Introduction
In general, direct energy consumption of an operation (expressed in MJ) and the consumption of raw materials for its execution (expressed in kg) are discussed in order to outline the quantitative profile of the needs of a specific process. Just as for environmental effects, it is necessary to establish an index that can correlate the effects produced by consumption with the overall situation of availability of natural resources. For this purpose, it is essential to make a distinction among resources: intrinsically renewable resources (defined as flow resources) and non-renewable resources (defined as stock resources). In particular:
- Flow resources include air, water, solar radiation, and biotic resources;
- Stock resources include mineral raw materials and land.
According to mining conventions, the concept of a resource is closely related to that of a reserve; in general, resources are transformed into reserves as knowledge about their physical availability and the economic conditions for their use become compatible with their industrial exploitation. Resource consumption is thus linked to the idea that reserves, flow resources, and stock resources decrease as a result of human activities in such a way that in the future they can no longer be used as inputs by the production system.
Calculation Methods
A first quantitative approach to evaluating resource consumption, especially of non-renewable resources, may be to compare current annual consumption with the total confirmed reserves. However, this approach was found to be incorrect (following studies conducted in the '80s by Meadows) because reserves cannot be confused with resources and especially because both are “dynamic” meaning they vary over time. Concerning reserves, it should be noted that they result from the research activities of mining companies, which, as is known, do not invest in this sector more than necessary to guarantee production for a certain number of years. Estimated reserves also do not take into account new discoveries or the contribution of any recycled materials (secondary raw materials) such as methane hydrates.
A second quantitative approach is to consider the annual production of the raw material in question. The ratio between the consumption of a resource and its annual production expresses a reference parameter (in years) that can indicate a measure of the use and availability of a resource while keeping all surrounding variables constant. However, this method leads to neglecting the contribution of reserves and therefore excludes the concept of the resource's finiteness.
An enhancement of the approach was introduced by Guinee et al in 1992 with the following relationship:
D (Depletion) = Ci * Pi / Ri
where:
Ci represents the consumption of mineral expressed in kg
Ri represents the amount of its reserves expressed in kg
Pi represents its annual production expressed in kg / year
The relationship provides an estimate of reserve availability or a measure of the number of years for which current reserves are compatible with current levels of production and consumption. An additional improvement in the calculation was made by Guinee et al in 1992 by assuming to use as a weight the product of reserves and the reserves/production ratio. The relationship becomes:
D' (Depletion) = Pi * Ci / Ri^2
In both cases, the values of D and D' provide an “effect score” (effect score) indicating the evolution of the consumption of a resource over time.
