, resulting in far less unspent calcium carbide in the waste therefore, the wastes may not be classified as reactive hazardous waste. 

The first option is to eliminate the generation of the reactive desulfurization slag by substituting calcium carbide with some other material. A few large foundry companies have made major advancements in new desulfurization technologies over the past years. One such process involves the use of a mixture of calcium oxide, calcium fluoride, and two other materials. One foundry reports that, not only is the product quality satisfactory, but the plant has eliminated the generation of a major problem hazardous waste, and the economics of the process are actually better than calcium carbide desulfurization. 

We have seen that the state-of-the-art methods for spraying or immersing the desulfurization slag in water have the potential to cause difficult air emissions and industrial hygiene problems. Several foundry companies and research groups have evaluated three different basic types of reactors for treatment of calcium carbide desulfurization slag. 

A second major type of reactor involves thermal destruction of the calcium carbide. At about 1,S00°F, both calcium carbide and acetylene are thermally oxidized. Therefore, a system such as a rotary kiln could be used for thermal destruction of the reactivity characteristics. The additional benefit of thermal destruction is that it will also effectively deal with potential sulfide reactivity problems. Large chunks of metals often included in the desulfurization slag will tend to be a problem for many types of thermal units. Concern over air emissions and cost are other hurdles to the use of thermal systems for calcium carbide desulfurization slag. 

In the United States calcium carbide-based acetylene is mainly used in the oxyacetylene welding market although some continues to be used for production of such chemicals as vinyl ethers and acetylenic alcohols. Calcium carbide is used extensively as a desulfurizing reagent in steel and ductile iron production allowing steel mills to use high sulfur coke without the penalty of excessive sulfur in the resultant steel (see Sulfurremoval and recovery). Calcium cyanamide production continues in Canada and Europe (see Cyanamides). 

Reaction With Sulfur. An important use of calcium carbide has developed in the iron (qv) and steel (qv) industries where the carbide has been found to be an effective desulfurizing agent for blast-furnace iron. Calcium carbide and sulfur present in the molten metal react... 

The largest use for calcium carbide is in the production of acetylene for oxyacetylene welding and cutting. Companies producing compressed acetylene gas are located neat user plants to minimize freight costs on the gas cylinders. Some acetylene from carbide continues to compete with acetylene from petrochemical sources on a small scale. In Canada and other countries the production of calcium cyanamide from calcium carbide continues. More recentiy calcium carbide has found increased use as a desulfurizing reagent of blast-furnace metal for the production of steel and low sulfur nodular cast iron. 

The filtered cake produced from the manufacture of dicyandiamide contains about 86% calcium carbonate. American Cyanamid Co. blends the dried waste for the manufacture of calcium carbide-based desulfurized reagents as a gas releasiag agent. 

Desulfogypsum, 4 591-593, 595 Desulfurization, 1 650 10 785 Desulfurizing reagents, calcium carbide application, 4 549-550 Deswelling process, for solvent removal, 28104... 

Calcium carbide used for desulfurization is milled to a powder in laige multichamber ball or rod mills. Performance-enhancing ingredients such as limestone and graphite may be added during the milling process or added later in a post blending operation. 

In the production of ductile iron, calcium carbide is often used as a desulfurizing agent. When added to the iron, it reacts with the sulfides and forms calcium sulfide. This material floats on the surface of the molten iron, where it is skimmed off the surface and placed in a hopper. Since excess carbide is employed to ensure removal of the sulfur, the resulting slag contains high levels of unreactive carbide and must be handled as a reactive waste. 

To eliminate the use of calcium carbide, several major foundries have investigated the use of alternative desulfurization agents (Stephens 1988). One proprietary process employs calcium oxide, calcium fluoride, and two other materials. The foundry reported that not only was the quality of their iron satisfactory, but that the overall process was economically better than carbide desulfurization. 

One common practice is to treat the desulfurization slag with water (Stolzenburg, ec al., 1985). This is done to generate and release acetylene gas from the unreacted calcium carbide. The other major reaction product is solid calcium hydroxide or lime. 

Desulfurizing reagent -calcium carbide as [CARBIDES - CALCIUM CARBIDE] (Vol 4)... 

Desulfurization of hot iron by addition of calcium carbide leads to formation of a slag containing calcium sulfide and hydroxide as well as substantial amount of iron oxides. This slag is completely different than other metalurgical slags and may be effectively used for treatment of various acidic dr neutral wastewater containing heavy metal cations by hydroxide/sulfide precipitation, and, by removal of several components from wastewater by sorption and other interactions with iron oxides. 

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