Process for solubilizing, reducing and fixing hexavalent chromium contained in chromite ore processing residue into trivalent chromium

1. A method for determining compositional hexavalent chromium (Cr(VI)) level in a sample of a particulate COPR waste matrix treated with reducing agents to reduce hexavalent chromium in the waste matrix, the method comprising the steps of:

(a) removing from the treated COPR waste matrix particles free of Cr(VI) while retaining other particles;

(b) abrading and washing the retained particles to remove particulate reducing agents from the retained particles and unbound reducing agents; and

(c) determining the compositional Cr(VI) level in the washed particles.

2. The method of claim 1 , wherein the removing step comprises passing the Cr(VI)-free particles through a sieve to remove them from other matrix particles.

3. The method of claim 2 , wherein the sieving entails passing the Cr(VI)-free particles through a 0.25 mm sieve.

4. The method of claim 1 , wherein the particulate reducing agents are characterized by a dark black coating on the particles and wherein removal of the particulate reducing agents is evidenced by removal of the coating.

5. The method of claim 1 , wherein the determining step comprises an alkaline digestion test of the washed particles.

6. The method of claim 1 , wherein the method further comprises the step of determining a total dose of all reducing agents and a relative dose of each reducing agent for treating the compositional Cr(VI) in the particulate COPR waste matrix.

7. The method of claim 1 , wherein the reducing agents comprise ferrous iron and sulfide ions.

8. A method for reducing compositional hexavalent chromium in a particulate COPR waste matrix, the process comprising the sequential steps of:

(a) at least partially solubilizing the particulate matrix and at least partially reducing Cr(VI) in the matrix to Cr(III) in the presence of a first reducing agent; and

(b) further reducing Cr(VI) in the matrix to Cr(III) in the presence of a second reducing agent, wherein the first and second reducing agents together are provided in stoichiometric excess relative to the amount of compositional Cr(VI) in the matrix and wherein the second reducing agent is provided in stoichiometric excess relative to the first reducing agent.

9. The method of claim 8 , wherein the amount of compositional Cr(VI) in the particulate COPR waste matrix is determined by a method comprising the steps of:

treating a sample of the particulate COPR waste matrix with reducing agents to at least partially reduce Cr(VI) in the waste matrix;

removing from the treated waste matrix particles free of untreated Cr(VI) while retaining other particles;

abrading and washing the retained particles to remove dissolved and particulate reducing agents on the surface of the retained particles; and

determining the compositional Cr(VI) level in the washed particles.

10. The method of claim 9 , wherein the removing step comprises passing the Cr(VI)-free particles through a sieve to remove them from the remaining particles.

11. The method of claim 10 , wherein the sieving entails passing the Cr(VI)-free particles through a 0.25 mm sieve.

12. The method of claim 9 , wherein the particulate reducing agents form a dark black coating on the particles and wherein removal of the particulate reducing agents is evidenced by removal of the coating.

13. The method of claim 9 , wherein the determining step comprises an alkaline digestion test of the washed particles.

14. The method of claim 9 , wherein the method further comprises the step of determining a total dose of all reducing agents and a relative dose of each reducing agent for treating the compositional Cr(VI) in the particulate COPR waste matrix.

15. The method of claim 8 , wherein the stoichiometric excess of the reducing agents together relative to the compositional Cr(VI) is between about 1.5:1 and 2:1.

16. The method of claim 8 , wherein the stoichiometric excess of the second reducing agent relative to the first reducing agent is between about 5:1 and about 10:1.

17. The method of claim 8 , wherein the method is carried out ex situ.

18. The method of claim 8 , wherein particles of the particulate COPR waste matrix pass through a sieve having an opening size of between about 0.5 inch and one inch.