This is what the Silver Users Association say about the subject :
Between 1966 and 1970, U.S. Treasury sales of silver were a major secondary source of supply. Because silver had been a U.S. monetary standard along with gold, the U.S. government held the world's largest source of secondary supply in an effort to meet a growing production/consumption deficit. In 1965, it appeared that in less than two years the Treasury would effectively lose control of the price of silver. If silver had been allowed to rise above $1.40 per ounce, the silver content of U.S. coins would have been worth more than their face value, causing them to disappear from circulation. Under the Coinage Act of 1965, Congress eliminated the use of silver in coins and authorized the mining of cupro-nickel substitutes and the sale of silver to the public. The right of holders of U.S. silver certificates to redeem them for silver was suspended in 1968. The following year, a federal ban on the melting of U.S. coins was lifted, freeing anywhere from 400 to 700 million ounces for secondary recovery.Read more...
And the report here