Consider a low-grade disseminated base metal sulfide orebody being mined as an open pit with no stockpiling of low-grade ore. The implication with the valuable component being disseminated is that the same mining methods and mining fleet are used for all the material mined in the pit: there is no distinction in what we colloquially call mining between ore and waste. The no-stockpiling condition (which is not uncommon for mines in mountainous regions) implies that the ore–waste decision, once made, is irrevocable. This helps us to understand the development of the theory initially, but these conditions will be relaxed later in the discussion.
Although in practice grade control procedures will have identified which regions are ore and which are waste and these will be drilled and blasted separately, for the development of cut-off theory this early knowledge is irrelevant. In one sense, the operators do not need to know whether they are dealing with ore or waste; their mining practices are not impacted by this distinction. For cut-off derivation purposes, operations in the pit, which we would usually call mining, generate rock only. All in-pit operations are therefore mining as defined by Lane. Separating ore from rock does not occur until a truck has to branch to the treatment plant or the waste dump on surface after it has exited the pit.3 Mining, whether the normal meaning of the word or Lane’s
definition, is the same thing.
Crushing, grinding and all other ore-related processes are treatment. Lane’s specific usage corresponds with the normal use of the term.
Product becomes separated from ore at the flotation banks where concentrate overflows into the launders and the ore stream becomes physically separated into concentrate and tailings streams. For now we will assume that the tonnages of tailings and ore feed are the same, so the tailings stream may be thought of as a continuation of the ore stream after the product stream has been separated from it.
2. In Lane (1964) mineralised material is simply referred to as material, treatment is referred to as concentrating and marketing is referred to as refining. Lane identifies two product stream subcomponents: concentrate moving from concentrator to refinery, and product from refinery to market.
3. In some operations, this is the case in practice. For example, in some uranium mines classifying a truckload as ore or waste won’t occur until the truck has passed through a detector to measure the level of radioactivity and the truck’s destination is signalled to the driver. In most mines, however, ore and waste are identified prior to drilling and blasting, so that they are not mixed when blasted.
CHAPTER 5 | Lane’s Methodology Concentrate pumping, thickening, filtration, drying and transport are all processes dealing solely with product, and are therefore classified by Lane’s terminology as marketing. While smelting, refining and selling are identified by Lane (and correctly by many users of the theory) as marketing, Lane’s omission in discussing the inclusion of the concentrate (or other product) handling activities within the concentrator or the mill has caused confusion to many of his readers.
The concentrator or mill – because of its primary purpose of separating product from ore – must have some activities that are ore-related and classed as treatment, and other activities that are product-related and hence classified in a Lane-style cut-off derivation as marketing. The mill is not treatment and treatment is not the mill. It is not uncommon in practice to find a milling constraint expressed as a tonnage of ore per unit of time that is constant up to a specified head grade, above which the milling constraint reduces with increasing grade. This is a practical recognition that, above a certain grade, the product- related parts of the circuit will be overloaded if treatment proceeds at the maximum rate at which ore could be handled at lower grades; hence, the treatment rate of ore must be reduced so as not to overload the product circuits. Similarly, all costs in a base metals concentrator are, in many mine cost-reporting systems, recorded as a cost per tonne of ore. This ignores that some costs are driven by ore tonnages and others by product quantities. The average cost per tonne of ore can therefore vary with grade, even though the underlying cost driver relationships have not changed.
So while for a simple open pit operation the rock–ore interface is easy to identify and the cut-off-specific meanings of the terms mining and treatment correspond with the normal meanings, the ore–product split in the concentrator, while fully appreciated operationally and technically, is not always well recognised as the treatment–marketing interface when using those terms in their cut-off-specific context. Often this interface is perceived to lie in the downstream part of the process flow, perhaps starting with concentrate transport after the concentrate has left the physical boundaries of the mill. The product constraint specified will then only be based on the limitations of the downstream concentrate-handling processes. But the real constraint on concentrate production or handling will frequently be located within the mill, and may be lower than the downstream concentrate handling capacity, so the product or marketing constraint will be overstated.4 Additionally, because the starting point of the product stream is not
correctly identified, costs in the concentrator will be allocated incorrectly between ore and product. For a Lane-style analysis, we need to distinguish between ore and product- related processes, and hence costs and capacities associated with each stream.
Note that for an open pit, rock and ore are mined (in the colloquial sense) simultaneously. Ore is merely reclassified rock, and is separated from the rock stream by directing trucks carrying rock to different ore and waste destinations. In exactly the same way, product is reclassified ore that is separated from the ore stream for further processing, if appropriate, and sale, while the remaining ore is disposed of as tailings.
4. From time to time one will see a technical paper that declares: ‘We are doing a Lane-style cut-off optimisation. We can sell everything we produce; therefore we do not have a marketing constraint’. this will usually be incorrect, and indicates that the team conducting the analysis has missed the point. If a mine is not one of the relative few that does not separate a product from the ore stream, there logically has to be a limitation on the amount of product that can be dealt with somewhere in the production process. For a base metals operation, it will typically be one of the concentrate pumping, thickening, filtration, drying and transport activities noted in the text, or, at a gold mine, one of the activities associated with the activated carbon into which gold is adsorbed.