# Difference between revisions of "Cut-off grade estimation"

Cut-Off grade is the minimum amount of valuable mineral in a given mining unit required to mine (or process) at a profit. Material above this grade is considered to be ore, and material below this grade is considered to be waste. Usually expressed as:

• g/t
• \$/t
•  %

### Break-Even and Internal

The most simplistic way to determine cut-off grade utilizes the following formula: Equation 1. G=(c+m_o-m_w)/(y(s-r)) Table 1: Basic Cut-Off Grade Parameters mo cost to mine ore mw cost to mine waste r refinery unit cost c cost to process ore y metal recovery s unit metal sale price

For the Internal (Milling) cut-off grade, mw = mo. This is simplified in Equation 2. Material below this grade should not be processed whether or not it has already been mined. G_Mill=c/(y(s-r)) For the Break-even (Mining) cut-off grade, mw = 0. This is simplified in Equation 3. Material below this grade should not be mined. G_Mine=(c+m_o)/(y(s-r))

These cut-off grades are the fastest way to classify ore and waste, however, in a more detailed analysis they should not be relied on.

### Lane’s Method

Lane’s Method offers a more accurate and complex way to calculate cut-off grade. This method also requires more information, so it may not be the first choice for a preliminary calculation. Although there are many intermediate grades determined through Lane’s Method, there is ultimately only one output cut-off grade.

Table 2: Lane’s Method Cut-Off Grade Parameters y Recovery C Concentrator capacity R Refining capacity f Fixed costs s Sale price c Concentrator costs r Refining costs V Present value d Discount rate

g_c=(c+(f+dV)/C)/(y(s-r))

g_r=c/(y(s-r-(f+dV)/R))

gmc = f(Qm,Qc,gradedistribution) grc = f(Qr,Qc,gradedistribution) gmr = f(Qm,Qr,gradedistribution) Qm Mine capacity Qc Concentrator capacity Qr Refining capacity

The following formula can be used to classify the equivalent grade for a mining unit: G = g1 + f2g2 + f3g3 + …

$\displaystyle G = g1 + f2g2 + f3g3 + …$ This is necessary for polymetallic deposits in which the value of a block may be influenced by more than one valuable mineral. It simplifies the value of the block so that cut-off grade analysis can be performed, and a decision of whether or not to mine the block can be made.

## Mineral Classification

Cut-off grades are used to determine the point at which material becomes economically valuable. Therefore, they are necessary to define the amount of ore in a deposit. The classification of ore depends entirely on cut-off grade. Increasing the cut-off grade places a stricter requirement on what can be considered ore. This decreases the amount of material that can be considered ore, and with it the reserves and life of mine.

Grade-Tonnage curves are a visual representation of the impact of cut-off grades on mineral reserves.

Figure 1: An Example Grade-Tonnage curve (J.M. Rendu) Metal value is not the only factor affecting the profitability of a block. The presence of unwanted (often hazardous) material in a block may increase the processing cost [SME reference]