All soda ash producers do not use the Solvay process. The lowest cost method of producing soda ash is by mining Trona. ~66% soda ash capacity of Tata Chemicals is of this natural method i.e. in the US and Kenya.
I got this cost curve from a British soda ash company’s IPO filings. I think we can at least guess that Tata Chemicals lies between the 10-30 percentile range i.e. costlier than Kazan Soda, which is the turkish Ciner group company and cheaper than the other producers using the Solvay process due to economies of scale ?