Carbide Desulfurization Slag

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. 

Melting Emission Control - Melt materials which contain significant amounts of certain heavy metals (such as lead, cadmium, and chromium) may result in wastes which are classified as hazardous due to EP Toxicity. 

Molding - Nonferrous alloy castings, such as brass and bronze, contain lead that may generate wastes which are classified as characteristic hazardous waste due to EP Toxicity. 

The basic management options for minimizing both the amount and degree of hazardous waste associated with foundry waste are as follows  

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. 

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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). 

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. 

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. 

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. 

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. 

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. 

Quite often, the specifications of a product are based not on the requirements of that product but on what is achievable in practice. When total sulfur removal is required, it is not uncommon that 20 to 30 percent excess carbide is employed. All of this excess carbide then ends up as slag and creates a large disposal problem. If the iron were desulfurized only to the extent actually needed, much of this waste could be reduced or eliminated (Stephens 1988). 

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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