Prior changes:
- 2021 = High $630/ton on 07-09-21, Low $480/ton on 01-11-21
- 2020 = High $320/ton on 06-04-20, Low $270/ton on 04-13-20
- 2019 = High $375/ton on 01-09-19, Low $290/ton on 02-08-20
- 2018 = High $405/ton on 07-06-18, Low $365/ton on 01-08-18, End $405/ton on 12-06-18
- 2017 = High $370/ton on 03-07-17, Low $305/ton on 02-08-17, End $345/ton on 12-07-17
- 2016 = High $275/ton on 05-06-16, Low $180/ton on 01-01-16, End $275/ton on 12-08-16
- 2015 = High $355/ton on 01-08-15, Low $160/ton on 11-06-15, End $160/ton on 12-07-15
- 2014 = High $440/ton on 01-10-14, Low $349/ton on 11-07-14, End $351/ton on 12-05-14
- 2013 = High $430/ton on 11-12-13, Low $380/ton on 05-03-13, End $430/ton on 12-12-13
- 2012 = High $520/ton on 01-09-12, Low $337/ton on 07-10-12, End $390/ton on 12-06-12
- 2011 = High $515/ton on 07-08-11, Low $434/ton on 01-07-11, End $500/ton on 12-07-12
- 2010 = High $480/ton on 04-08-11, Low $390/ton on 11-03-10, End $440/ton on 12-07-10
- 2009 = High $345/ton on 12-07-09, Low $170/ton on 04-16-09, End $345/ton on 12-07-09
- 2008 = High $890/ton on 07-17-08, Low $125/ton on 11-06-08, End $260/ton on 12-04-08
- 2007 = High $370/ton on 04-02-07, Low $265/ton on 01-08-07, End $340/ton on 12-10-07
- 2006 = High $345/ton on 06-14-06, Low $220/ton on 11-09-06, End $235/ton on 12-07-06
- 2005 = High $370/ton on 01-04-05, Low $140/ton on 06-03-05, End $280/ton on 12-01-05
- 2004 = High $430/ton on 11-08-04, Low $265/ton on 04-21-04, End $390/ton on 12-08-04