Ore Transfer Case Study
A study of the impacts on ore transfer via gantry cranes and AGVs when increasing the number of ore types from one to two using Flexsim Simulation Software
In an attempt to decrease emissions, increase production by 20%, and improve the quality of the final product, a mine was evaluating the feasibility of producing two types of ore from reduction kilns. The plant being studied had five reduction kilns and four smelting furnaces. The reduction kilns were used to heat the ore to approximately 1200 degrees Fahrenheit to reduce the moisture content and reduce the ore oxide. To produce the new ore types in the correct ratio, four of the reduction kilns would produce the primary ore type and the remaining reduction kiln would produce the secondary ore type.
The smelting furnaces melted the ore into slag and a metal matte that was further refined at a later process not included in the simulation model. The two new types of ore produced by the reduction kilns would be combined at the smelting furnaces to produce a single type of metal matte.
Transfer of the ore from the reduction kilns to the smelting furnaces was done through a combination of gantry cranes and AGVs. Each of the five AGVs travelled along its own unique set of rails that ran between one of the reduction kilns and the corresponding hoist well. There were five gantry cranes that all traveled along the same set of rails that were available to transport the containers from the hoist wells to the feed bins. The ore transfer process is outlined below:
- Ore is loaded into a brick lined container on an AGV at a reduction kiln
- The AGV transports the full container to a hoist well
- A gantry crane places an empty container onto the AGV and then removes the full container
- The AGV takes the empty container back to the reduction kiln to be refilled
- The gantry crane hoists the full container up to the feed bin level of the plant
- The gantry crane transports the full container to a feed bin that is ready and empties the container into the feed bin
- The gantry crane transports the now empty container back to a hoist well
- The gantry crane hoists the container down to a ready position and waits for the AGV to return with a full container
Issues to Solve
The initial capital costs required to produce two types of ore from the reduction kilns was high; as a result, it was important to quantify the impacts of those changes to the current system and the benefits of any modifications to the system. While the original operating procedures for the gantry cranes allowed for some cross crane transfers the general rule was to assign one crane to one reduction kiln/AGV. With one reduction kiln now proposed to supply all of the furnaces with the secondary ore type, crane interference became the main area of concern.
The overriding question was whether or not the gantry cranes would be able to maintain and even increase the plant throughput while having to deal with the increased interference resulting from transferring ore across the paths of multiple other gantry cranes.
The first step to solving these issues was to create a base model of the current system producing one ore type. The purpose of this model was to identify the current limitations and performance of the system. The next steps to solving these issues was to produce a series of scenario models that would simulate the proposed changes that would allow the mine to produce the two ore types at the proposed production levels.
The base model started at the output of the reduction kilns into the kiln surge bins that acted as small storage buffers for the ore. The model ended at the output of the furnace feed bins as the ore was released into the smelting furnace. Knowing that the model would see several modifications (one for each scenario) over the life of the project, the model was built to allow for all minor and most major changes to be made easily.
One method employed was the use of an Excel spreadsheet as the interface for changing most of the model parameters. This method was used because it is faster and generally a more recognizable interface for changing large amounts of data when compared to custom GUIs.
Another method employed was the use of containing objects to control the interface with other object types and some of the general behaviors for the object type that they contained. This method was used to simplify any code that needed to be written and to allow the number of any particular object type to be changed easily.
The purpose of the scenario models was to simulate the changes that would be made to the plant to allow for the production of the two types of ore. To simulate anticipated upgrades to the reduction kilns and smelting furnaces the production capacities were increased in each of the scenarios. In addition to the changeover of the center (3rd) reduction kiln to a secondary ore type, there were two major changes that resulted in four scenario models:
- Secondary ore transfer
- All secondary ore was lifted through the center (3rd) hoist well and then either transported directly to a furnace feed bin or to a transfer point to allow a second crane to finish the transfer process.
- Secondary ore destined for the center smelting furnaces (2 and 3) would be transferred the same as above. Secondary ore for the outside smelting furnaces (1 and 4) would use a special conveyor to be transferred to secondary surge bins that were capable of loading the outside AGVs (1 and 5). The secondary ore destined for the outside smelting furnaces would then be lifted through the outside hoist wells.
- Total number of reduction kilns
- The number of reduction kilns would remain at five with the center reduction kiln (3rd) being converted to secondary ore. The percentage of secondary ore required by the smelting furnaces would be roughly 18%.
- An additional reduction kiln, AGV, hoist well, and gantry crane would be built. The fourth reduction kiln would be converted to secondary ore. The ore types would be adjusted so that the percentage of secondary ore required by the smelting furnaces would be roughly 16%.
Initially it was suspected that the ore transfer system (AGVs and gantry cranes) was the main bottleneck to the current production capacity of the plant. Accordingly, it was assumed that the ore transfer system would be main limitation for the changeover to multiple ore types. During the validation of the base model it was determined that the actual bottleneck for the current system was the interactions of the reduction kiln and smelting furnace delays and the misalignments of their production capacities at any given time. After validation the base model showed that the transfer system was Idle 23% of the time and that after scheduled and unscheduled delays the reduction kilns and smelting furnaces were utilized 84% and 90% of the time respectively.
Similar to the base model, the scenario models showed that the current ore transfer system was more than capable of meeting the production requirements for the plant. Even with the increase in production and the added complexity caused by having two ore types, the ore transfer system had excess capacity. As a result, the methods for transferring the 2nd ore type from the reduction kilns to the smelting furnaces showed no significant differences between one another.
The scenarios also showed that increasing the number of ore types from one to two greatly increased the negative impacts of the delay interactions and production misalignments between the reduction kilns and smelting furnaces. The scenarios that did not include the additional reduction kiln showed that the reduction kiln and smelting furnace utilization (after delays) dropped to approximately 68% and 78% respectively. In both scenarios the plant was not capable of meeting the increased production goals.
Adding an additional reduction kiln increased the primary ore capacity to a point where the plant was capable of meeting the increased production goals. In these scenarios the reduction kiln and smelting furnace utilization (after delays) were approximately 65% and 85% respectively. The addition of the extra reduction kiln is the reason that the reduction kiln utilization does not increase.
The impact of a 2nd ore type was such that without the increase in reduction kiln and smelting furnace capacities, included in the scenario models, the plant would be incapable of overcoming the lower utilization values to meet the increased production goals.
Model and case study completed by Brandon Peterson with help from AJ Bobo, Dustin Derrick, and Brenton King. Brandon is a simulation engineer with Flexsim Software Products and can be reached at [email protected] or 801-224-6914.