Previous * Next
Blanket Gold Mine
The current Blanket mining area has eight ore shoots in the producing section of the mine. The majority of the mine production is sourced at present from the AR Main and AR South ore bodies with a lesser contribution from the Blanket, Eroica and Lima reefs. AR Main and AR South are massive ore bodies up to 30 m wide and are ideally suited to the long-hole open stope mining method, while the remainder of the Blanket ore bodies are tabular and better suited to underhand stoping methods.
Following the successful commissioning of the No. 4 Shaft Expansion Project in September 2010, the underground workings have increased production to 1,000 tonnes of ore per day using both long-hole open stoping and underhand stoping methods. Broken ore is trammed along the 22 Level rail system by battery locomotives and the ore cars trains are self-tipped onto one of three grizzlys above the ore bins which are located between 22 Level and the 765m level crushing station. The minus 300 mm rock held in three underground storage bins, Payable ore and waste ore are held in separate storage bins and handled accordingly. Ore is gravity fed from these ore bins onto the 765m Level crushing station conveyor which discharges the ore onto a vibrating grizzly feeder which discharges the oversize into a 30 x 20 Telsmith jaw crusher. The underground crushing station ensures that all the run-of-mine ore is reduced to minus 150 mm in size as this provides for the optimisation and greater efficiency of the automated skip loading and hoisting operations. This allows mining and hoisting activity to continue without interruption. Blanket No. 4 Shaft has been equipped with the first automated loading system in Zimbabwe which sequentially fills the two six tonne ore skips which are hoisted from the 789m level to surface. The use of this state of the art automation reduces the risk of ore loading accidents and injuries, reduces manpower costs, minimises spillage, reduces skip loading times, increases hoisting capacity, ensures precise ore tonnage accountability, and enhances winder efficiency while lowering loading and hoisting costs. The double compartment No. 4 Shaft is Blanket's main shaft for hoisting ore to surface from the loading stations at 510m and 789m below surface, and it has a proven hoisting capacity of 110 tonnes per hour from 789m. The Jethro and Eroica Shafts and the No.5 and No.6 Winzes are used for transporting personnel and materials underground, and the No.2 and Lima Shafts are also used for hoisting ore to surface.
The entire underground and surface operations of the Blanket mine, except for the Lima Shaft, including the surface compressors and the No 4 Shaft Winder can be operated by the 10,000kVA standby diesel powered generating sets which were installed and commissioned in May 2011. This standby generating station ensures that all mining and metallurgical operations continue notwithstanding any interruptions to the electrical power supply from the grid. The level of interruptions to Blanket's power supply has diminished considerably following the agreement of an un-interrupted power a supply agreement between Blanket and ZESA. In the year to 31 December 2012, the standby generators were used for a total of 108 hours (2011, 121 hours).
The skips automatically tip ore hoisted to surface into the Shaft Bins on the No4 Shaft headgear. Ore is gravity fed from the Shaft Bins onto the No1 belt which conveys the ore over the automated belt scale and to vibrating screens and 14x24 Telsmith jaw crushers. This crushing circuit reduces the ore to minus 50 mm and it is then deposited by the No 2 belt stockpile conveyor onto the coarse ore stockpile which has a live capacity of approximately 2,000 tonnes of material. Ore from the coarse ore stockpile is then fed onto the triple-deck vibrating screen with the oversize being crushed to minus 12 mm by one of two 38H Telsmith Gyrasphere crushers. The 12mm ore is then fed into the 600 tonne Mill Bin which feeds the two (of the three installed) 1.8 x 3.6 m rod mills where it is milled down to approximately 70% passing 75 microns, before being passed through two 30 inch continuous Knelson Concentrators where approximately 49% of total gold production is recovered. The Knelson Concentrator tails are pumped through cyclones and into a 3.66 x 4.9 m x 750kW (1,000 HP) regrind ball mill. As part of the No.4 Shaft Expansion Project, the capacity of the secondary crushers was increased to over 2,000 tpd and the capacity of the rod mills was increased to 1,800 tpd. The slurry from the regrind mill is pumped into a carbon in leach ("CIL") plant consisting of eight, 600 cubic meter leach tanks equipped with 45 kW agitators where leaching at 50% solids and simultaneous adsorption of dissolved gold onto activated carbon takes place. The CIL plant has a nameplate capacity of 3,800 tonnes of milled ore per day. Elution of the gold from the loaded carbon and electro winning is done on site. Gold is deposited onto steel wool cathodes, the loaded cathodes are acid-digested and the resultant gold solids are smelted in an induction furnace to produce gold bullion of approximately 90% purity, after which the bullion is delivered as required by Zimbabwean law to Fidelity Printers and Refiners (Fidelity) in Harare for sampling, prior to Blanket exporting it directly to Rand Refinery in South Africa for final refining and sale. The full proceeds of sale (i.e. before payment of any royalty) are paid directly into Blanket's foreign currency account with its commercial bank in Zimbabwe within approximately 7 days of receipt of the gold by Rand Refineries.
Overall gold recovery rates have been increased from 85% at the time of acquisition by Caledonia to over 93% as a result of the re-design of more efficient CIL agitators and the installation of an automated liquid sodium cyanide facility which allows for multiple stage cyanide dosing and monitoring of the CIL. The PSA (Pressure Swing Adsorption) Oxygen Generator should be re-commissioned after its overhaul during March 2012 and the controlled sparging of oxygen into the CIL is expected to result in an increase in leach recoveries.
Tailings from the CIL circuit contain less than 30 ppm of cyanide, and are pumped to one of two tailings dams which are operated and maintained by Fraser Alexander, and are inspected and monitored daily by Blanket. Blanket is the sole Africa-based signatory to the Cyanide Convention and continues to adhere to its cyanide discharge standards of less than 30ppm. The Blanket tailings facilities have been rated as "Yellow" (the second highest rating) by the Environmental Management Authority of Zimbabwe.
Consumables used in the metallurgical plant (e.g. grinding media, reagents etc) and the mining operations (explosives, detonators, drill steels etc.) are currently sourced primarily from South Africa.
Historical operating statistics for the Blanket Mine are available in Caledonia's MD&A, which can be found in the Investor section of Caledonia's website. Blanket's historic financial performance up to February 2009 was accounted for in Zimbabwean dollars until 2009. Due to the extreme hyper-inflationary environment which prevailed in Zimbabwe until early 2009 and the resultant devaluation of the Zimbabwean dollar, Blanket's stated historic financial statements are unhelpful for the purposes of evaluating Blanket's historic financial performance. The Zimbabwean dollar was abolished in February 2009 and all financial transactions in Zimbabwe now take place using other currencies, including the US Dollar, the South African Rand and the Botswana Pula. With effect from 1 January 2009, Blanket has prepared its accounts in US Dollars.
Previous * Next