Source: United States Geological Survey (USGS) Minerals Resources Program
Note: Check source table for details and footnotes
| Year | Production | Unit of Measure | % Change |
| 2003 | 309.350006103516 | Thousand metric tons | NA |
| 2004 | 263.358001708984 | Thousand metric tons | -14.87% |
| 2005 | 250 | Thousand metric tons | -5.07% |
| 2006 | 250 | Thousand metric tons | 0.00% |
| 2007 | 250 | Thousand metric tons | 0.00% |
| 2008 | 250 | Thousand metric tons | 0.00% |
| 2009 | 215.11799621582 | Thousand metric tons | -13.95% |
| 2010 | 240.897994995117 | Thousand metric tons | 11.98% |
| 2011 | 230 | Thousand metric tons | -4.52% |
| 2012 | 225 | Thousand metric tons | -2.17% |
| 2013 | 235 | Thousand metric tons | 4.44% |
