Can you tell me what china clay is? 1) clay soil from China, 2) clay for making ceramic dishes, 3) another material.
How much "china clay" is contaminated? 1 ton, 100 tons, 10000 tons?
What is the waste oil content of the "china clay"? 0.1%, 1%, 10%?
Answers to these questions will help a lot.
Waste oil can be removed from soil using heat in a thermal desorption unit. However, small quantitities of soil with non-hazardous oil can be treated with "bio piles."
I can give better advice with answers to above questions.