Grade Control and Blending at a Sundargarh Manganese Lease
At a small manganese lease in Sundargarh district, lateritic ore occurs in three zones over 4 ha. Drilling and pit sampling give: Zone A, 0.15 Mt at 40% Mn, 10% SiO2 and 0.18% P; Zone B, 0.20 Mt at 30% Mn, 16% SiO2 and 0.12% P; Zone C, 0.10 Mt at 28% Mn, 20% SiO2 and 0.10% P. The previous estimate quoted a single grade of 34% Mn and 12% SiO2, apparently from the 12 pits closest to the access road. A buyer’s contract requires at least 33% Mn, no more than 14% SiO2 and no more than 0.20% P; there is a 5% price penalty per percentage point above 14% SiO2. The mine has a 0.45 Mt reserve, an open-pit permit to 15 m depth, and a wet season from June to September. Zone C lies 300 m from the plant and is clay-rich; Zone A is 1.2 km away. The company wants to ship 0.10 Mt this year without a beneficiation plant.
1. Test the previous grade estimate against the whole reserve and explain the likely source of error.
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The previous estimate is biased toward the accessible road-side pits. If those pits are mainly in Zone A, the quoted 34% Mn and 12% SiO2 describe A, not the lease. Whole-reserve weighted averages are: Mn = (0.15×40 + 0.20×30 + 0.10×28)/0.45 = 32.9%; SiO2 = (0.15×10 + 0.20×16 + 0.10×20)/0.45 = 14.9%; P = (0.15×0.18 + 0.20×0.12 + 0.10×0.10)/0.45 = 0.136%. The reserve therefore fails both the 33% Mn and 14% SiO2 limits, even though the road-side sample passes. The error is not the analytical method but the sample support: a grade from one zone cannot be applied to the whole deposit.
2. Calculate a compliant blend and determine how much Zone C can be included without exceeding the silica limit.
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Blend A and B only: mass 0.35 Mt; Mn = (6.0 + 6.0)/0.35 = 34.3%; SiO2 = (1.5 + 3.2)/0.35 = 13.4%; P = 0.146%. This meets all three limits. For 0.10 Mt, use about 0.091 Mt A+B with no C, or test C carefully. Maximum C fraction is set by SiO2: 13.4(1−x) + 20x ≤ 14 gives x ≤ 0.087. The Mn constraint, 34.3(1−x) + 28x ≥ 33, gives x ≤ 0.205. Silica is therefore the binding constraint: C can be at most about 8.7% of the blend, or 8,700 t in a 0.10 Mt shipment. Because C is clay-rich and wet-season handling will raise moisture and variability, exclude C unless homogenization is assured.
3. Recommend a grade-control and shipping plan for the first 0.10 Mt.
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Use selective open-pit mining to 15 m with grade control by face sampling every 2 m, stockpiling A, B and C separately. Do not blend by visual colour. For the first 0.10 Mt, target roughly 60% A plus 40% B to keep Mn above 34% and SiO2 near 13%; add C only as a controlled 5–8% component after assay. Dewater and cover C during the monsoon, and dry it before blending. Ship in lots of 5,000 t with assay certificates. Because the buyer penalizes SiO2 above 14%, keep a 0.5% safety margin rather than aiming exactly at the limit.