You need to find an analytical lab to perform a "TCLP" from "grab samples" to help you classify the material. I had to do this very same thing so that we could dispose of used shot blast dust.

Here is some more info that is in one of our forms for "Waste Determination and Characterization" of substances.

Documents used were EPA's "Hazardous Waste Identification Flowchart"
MSDS of the materials that were cut to create the slag(Steel)
Lab reports "Analytical Summary"

Methods for determining this are set forth in subpart C of 40 CFR part 261

edit* added link
http://www.epa.gov/osw/laws-regs/regs-haz.htm

Andrew, and John,

It will be important to remember that after being processed with the plasma the slag is no longer just a remnant of the steel as far as using it's MSDS.  You have changed it's structure metallurgically, chemically, and physically.  Without the analysis suggested by John it will be impossible to know it's exact nature as to rather it is Hazardous or not.  And it is definitely impregnated with dust that may, depending largely upon original material ID, be high in hexavalent chromium content. 

Plasma fumes were deemed in many areas (pronounced 'CALIFORNIA') years ago to be highly hazardous and subject to operator and other personnel respiratory protection above and beyond most other welding and cutting processes. 

Just my two tin pennies worth.

Have a Great Day,  Brent

Here is an example(we used this to characterize shotblast dust)

1) Is this or will this become a solid waste(see 40 CFR 261.2)?
Yes-plan to dispose of material.
2) Is this excluded from the hazardous waste regulations(see 40 CFR 261.4)?
No.
3) Is this a listed hazardous waste(see 40 CFR 261, subpart D)?
No.
Is this a characterized hazardous waste(see 40 CFR 261 subpart C)?
Complete the following by either (1) Testing the waste according to the methods set forth in Subpart C of 40 CFR part 261, or (2) Applying knowledge of the hazard characteristic of the waste in light of the materials or process used.

Ignitability (40 CFR 261.21) - No, the MSDSs for steel shot and structural steel show no evidence of iginitability.

Corrosivity (40 CFR 261.22) - No, the MSDSs for steel shot and structural steel show no evidence of corrosivity.

Reactivity (40 CFR 261.23) - No, the MSDSs for steel shot and structural steel show no evidence of reactivity.

Toxicity (40 CFR 261.24) - No, the MSDSs for steel shot and structural steel shows the presence of potential tixic metals. The shot blast dust has been tested for TCLP metals and found not hazardous.

5) Is this a hazardous waste mixture(see40 CFR 261.3(a)(iii) and (iv))?
No the material is not mixed with hazardous waste.

6) Is this derived from a hazardous waste(see 40 CFR 261.3(c) and (d))?
No, the material is dust collected from steel shot blasting.

Hope this helps you characterize this waste.

BTW...

WELCOME TO THE AWS WELDING FORUM!!

Have a Great Day,  Brent