14 the slag, which flows continuously into slag beds or may be tapped batchwise into slag pans. The slag contains calcium from the ore and silica from the gravel and forms a by-product of slightly positive value used mainly in road construction. The production of phosphoric acid thermal treatment is no longer in use as it requires too much energy. However, this production method has gained renewed interest for recycling of P for sewage sludge and manure. 2.2 Global phosphate reserves Reserve and resource definition As mentioned above, any naturally occurring material containing one or more phosphate minerals and possessing chemical and physical characteristics that make it acceptable for commercial use as a source of phosphate, may be considered as a phosphate ore in the broadest sense. The reserves of any ore are subdivided into various categories depending on the degree of exploration carried out on the geological formation containing the ore in question. Proven or measured reserves are defined by a prescribed drilling pattern and are considered mineable under the economic conditions at the time of definition. Proven reserves form the basis for the considerable investment necessary for the development of the ore body into a mine, and strict rules, which are imposed by stock exchanges and financial institutions, apply to their definition. Indicated and inferred reserves, also referred to as probable and possible, are less well defined but may be expected to be mineable within a certain margin of error. All these reserves involve ore bodies that have been discovered and, in many cases, are actually being mined. More detailed exploration will upgrade at least a portion of the indicated and inferred ore to the category of proven reserves. Where the previous categories all refer to discovered ore deposits, considerable potential for exploitable ore may exist in geological environments similar to those containing the already known ore bodies. One example is the oil and gas fields assumed to exist beneath the North Pole ice. Interpretation of the geology in this area indicates a high degree of probability of the existence of oil and gas fields, yet no specific deposits have actually been discovered. Similarly, such hypothetical and speculative resources of phosphate can be defined. In fact, much exploration leading to the discovery of new reserves is based on hypotheses and sometimes on speculation. From an economic point of view, reserves are subdivided into the categories economic, marginally economic and sub-economic. Marginally economic reserves are expected to be exploitable at marginal profit and sub-economic reserves may be exploitable in the near future at a given price development. The total number of measured and indicated reserves in the economic and marginal category is called the reserve base. Depending on expectations Pagina 23

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