Method of making proppant used in gas or oil extraction

1. A method of making a spherical proppant having a selected grade from a naturally occurring mined mineral having a hardness of over 6.0 Mohs, said method comprising:

(a) preprocessing said mineral mechanically into a semi-dry feedstock comprising a mass of particles with initial sphericity values generally less than 0.60;

(b) subjecting said feedstock to abrasive attrition for a process time; and,

(c) controlling said process time to a value wherein said particles of said feedstock are converted to processed particles having a sphericity greater than 0.60 by increasing the sphericity of said feedstock particles by at least 0.10.

2. The method of claim 1 further including:

(d) washing said processed particles to remove fines;

(e) drying said processed particles; and,

(f) screening said processed particles to obtain a proppant having said selected grade.

3. The method as defined in claim 2 wherein said controlled selected time is greater than about 12 hours.

4. The method as defined in claim 3 further including:

(g) measuring the sphericity of said process particles after a first controlled time; and,

(h) converting said first controlled time to the process time if said measured sphericity exceeds a given value.

5. The method as defined in claim 4 where said given value is in the range of 0.75 to 0.90.

6. The method as defined in claim 2 further including:

(g) controlling the preprocessing operation by the size of said selected grade.

7. The method as defined in claim 1 wherein said controlled selected time is greater than about 12 hours.

8. The method as defined in claim 7 further including:

(d) measuring the sphericity of said process particles after a first controlled time; and,

(e) converting said first controlled time to the process time if said measured sphericity exceeds a given value.

9. The method as defined in claim 8 where said given value is in the range of 0.75 to 0.90.

10. The method of claim 1 wherein said mineral is silica sand and said proppant is frac sand.

11. The method as defined in claim 10 wherein at least 99% of said processed particles have a sphericity in the range of 0.75 to 0.90.

12. The method as defined in claim 10 wherein said selected grade is a grade with a maximum value of 6 to 35 mesh and a minimum value of 30-140 mesh.

13. A method as defined in claim 10 wherein said silica sand has a SiO 2 content of at least 80% and a general particle shape classified as between round and angular and a particle size of less than about 8.0 mm.

14. A method as defined in claim 10 wherein said preprocessed feedstock has a particle size coarser than about 350 mesh.

15. The method as defined in claim 1 wherein said feedstock is combined with water before being subjected to abrasive attrition.

16. The method as defined in claim 15 wherein said water is in the range of 5-20 percent by weight of said feedstock and water.

17. The method as defined in claim 1 wherein at least 99% of said processed particles have a sphericity in the range of 0.75 to 0.90.

18. The method as defined in claim 1 wherein said selected grade is a grade with a maximum value of 6 to 35 mesh and a minimum value of 30-140 mesh.

19. A method as defined in claim 1 wherein said preprocessed feedstock has a particle size coarser than about 350 mesh.

20. The method as defined in claim 1 wherein said semi-dry feedstock has at least 5 percent moisture content.

21. The method as defined in claim 1 wherein said semi-dry feedstock has less than 20 percent moisture content.

22. The method as defined in claim 1 further including conversion of said process from batch to continuous before said feedstock is washed.