Ultimate Pit And Mining Plan Optimization With Ecological Costs For Open Pit Metal Mines

Mining industry can strongly affect the ecological environment.In order to minimize this environmental impact,it is not only required to recover the ecological function by land reclamation,but also necessary to thoroughly carry out the mine design based on the concept of Designing for Environment,and realize the resource reduction of ecological and environmental stress.However,both domestic and overseas mining engineering designs often consider the ecological and environmental damages as external influences and ignore them,and only take the economic benefit maximization as the core target in most mining designs.Although many laws of various countries demand to conduct environment assessments for mine projects,the environment assessments fail to feed back to the mine designs due to the independent characteristics of them.This thesis considers the open pit metal mine as a research object,and studies the superiority of mine designs incorporated with ecological costs from the perspectives of ultimate pit and mining plan.Based on the characteristics of mining in open pit metal mine,this study applies the concepts and methods of ecological footprint,and quantifies the ecological stress of mine production to be the direct stress and indirect stress.The former is the area of excavation and land occupation,including open pit,discharged field,tails pond and special road,which damage the ecological system directly.The later is indirect damages caused by pollution discharged from energy consumption during mining processes,such as air pollution,greenhouse effect,and so on.which can be quantified by the amount of forestland used to absorb the pollutants.Ecological stress can be transformed to be ecological costs based on the previous literatures in the ecological costs field.The current work defines the ecological costs as "the loss of ecological values companied with ecological function loss and the economic cost used to recover ecological function".For an open pit metal mine,ecological costs can be divided into four parts:the direct eco-services is the value loss of products(such as crop,wood and livestock)with some market value provided by the ecological system;the indirect eco-services is the value loss of other eco-services(such as conservation of water and soil,air purification,oxygen releases,and so on.)provided by the ecological functions;the prevention and restoration costs are used to pay for land reclamation;and the ecological cost from energy consumption,which is used to absorb or dispose the pollutants from energy consumption.The first three costs correspond to the direct ecological stress of mine,and the last one is related with indirect ecological stress.Based on this classification,the estimation models of ecological costs for the open pit metal mine is built in this thesis.Optimizing the ultimate pit incorporated with ecological costs can be realized by combined the ecological costs with the algorithm of ultimate pit optimization.The volume of pits determines the quantities of ore and rock.The areas of open pit,discharge field and tails pond are different for different pits,which leads to various direct ecological stress.Meanwhile,different pits correspond to different quantities of energy consumption,resulting in different indirect ecological stress,so that the ecological costs may vary with different pits.However,the ultimate pit should be designed before calculating the ecological costs.In order to resolve such contradiction,an iterative pit optimization algorithm incorporated with ecological costs is designed based on cone exclusion method.Firstly,an initial pit without considering the ecological costs is optimized,followed by calculating the ecological costs of this pit,and then allot them to mining,stripping and beneficiation according to their corresponding objects,and conduct the pit optimization again.Finally,repeat the pit optimization and ecological costs calculation procedures until the algorithm converged,then the ultimate optimum pit with ecological costs is found out.Optimizing mining plan incorporated with ecological costs can be obtained by combining the ecological costs with the algorithm of mining plan optimization.Optimizing the mining plan in a designed pit requires to determine the quantities of mining ore and stripping rock,mining location and mining life,so that to maximize the net present value(NPV).Obviously,different mining plans can lead to different land damage areas and different quantities of energy consumption during different periods.In other words,different mining plans have different ecological costs reflected in the ecological costs-time distribution.Ecological costs could directly affect the NPV of mining plan,and therefore impact the strategy of optimum mining plan.In this thesis,a geologically optimum pushback sequence is generated according to a sufficient small increment based on the work-slope angles.Any pushback in this sequence has the biggest metal content among all of pushbacks with the same volume.Then the pushbacks are dynamically ordered to establish a subsequence to guarantee the highest NPV.The pushback i in subsequence indicates the best position for mining plan in the ith year.In the dynamic ordering model and algorithm,according to the quantities of ore and rock and the areas of surface,ecological costs are calculated and corresponded to different states in different ordering paths.By conducting this strategy,the ecological costs can play a similar role as the normal costs in mining plan optimization,and incorporate the mining plan optimization with ecological costs.Pit and mining plan are correlated rather than independent,so that it is not easy to obtain a global optimum solution by presupposing one element to optimize the other,but to optimize them together.In this thesis,a geologically optimum work-slope sequence is generated based on the final-slope angles.Any pit in this sequence has the biggest metal content among all of pits with the same volume.All of the pits obtained are the best candidates.The mining plan in each candidate pit is optimized depending on the model and algorithm of mining plan optimization mentioned above.Then the NPV and the corresponding mining plan of each candidate pit can be obtained.The candidate pit with the highest NPV should be the optimum pit and the corresponding mining plan is the optimum strategy.Then the global optimization of pit and mining plan is obtained by incorporating the ecological costs with mining plan optimization of each candidate pit.Based on the real geological model and technical economical condition of NanFen open pit mine,the optimization models and algorithms mentioned above are used to optimize the ultimate pit both with and without ecological costs.The pit optimization results depending on only mining plan in designed pit and on both optimized pit and mining plan are compared.The results showed that the optimization model and algorithm incorporated with ecological costs are reasonable and practicable.The influence of ecological costs on the ultimate pit and mining plan of the open pit metal mine cannot be neglected.Whether considering the ecological costs or not,optimizing pit and mining plan together should be superior to optimizing the pit or mining plan independently.