A new approach to waste dump site selection according to the fuzzy decision-making process
During the process of open-pit mine development, some sites are considered for dumping of surface soils and for waste disposal. Mine spoils include overburden, waste rock, low-grade materials, and tails from the process plant, each of them having their own unique characteristics. For each type of overburden, waste rock, low-grade materials, and process tail, separate storage sites are considered so that it becomes possible to transport or reuse the individual items. Parameters that affect waste dump site selection are environmental, operational, and social factors. Decision-making is commonly explained as a selection process, in which the best alternative is chosen in order to reach a goal. Decision theory, as a specialized field of operation research (OR), is the process of specifying a problem or opportunity, identifying alternatives and criteria, evaluating alternatives, and selecting a preferred alternative. The Society for Judgment and Decision Making (SJDM) defines decision theory as: “… a body of knowledge and analytical techniques of different degrees of formality designed to help a decision-maker choose among a set of alternatives in light of their possible consequences.”
The decision theory offers a rich collection of techniques and procedures to reveal preferences and to introduce them into models of decision. Decision theory is not concerned with defining objectives, designing the alternatives, or assessing the consequences; it usually assumes they are known. Given a set of alternatives, a set of a consequences, and a correspondence between those sets, decision theory conceptually offers simple procedures for choice. Many methods of decision-making may be considered, such as: ELECTRE, MAUT, PROMETHEE, Multiple Objective Mathematical Programming, among others. The focus of this paper is on the inspection of suitable criteria for finding a waste dump site and the presentation of a method, based on the Yager method. This method is one of fuzzy multiple attribute decision-making (FMADM). The aim of FMADM is to obtain an optimum alternative that has the highest degree of satisfaction for all of the relevant attributes. This technique has been used to solve selection problems of decision makers in different areas such as politics, town planning, communication, and mining engineering.
The iron ore district of Gol-E-Gohar (GEG) is located about 60 km southwest of the city of Sirjan, in the Kerman Province of the Islamic Republic of Iran. This complex lies at a point approximately equidistant from the cities of Bandar Abbas, Shiraz, and Kerman, each of these points being approximately 280 km away. The mentioned complex is situated on the Sanandaj-Sirjan metamorphic zone, which has played an important role in the tectonic development of Iran’s plate margin. Iron ore has been mined for at least the past 900 years. Some historians believe that mining activity was carried on in the district of study as far back as 2,500 years ago, during the time of the great Persian Empire at Persepolis. Modern exploration activities, predominant since 1974, have focused on six magnetic anomalies in the district. For ten years, Area 1 ore has been mined and processed in order to achieve recovery of dry as well as wet concentrate. The proven reserves of Area 1 are about 265 million tonnes, whereas nearly 100 million tonnes have already been mined. About 5 km north of Area 1 is the Area 3 deposit (Goharzamin Mine-GZ). Deposit No. 3 is located approximately 1,728 m above sea level in an area of planar desert topography. The landscape is interrupted by ridges and mesas of folded and uplifted metamorphic rocks of Paleozoic and Mesozoic ages, which rise 300 to 400 m above the surrounding plain. The general shape of Deposit No. 3 GEG is generally semi-lenticular. Overall, its dimensions are 2,200 x 2,400 m. The maximum vertical thickness of the orebody ranges from 80 to 100 m and is 40 m thick in the centre. This orebody has, and continues to be, explored by core drilling techniques, whereby the 28,000 m of proven reserves hold 586 million tonnes of ore, and about 200 million tonnes can be mined via open-pit mining.
In order to determine initial alternatives to selecting the best dump site, existing maps of the GZ mine were studied. Analysis of these maps allowed for four locations to be selected for waste dumps and, in accordance with FMADM, the most suitable one was chosen. At the end of the evaluation, the waste dump will be located north of the pit at a distance of 200 m from the crest of the pit. The dump will be around 353 Mt of waste and overburden by the end of the first 15 years of mining. The dump will have a considerably higher capacity suitable for storing waste from future mine expansions.