IRANIAN JOURNAL OF MINING ENGINEERING (IRJME)
Year:2016 | Volume:11 | Issue:32|Papers Count:12

Power draw is one of the key parameters to control mill performance. It is dependent on various factors such as charge density, ore characteristics, ball sizes and type of the mill (AG, SAG, ball mill). In calculation of mills power draw, voidage and the fraction filled by the voidage in the charge is essential. In estimation of power draw, for the voidage between the balls a value of 40% is assumed for all the ball sizes. In this study it was found that the voidage between balls largely depends on the balls shapes and size distribution. The voidage between balls for 8, 9, 10, 11 and 12 mm ball sizes in both single size and seasoned balls was measured. By decreasing the ball size from 12 mm to 8 mm, the voidage decreased from 39.0% to 37.8%. The voidages with Bond chain with 11.4 and 7.6 mm top size balls were measured 36.0 and 35.2%, respectively. The effect of charge density on power draw was studied through scale-downing the ball size distribution of the Sarcheshmeh ball mills at the start up and the crush stop in a laboratory container. The voids of the two distributions were measured to be 33.5 and 36.1%, respectively. The difference in the amount of calculated power draw for these values of voidage was equal to 164 kW, which is equivalent to the weight of 15 tons of balls. An equation was proposed to correct the effect of bulk density on calculation of ball mill power draw.

Failure of many concrete and rock structures on the development of interior crack due to stress concentration at the crack tip shows the prominence of the fracture mechanism investigation of these structures. Using conventional methods for the design and analysis of steel, concrete and rock structures based on the criteria of stress and strength has raised many problems for the structure stabilities. Today, with increasing application of nanoparticles, studies focused on the effect of nanosilica on failure mechanism are rare. Addition of nanosilica to the concrete results in higher compressive, tensile and flexural strength; lower setting time and water permeability and higher resistance to chemical attacks. In this research, the internal mechanism of resistance and failure of concrete reinforced with silica nanoparticles was studied to achieve a better understanding of their mechanical behavior. For this purpose, various standard samples were studied including disc with a central crack (CSCBD) and cubes with similar mixtures (water, cement and aggregates), but different in the amount of silica nanoparticles (0, 0.5, 1, 1.5) in 7 and 28 days of curing time. Acoustic emission techniques (AE) was used for the monitoring of the failure process. It was revealed, parameters such as the number of count, stable and unstable crack growth, average frequency and RA index are also affected by varying the amount of silica nanoparticles in the concrete composition. The results showed that concrete with silica nanoparticles can withstand higher stress without unstable crack propagation. It is concluded that the addition of silica nanoparticles improves the fracture parameters of concrete.

The key block is a well-known method in stability analysis of jointed rock slope. This method is based on the finding & analysis of the key block. In this method if none one of the blocks were unstable we can conclude the rock mass is stable. Sometimes a collection of some stable blocks makes an unstable group, therefore the stability analysis of jointed rock masses leads to investigation of the groups of blocks that are potentially dangerous for the stability of a rock slope. The key-groups method detects the critical groups and focuses the stability calculations on them. For three dimensional development and also increase the efficiency, accuracy and speed of key-group method, combining it with numerical methods is focused. This objective can be achieved in two parts: A: finding the critical key-group and analysing the interaction between the blocks of a key-group. B: analysing the interaction between the critical key-group and rock mass and analyse the stability of rock mass. In this research the blocks interaction of key-groups was analysed, firstly to evaluate the performance of the key-groups method and then to choose the best numerical methods for method development based on fuzzy multi criteria decision maker. At the end, discontinuous deformation analysis was selected as the compatible numerical method with standards. These two methods were combined and the algorithm as well as the primary program were prepared in mathematica software. The results reveals combining the key-groups method with numerical methods results in good performance of identifying and assessing the sustainability of key-groups.

In this paper, the effect of the ratio of tensile strength to confining pressure on the penetration of U Shape disc has been investigated using Discrete Element Method. For this purpose, three numerical models with different tensile strength of 5 MPa, 15 MPa and 25 MPa were built. From each model two similar samples were prepared and compressed by two different confining pressures of 5 MPa and 25 MPa, respectively. The U shape cutter penetrates in the model by the rate of 0.02 m/s till 4 mm of disc is penetrated. In total, 6 simulations have been done. The rock materials, below the cutters, show three different mechanical behavior including failure, plastic and elastic behavior. The failure zone is fully fractured while the plastic zone is consisted of partially micro crack with several major fractures. The elastic zone has remained undamaged. The effect of the ratio of tensile strength to confining pressure has substantial effect on the extension of three introduced zones. When tensile strength is 5 MPa, the initiation force and failure stress is increased by increasing the tensile strength but the extension of failure zone and chipping thickness is decreased. When tensile strength is 25 MPa, the initiation force, failure stress, extension of failure zone and thickness of chipping is constant by increasing the tensile strength.

Electrocoagulation process is one of the new and progressing methods to remove suspended matter. In this study, electrocoagulation was assessed to reduce total suspended solids (TSS), turbidity and hardness of the thickener overflow of hematite in Gole-Gohar Sirjan. Therefore, in a laboratory scale, a glass container with 3 liter volume was prepared and equipped with 6 electrodes made of iron, steel and aluminum. Parameters such as electrode material, contact time and voltage were studied. The aluminum electrode showed the highest TSS removal comparing to the iron and steel electrodes. Aluminum electrode in a voltage of 6V and retention time of 5 minutes removed 88.5 percent of TSS while the steel electrode in voltage of 6 V and retention time of 15 minutes have maximum turbidity equal to 81% (NTU 21). Also in various voltages and different retention times, there was no significant change in the hardness. Based on the technical and economic studies, capital, operating and maintenance costs for electrodes of aluminum, iron and steel were calculated separately. The results illustrated that iron electrode with respect to the total costs (878 Ria) is preferred in comparison with steel (2978 Ria) and aluminum electrodes (3160 Ria). The iron electrode is selected based on the ease of operation, the settling of solid sludge, dewatering capability suitable and less sludge production volume.

Today, large dams are constructed with the different purposes of hydroelectric energy production, tourist attraction, increasing downstream life dams, etc. Concrete arch dams are one of the most affordable large-scale dam construction methods. Much of the reservoir pressure are supported by the abutments in concrete arch dams. This, in turn, highlights the significance of the stability of concrete arch dam abutment. Traditional method of abutment stability analysis calculates the safety factor of potentially slipping wedges using limit equilibrium. In this method rock wedges are considered rigid with no internal discontinuity. In this research, after a short review of geological structure of Bakhtiary dam site and recognition of potential slip wedge, the load distribution of abutments are estimated using finite element method. In numerical modeling, a least simplification in dam body and abutment geometry is purposed. Infinite elements are used to reduce effect of boundary condition. Finally, stability of Bakhtiary arch dam abutments slip wedges has been approved by the application of upper bound theorem. Despite the limit equilibrium method, the proposed method is based on energy-work balances and is able to account for the effect of internal discontinuities yield.

Deposit zoning is one of the significant issues in the field of modeling, evaluation and exploitation planning in the mining sector. In ore modeling, the site is divided into zones based on the physical features affecting mineralization or the spatial distribution of ore grade. In this research, self-organizing map (SOM) has been used for three-dimensional zoning of the mineral deposits. The cluster validity indices has been applied to define the optimal number of zones. The proposed algorithm verified using the data of Choghart iron deposit. The Clustering validation indices were executed based on assay data (iron and phosphorus) of exploratory boreholes and optimal number was resulted in two zones. The SOM clustering algorithm was utilized to determine the confine of each zone and assigning samples within the two zones. As a result, the output of SOM and K-means algorithms illustrated that about 90% of the data was similarly assigned to the same cluster. In SOM clustering, the discrimination surface of the two zones has the northeast-southwest orientation with the southeast slope, while the K-means algorithm determined the surface with east-west orientation and south slope. The resulting surface of SOM is in accordance with the dimensional and directional properties of the structural features (especially fault system) in Choghart iron deposit.

In this research the effect of feed characteristics on AG mill performance of Gole-Gohar iron ore processing plant was studied. For this purpose, 11 surveys in industrial scale were conducted on the circuit. At each survey for 2 hours, samples from AG mill feed and product were collected and mill data such as feed rate and power draw were recorded. Also for the evaluation of the effect of feed size distribution, in 3 surveys, feeder conveyor was stopped and all material on 25 meters of the belt were collected. SAG mill power index (SPI), was determined as the index of feed materials hardness. The result showed with increasing SPI, the product particle size decreased. A harder feed material produces more critical particles in the mill which elevates the mill volume content. It is expected that with increasing ore hardness, abrasion and attrition breakage mechanisms dominate and the product will be finer. Also the results showed, the SPI indexes increased with increasing the ratio of coarse particles (+100mm) to medium particles (-100+40mm).

In flotation circuits, staged separation is used to produce a product with desired recovery and quality. Since usually million tons of mineral ore are upgraded in flotation circuits annually, achieving small improvement through the circuit re-arrangement can have substantial economic effect. In the other hand, there are various designs for a flotation circuit and finding the best combination experimentally is time consuming and costly. In this research, the principles of linear circuit analysis is used to develop a mathematical correlation between the rows of flotation circuit cells. This is performed by considering the transfer function for each cell row and then apply the appropriate criteria for evaluating the circuit transfer function. Since, signal flow graphs can be more easily and efficiently used for analytical method, it is replaced in block diagram for transfer function. This facilitates examination of the large and complex arrangements of flotation circuits. The arrangement of the Qaleh-Zari copper flotation circuit was evaluated by this method. The results showed that by modifying the flotation circuit arrangement, separation sharpness increased from 1.62 to 1.83. This increase can improve the separation of intermediate material in the flotation circuit. It was also shown that if the recovery is determined for each bank, the flow rate of output can be assessed and calculated precisely, and consequently the probable errors can be prevented. The results of this method can be used in circuit design or modification of plant circuit layout.