I have a multi-entry Schengen Type 'C' visa issued for Business training trip by Spain, which I have recently completed and came back to my own country. Is it possible to use this visa for purely tourism related purposes (no business) for another EU country within its validity. The visa sticker just mentions 'Estancia' in the remarks and doesn't mention anything related to 'Business' or 'Tourism'.
Yes, this is fine. There is only one type of Schengen short-stay visa (uniform visa) covering all purposes (even work, see below). The list of purposes you find on the application form and in other places is not exhaustive and doesn't define separate types of visa.
While a single-entry visa is implicitely tied to a specific trip and using it for an entirely different purpose could be problematic, a multi-entry visa is inherently more flexible. Using it for tourism is perfectly fine.
The one purpose that is restricted is work. If you wanted to come to the Schengen area to work for less than 3 months, you could still do it on your visa but you would have to secure a separate authorisation to work from the state where you want to work.