Note 6 - Business Segments and Sales of Products - Sales from Continuing Operations to Significant Metals Customers As a Percentage of Total Sales (Details) - Customer Concentration Risk [Member] - Revenue from Contract with Customer Benchmark [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
CIBC [Member]
Significant metal customers	31.00%	36.00%	31.00%	24.00%
Ocean Partners [Member]
Significant metal customers	25.00%	0.00%	10.00%	0.00%
Korea Zinc [Member]
Significant metal customers	15.00%	9.00%	16.00%	16.00%
Teck Metals Ltd [Member]
Significant metal customers	12.00%	3.00%	12.00%	7.00%
Scotia [Member]
Significant metal customers	0.00%	18.00%	4.00%	26.00%
Cliveden [Member]
Significant metal customers	0.00%	12.00%	0.00%	4.00%