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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, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
CIBC [Member] |  |  |  |
Significant metal customers | 36.00% | 26.00% | 24.00% | 34.00%
Percentage of sales | 36.00% | 26.00% | 24.00% | 34.00%
Scotia [Member] |  |  |  |
Significant metal customers | 18.00% | 19.00% | 26.00% | 13.00%
Percentage of sales | 18.00% | 19.00% | 26.00% | 13.00%
Korea Zinc [Member] |  |  |  |
Significant metal customers | 9.00% | 15.00% | 16.00% | 22.00%
Percentage of sales | 9.00% | 15.00% | 16.00% | 22.00%
Teck Metals Ltd [Member] |  |  |  |
Significant metal customers | 3.00% | 11.00% | 7.00% | 11.00%
Percentage of sales | 3.00% | 11.00% | 7.00% | 11.00%
Cliveden [Member] |  |  |  |
Significant metal customers | 12.00% | 0.00% | 4.00% | 0.00%
Percentage of sales | 12.00% | 0.00% | 4.00% | 0.00%
Trafigura [Member] |  |  |  |
Significant metal customers | 0.00% | 10.00% | 4.00% | 3.00%
Percentage of sales | 0.00% | 10.00% | 4.00% | 3.00%

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