Optimizing Melting Expansion at ME Elecmetal
Adding capacity is not as simple as adding a furnace.
Paul Henriksen and Travis Needham, ME Elecmetal, Duluth, Minnesota
(Click here to see the story as it appears in the August issue of Modern Casting.)
ME Elecmetal’s Duluth, Minn., casting facility produces chromium-molybdenum steel and white iron wear parts for the mining industry. Parts range from 200 to 10,000 lbs. and are produced from a unique vacuum molding process. Among vacuum process metalcasting facilities, Duluth produces the world’s largest tonnage of abrasion resistant castings.
In late 2012, the Duluth plant expanded its capacity by installing a third electric arc furnace identical to the two existing furnaces. The furnace was installed to upgrade the foundry’s capacity from 130 to 160 tons/day, with the potential for higher output depending on market demand (Fig. 1). The furnace addition resulted in a more continuous process, producing a heat every 65 minutes improving upon the 90 minute cycle times for two furnaces.
Just adding furnaces to a melt process will not always increase plant capacity. The entire metalcasting facility also was augmented to keep pace with the furnace install. Staffing was increased from 140 to 175 employees, and the company added heat treat ovens to match the production rate. Molding and finishing processes were expanded from two to three shifts. Minor process changes also were required to eliminate potential delays the melting department may have encountered transitioning to 160 tons/day.
Other minor issues regarding slag accumulation, refractory consumption and energy usage have appeared as a result of the decrease in furnace utilization. Efforts to correct these issues and further optimize the plant currently are being realized and implemented.
Process Comparison of Two vs. Three Furnaces
Multiple processes in a metalcasting facility rely on the output of another process to function. The Duluth plant can be summarized into four main departments, each critical for the facility to operate:
Melting: the processing of scrap metal into molten metal of specific chemical identity.
Pouring: pouring the molten metal into molds.
Molding: making the molds to pour into.
Finishing: heat treating and cleaning the castings.
Prior to the 2012 expansion, melting was the main step that limited the rate of production, and the component of melting that caused the longest delay was melt down (Fig. 2).
ME Elecmetal’s Duluth plant can pour only one heat at a time, so only one heat is processed at a time. Ideally, a heat will be sent to pouring as soon as the prior heat finishes being poured. This allows continuous use of the pouring department while also maximizing output from the melt department. In a two-furnace process cycle, once a heat was finished being processed, the following heat was not yet in melt down, resulting in a period of time where no processing occurred. The third furnace was installed to eliminate this bottleneck and allow continuous processing. Figure 2 shows the continuous process cycle. As soon as a furnace finishes being processed, melt personnel immediately move onto the next furnace to begin processing.
Oxygen refinement occurs during early stages of the processing step. After refinement, the heat is not allowed to sit idle for a specific length of time without additional refinement due to quality concerns, which constrains processing to a single heat at a time. Additional refinement incurs additional alloy cost for materials that are oxidized out of the bath, so the practice is avoided. Because of the oxygen refinement constraint, multiple heats are not processed simultaneously. Any additional furnaces would only result in longer wait times until the cycle returns to processing; as a result, a fourth furnace would not increase production rates.
In order to achieve a continuous process in the three-furnace cycle time, the processing time of 65 minutes is staggered, causing a 15-minute wait time. The primary reason for this is to level-load the plant and maintain low man-hours per ton.
With the installation of the third furnace, staff was increased by 35 employees. When the melt department operated just two furnaces, staffing was limited to five individuals: two operated cranes and three worked on the floor maintaining furnaces and processing heats. In the three-furnace process, staffing was increased to six individuals: two who operate cranes and four who work on the floor. Additionally, processing a heat requires two to three individuals on the floor, depending on the stage of processing, with the latter stages (tapping a heat) requiring three.
The majority of the additional staff hired for the expansion were directed to molding and finishing to transition from two to three shifts per day and meet production demands.
ME Elecmetal has explored optimization to reduce the processing time by five minutes, which would result in a heat produced every 60 minutes and a drop in wait time. The metalcasting facility has processed 24 heats in one day on occasion, but rate-limiting steps begin to appear, and often other departments cannot keep up with this pace unless under near perfect conditions.
With the described plant staffing and melt producing a heat every 65 minutes, the 160 ton/day goal is fairly easy to meet consistently. In 2013, the plant averaged 157.8 net good tons/day with planned tons equal to 157.6 tons/day. Market demand decreased slightly toward the end of 2013 resulting in lower production quotas.
Melt Process Optimizations for the Third Furnace
Prior to the third furnace being installed, the melt department had to initiate changes to handle continuous processing and proactively targeted potential bottlenecks in the process.
Scrap inventory is a function of the melt department, and the scrap bay has its own crane specialized for unloading scrap and alloy trucks. In addition, the crane also builds charges—the scrap material being loaded into a furnace. The majority of the scrap bay’s crane time was spent unloading trucks with a magnet and depositing the scrap into its respective inventory bin.
To allow the crane to keep up with building a charge every 65 minutes, ME Elecmetal combined multiple scrap classes into single categories and restructured the bin layout to allow trucks to backup and dump their scrap loads into their respective bin.
Due to limited floor space for truck maneuvering, only the two most commonly used scrap categories were optimized for truck dumping (Fig. 3). Scrap deliveries also were scheduled over all three shifts to avoid crippling a shift with continuous truck deliveries. Since the scrap bay optimization, delays associated with waiting for charges to be built have not appeared.
Ladle responsibility, which had been handled in melting, was shifted to the pouring department to allow melt staff to focus on continuous processing. The ladle repair area shares the same bay as the melt furnaces.
The crane associated with the melt furnaces for transporting ladles, charging, etc., was occasionally required to pick up ladles in ladle repair. This took approximately 15-30 minutes per event. To eliminate the need for the crane for this event, a rack was designed to hold the ladle and allow it to be rotated for repair. The crane is only needed to move the ladle in and out of the rack (Fig. 4). Since the rack has been installed, no delays have been associated with waiting for the crane to return from ladle repair.
Process Optimization With Three Furnaces
After the expansion, delay propagation became more apparent with three furnaces. Processing, rather than melt-down, became the new rate-limiting step for melting. Any delay associated with processing the current heat also offset the processing times of subsequent heats by the duration of the delay.
For example, if the current heat being processed had an event that took 30 minutes to correct, processing the next heat would be 30 minutes later. Often with three furnaces, multiple events coincidently occur that are necessary and need to be completed; however, resources are limited and only one event can be performed at a time. Every situation is unique, but in most scenarios prioritizing the task that affects processing minimizes delay and propagation.
Examples would be back charging a furnace (adding additional scrap to the furnace that could not originally fit when the furnace was charged) or chilling (dropping rail track into the molten bath of the furnace to cool the bath by taking advantage of a thermodynamic phase change). Both tasks require a crane and need to be completed for the heats to progress, but transferring a heat to a ladle is the final step in processing and should be prioritized to begin processing the next heat as soon as possible. The back charge can wait because that will only extend the melt down time for the heat.
Two furnaces were more resilient to delay propagation. During processing of the current heat, the next heat would still be melting down, so the delay would not immediately result in lost production time.
For the first month of production with three furnaces, processing times averaged 85 minutes due to improper prioritization of resources. ME Elecmetal wrote a new critical path for processing heats that defined all the actions affecting the current heat that should take precedence over the other furnaces. Processing is always the prioritized event with three furnaces. Once staff became experienced with the critical path and realized melt down events were no longer the rate limiting step and did not need to be prioritized, processing times dropped to the 65-minute average goal.
Opportunities for Improvement With Three Furnaces
Due to the additional wait time included in the three-furnace process cycle, slag accumulation became more severe than with two furnaces. The additional slag accumulation led to an approximate 30% increase in the frequency of furnace repair events to maintain adequate furnace conditions while also causing more refractory damage during each heat. Furnace repair events also increased in frequency because working all three furnaces together became more cost effective than service a single furnace. Refractory patch use increased approximately 17% on a net good ton basis. Energy usage increased 4% due to the melt operators needing to run the furnace longer to compensate for cooling during the wait time.
The most obvious way to improve the melt process would be to optimize processing times to 60-minute averages, which would decrease the wait periods. Eliminating the wait periods would correct for some of the issues discussed above and increase production rates.
Other departments in the facility would need to be enhanced to account for the additional production levels. Under specific conditions of high mold count per heat or difficult molds, the melt shop may be on hold for molding to produce the correct molds needed to pour the next heat. Any optimization to molding will benefit melt. Additionally, space on the pouring floor is a limited resource and may require additional wait time for the floor to be partially cleared to allow upcoming molds to advance onto the floor. This time is not always available when heats are continually arriving from melt. A disadvantage of vacuum molding is the molds need to be hooked up to the vacuum system until the metal has solidified to ensure proper mold/casting dimensions are maintained. In turn, this results in a much more complicated pouring infrastructure than what is encountered in a typical sand casting facility. A larger floor would allow more heats to be available for pouring and give molding a larger head start. Plant-wide optimization would be needed to fully utilize the production capacity of the melt shop.
Molding and pouring at ME Elecmetal are over 30-year-old processes and have undergone countless improvements; most of the quick fixes to improve process performance already have been implemented. Adding additional capacity would require capital additions to the process.
This article originally was presented at the 2014 Steel Founders Society of America Technical & Operations Conference.