a bentonite clay - based polymerization catalyst is manufactured by adhering together a plurality of smaller acid - activated bentonite clay particles , using a strong mineral acid , such as h 2 so 4 , as a binder . an acid - activated bentonite clay having a particle size below about 60 mesh , u . s . sieve series , preferably calcium bentonite , having calcium as a predominant exchangeable action , is mixed with a strong mineral acid , preferably h 2 so 4 , and water to form the catalyst . the mixture is subjected to intensive mixing , curing and drying to form strong particles capable of grinding to a desired particle size distribution , e . g ., - 6 mesh to + 60 mesh , u . s . sieve series , to form the catalyst . fines resulting from the grinding step are completely recycled to the intensive mixing stage of the process to achieve 100 % yield . some adjustment of acid content of the mix may be needed to take into account the acid carried into the mix by recycled fines .