Complex sentence: Congress also mandated that 45,000,000 part-silver pieces be available for collectors; the Mint sold over half before withdrawing them from sale in 1986 and melting the remainder.
Split into simpler sentences: Congress also mandated that 45,000,000 part-silver pieces be available for collectors. The Mint sold over half of the part-silver coins before withdrawing them from sale in 1986 and melting the remainder.