Efficiency of the Casting Process Starts in the Melt Shop

Written by Ryan Brown, Director of Aluminium Sales, North America

First published in Light Metals 2018, The Minerals, Metals and Materials Series by The Minerals, Metals & Materials Society.  View the published article here.

Abstract: Though leading metal casting facilities are converting to newer, more efficient styles of melting furnaces, a majority of the metal casting industry in North America still uses reverberatory (reverb) furnaces. This paper/presentation compares the melting process of efficient shaft/tower style melting furnaces to the common reverb furnace, highlighting the numerous ways in which shaft/tower melting furnaces can save money for the owner and increase the bottom line, while improving metal quality.
The primary topic involves a study or comparison of gas consumption and metal loss, which are the two main cost drivers in the melting process. Further advantages of shaft/tower melting furnaces include a reduced environmental impact (CO² emissions), higher quality and more temperature stable metal, ease of cleaning, methods of automation, greatly reduced dross and sludge formation, and further savings through energy rebate programs.

Melting Process

The main advantage of using a shaft melter (also referred to as a stack or tower melter) to melt ingot, scrap parts, and returns is the utilization of waste gases from the holding bath to pre-heat the charge material. Not only does this reduce overall gas consumption in the melting process, but it allows for cleaner, more temperature-stable metal in the holding bath for use in the casting process.

Using a charging bin, the material is loaded into the shaft and held at optimal levels of 60% of the total shaft capacity or higher. It is important to achieve a high shaft density to trap the waste gases from the holding bath; this relates directly to the gas consumption and efficiency of the furnace. The main methods to ensuring that the shaft remains at a high charge density are to a) use a shaft filling laser to measure the physical height of the charge and/or 2) measure the temperature in the flue. When using the method of measuring the temperature in the flue, exceeding the typical set temperature of 250 - 300 °C will tell the furnace that the shaft density is too low and more charge material is required. In contrast, flue temperatures in reverb furnaces often exceed 1000 °C.

As the aluminum charge material rests in the shaft and is pre-heated (figure 2), it does not reach the melting point of ~ 660 °C, rather it only reaches a “plastic” state as it nears the bottom of the shaft before falling onto the melting bridge. Ideally, there is no direct flame on the material in the shaft. While all shaft melters pre-heat the charge material using waste gases only, many tower or stack melters utilize a direct flame to heat the material, which has drawbacks related to metal loss, cleaning practices, and increased gas consumption.

After the aluminum in “plastic” state fall onto the melting bridge, it requires very little energy to reach the melting point of ~ 660 °C. With proper burner placement, from above and not impinging on the metal, the process to transform the aluminum from “plastic” state into molten state is very short and efficient.

The molten aluminum now follows the contours of the refractory of the melting bridge and through the wonders of gravity, flows into the melting bath. Melting baths on shaft melting furnaces are typically sized to be 1-2 times the hourly melting capacity of the furnace, compared to typical reverb furnaces where the bath size reaches 10 times the hourly melting capacity of the furnace! The bath is held at a set temperature via on/off burners, which hold the bath to within +/- 5 % of the set temperature. Level probes measure the bath height. When the bath nears capacity, the melting burners automatically turn off to avoid new molten aluminum entering and overfilling the bath.

There are multiple advantages of shaft melting furnaces having a greatly reduced holding bath size. First of all, the molten metal inventory is a fraction of that which is held in a reverb furnace. For example, a shaft melting furnace with a 6,000 lb. melt rate would have a bath of approx. 9,000 lbs. A reverb furnace with the same melt rate would have a bath of roughly 60,000 lbs. Secondly, it requires much less energy to hold the bath to the set temperature. Gas holding burners in a reverb furnace are usually set to “high fire” or “low fire”. In a shaft melting furnace, the burners will turn on and off as needed.

Finally, the ability to hold temperature at a consistent temperature has an impact on various parts of the casting process and the use of a shaft melter vastly reduces sludge formation, dross formation, and the absorption of hydrogen as compared to reverb furnaces. Dry hearth reverb furnaces pre-heat and partially melt the charge material (typically T-Bars or sows) on the dry hearth before the remaining and unmelted core of the charge material is pushed into the bath to make room for new charge material. The process of pushing “cold” materials into the bath is often compared to putting ice cubes in a cup of coffee. The temperature in the bath reduces, causing the burners to work harder to make up for the temperature reduction. This is where the challenges begin. First, the drop in temperature in the bath causes aluminum sludge to form on the bottom of the bath - sludge is the common term for a semi-solid form of aluminum which many times contains contaminants and sinks to the bottom of the holding bath From a metallurgic standpoint, when the temperature of molten aluminum swings below 1225° C, aluminum sludge forms. Next, the bath burners switch to “high fire” to make up for the reduced bath temperature. As this happens, the surface of the bath is exposed to even more heat and air, causing dross to form at a rapid pace. As the layer of dross continues to thicken on the surface of the bath, the metal below the bath becomes more insulated and struggles to hold temperature. As the much larger bath in reverb furnaces is difficult to keep at temperature, the burners must superheat the metal from the top to reach the process temperature at the depth of the tap. 1220 F is the critical temperature at which aluminum begins rapidly absorbing hydrogen. The higher the metal is superheated, the more hydrogen is absorbed, which directly affects the density index of the aluminum. A higher density index, or simply a higher hydrogen content in the molten aluminum, causes further challenges in transferring and holding the metal temperature to the requirements of the selected casting process. The result could look like this for a casting process with high metal quality requirements: a higher hydrogen content means that a longer degassing time is required, the longer the metal is degassed, the more temperature is lost; the more temperature the metal loses in the process, the harder the holding furnace has to work to hold temperature, creating a situation where many die casters have problems keeping their metal up to the temperature required for their casting process.

Conversely, using a shaft melter which keeps the metal bath temperature in a range of +/- 5 %, such tighter control allows for reduced degassing times, less temperature loss during the transfer process, and metal with a temperature closer to the required process temperature being delivered to the holding furnace.

It is also important to mention that shaft melting furnaces typically range from as low as 750 lbs. /hr. to as high as 15,500 lbs. /hr. in terms of melting capacity.

Gas Consumption

One of the two major cost drivers in the melting process is the use of natural gas. Depending on the type, age, and condition of the furnace, gas consumption can range from as low as 850 BTU/lb. to well over 2,500 BTU/lb. Using information provided in a study that was prepared for the DOE [1], “stack melters” were recorded using 861 BTU/lb. and reverb furnaces were recorded using 2418 BTU/lb. Let it be noted that the study was conducted in 2004 and advances in technology from both types of furnaces has advanced and gas consumption improvements have been achieved, such as improved cascading controls and regenerative burners on reverb furnaces and improved shaft and body designs on shaft melters.

For the purposes of this exercise, however, let us use the gas consumption of 950 BTU/lb. for a common shaft melter and 1,750 BTU/lb. for a fairly efficient cold-air reverb furnace.

Gas consumption is typically calculated using the BTU/hr. required to melt the aluminum plus the BTU/hr. required to hold the metal bath at temperature. For this exercise, the above numbers include both the melting and holding gas requirements.

We will also assume the following details:

  • 20 hours production per day
  • 250 working days per year
  • $0.40 per Therm of natural gas (1 Therm = 99,976.1 BTU)
  • Melting furnace with 4,400/lbs. of melting capacity per hour

Annual aluminum production can be calculated below

Annual aluminum production can be calculated in Eqn. 1.
Finally, a savings in gas consumption also relates directly to a reduction of CO2 emissions. Permitting becomes easier as metal casting facilities who convert from reverb furnaces to shaft melting furnaces will only decrease their emissions, effectively creating additional melting capacity without the need to apply for new local and state permits.

Metal Loss

Metal loss is often referred to as the “silent killer” in metal casting facilities. The entire process of melting, transferring, and holding metal before the casting process greatly impacts the amount of metal loss that various metal casting facilities experience. For this exercise, we will focus on the metal loss associated only with the melting process as it is the most significant.

According the same study performed for the DOE in 2004, it was reported that reverb furnaces recorded a metal loss of 5.5 % whereas “stack” melters recorded a metal loss of 1.25 %. Again, as with gas consumption, there have been technological improvements in furnace technology that will have reduced the amount of metal loss experienced in newer reverb furnaces. This, combined with proper cleaning practices, could result in much lower metal loss numbers for reverb furnaces.

Metal loss during the melting process happens in a number of ways. The most significant is the formation of dross on the bath surface due to exposure to the gas burners and atmosphere. Sludge formation in the bath, as covered in previous sections, contributes to metal loss. Finally, it is common for metal casting facilities to dump all returns, including very thin-walled returns, back into the reverb furnace by means of a side pocket which exposes a section of bath to atmosphere. Thin-walled returns (flash or very thin parts, for example), can burn up or oxidize before being transformed into usable molten aluminum.

For the purpose of this exercise, we will assume a metal loss number of 1.5 % for shaft melting furnaces and 3 % for reverb furnaces. Furthermore, we will assume the price of $ 0.90/lb. of aluminum ingot. This price of course fluctuates throughout the year, and will vary from alloy to alloy. Using the annual production number from the gas consumption calculation, metal loss can be calculated below.

Using the annual production number from the gas consumption calculation, metal loss can be calculated in Eqn. 4.

The total gas consumption and metal loss savings equals


$70,400 gas savings + $297,000 metal loss savings = $367,400 annual savings


It is important to remember that these are annual savings, not one-time savings or incentives on the purchase of a shaft melter. With proper use and proper cleaning practices, these savings can be realized year after year.

Energy Rebate Programs

In 2014, Chicago White Metal (CWM) installed a shaft melting furnace in their die casting facility. With the help of the furnace supplier, CWM applied for a state energy grant (NICOR) which reimbursed the manufacturer a specific dollar amount (one-time payment only) for every therm saved over either incumbent or common industry equipment.

The process begins as the agency conducting the state rebate program reviews the customer’s current technology and gas consumption. These are then calculated against the gas consumption of the new furnace being installed to provide an estimate on annual gas savings and an estimate on the amount of rebate the customer can expect. After the new shaft melting furnace was installed, gas consumption numbers are verified and once again measured against the gas consumption of the incumbent technology. For CWM, the amount of therms saved was significant enough to warrant a rebate from the NICOR program in Illinois. In addition to this one-time energy rebate, CWM also experiences recurring savings due to reduced gas consumption and metal loss.

It is important to note that energy rebate programs vary from state to state and also vary in terms of availability. Consult with your furnace supplier to review if an energy rebate program is available in your state.

Cleaning Practices

Though not all shaft melting furnaces are designed in the same way, the process of cleaning the furnace is simplified insofar as the bath and melting chamber have a smaller footprint, and subsequently the tools required to clean the internal furnace walls are much shorter and easier to handle.

Shaft melting furnaces are typically cleaned once per shift, and the cleaning should last somewhere between 15-20 minutes. First, the shaft of the furnace is melted free, which is simply and shockingly called “free melting”. After the shaft is empty and no more metal can drop from the charge area, the door to the melting bridge is opened. Tools provided by the furnace manufacturer are used to scrape the walls free of any oxide build up. (It is imperative that the walls are cleaned on regular intervals to avoid the formation of corundum, which can quickly deteriorate the refractory lining and cause the furnace to become less efficient. If the corundum goes unchecked, it will ultimately expand and choke off the furnace, causing a larger scale repair to be required.)

After the walls are cleaned, there should be a mixture of dross/oxide and some residual aluminum left on the hearth of the melting bridge. This pile of material is raked over to separate dross from aluminum, with the dross being removed and collected in dross bins (figure 4), while the aluminum remains on the hearth to eventually be melted and transferred to the holding bath.

Removal of dross from melting bridge using shorter cleaning tools and dross roll for easy handling.

The next step is to clean the holding bath, which is typically a simpler task. The walls of the holding bath normally do not experience splashing or excessive corundum growth, though it is a real possibility. While the bath should be slightly drawn down (remember, we have not melted fresh metal into it for 10-15 minutes by now, and metal can continue to be drawn out of the furnace during cleaning), a band of oxide is normally present around the upper limit of the bath and the depth to which it is usually drawn down. This band of oxide should be cleaned, along with any wall build up. When this is complete, the dross is simply scraped off of the surface of the bath and also collected in dross bins.

For cases where corundum has already built up or for walls that are particularly hard to clean, fluxes can be used, however be aware that flux usage should be discussed with the furnace supplier to ensure that the flux is not damaging the refractory in the furnace.

There are two types of standard fluxes that are used during the furnace cleaning process: metal cleaning fluxes and furnace cleaning fluxes. The metal cleaning fluxes are typically used in the bath to remove impurities and can normally be used each shift during cleaning (ratio 1 lb. per ton of bath). Furnace cleaning fluxes are much more aggressive and should be used sparingly (2-3 times per month maximum) as to not damage the refractory. Furnace cleaning fluxes can superheat the furnace chambers, which overheat and dry out the refractory, causing it to become brittle. This brittleness then causes refractory to break off of the wall, requiring premature relining or patching. Proper usage of furnace cleaning flux involves sprinkling the flux onto the remaining material on the hearth of the melting bridge. The flux should be mixed into the residual material, after which the melting bridge door is closed. After waiting a few minutes for the flux to activate, which heats up the chamber, the door is opened and the wall cleaning can begin as described above. The walls should now be very easy to clean; the rest of the process remains the same as previously described.

If the furnace is cleaned according to the supplier’s recommendation, a refractory life expectancy of 5-7 years can be realistically achieved. Of course, other factors can influence the life of the refractory, but proper and persistent cleaning remains key to refractory life.

Automation

Metal casting facilities are continually searching for ways to become more efficient, be it upgrading technology, integrating tighter process controls to reduce scrap, or automating where possible. With regarding to shaft melting furnaces, there are different levels of automation that can be utilized to both keep the furnace running efficiently and to handle the charge material entering the furnace.

Charging Bins

With very few exceptions, shaft melters use charging bins to load ingot, factory returns, and scrap castings into the melting shaft. Common bins have dimensions 37” x 37”x 24” (not including wheels or fork lift slots) and are designed with 4 walls (for scrap and returns) or 3 walls (to load ingot stacks via forklift). 4-sided charging bins are placed underneath trim presses to collect trimmings, while 3-sided bins are staged near the furnace loaded with ingot.

The standard charging practice is to utilize 50% ingot and 50% returns/scrap in the shaft. At the beginning of each shift, a few bins of returns/scrap should be charged to create a “pillow” of material, upon which the heavier and higher density ingot can be charged. This practice lengthens the life of the refractory in the shaft by minimizing the direct impact of loading ingots on an empty shaft.

During normal continuous operation, the shaft should be kept at a minimum of 60-70% full to reach maximum efficiency.

Standard Charging Units

A standard charging unit uses only one charging bin which is either fixed into the charging unit or which can be exchanged for a full bin when empty. The most common practice is to offload the empty bin and reload a full bin via forklift. The furnace will call for the bin to charge automatically. In this process, the full bin is raised via an elevator on the charging unit and is tilted into the shaft from the top. The empty bin returns to the bottom of the charging unit, where it can be exchanged for a full one.

This is the most basic and most common form of charging utilized with shaft melting furnaces today.

Over/ Under Charging Conveyor

The so-called over/under conveyor (figure 3) acts as a buffer to feed the charging unit with full bins as soon as they are emptied into the furnace shaft. This 8-position conveyor includes 4 positions on the lower level for full bins and 4 positions on the upper level for empty bins. During the process, full bins are loaded via fork lift onto the lower conveyor and empty bins are removed from the upper conveyor. This system simply allows the furnace operator to load 4 full bins, which will be automatically charged into the furnace when the material in the shaft is moving to the melting bridge. Depending on the size of furnace and size of the bins, the furnace operator can then complete other necessary tasks in the metal casting facility while the furnace automatically charges.

Scrap Return Conveyor

Some metal casters opt to use a conveyor system in their facility to collect and return scrap (for example from the trimming process) to the melting furnace area via conveyor system, which is often times an under-floor system.

After the scrap arrives to the melting area, it is charged into 4-sided bins, which can then be indexed to the charging unit on the furnace for automatic loading. In this scenario, best practices call for loading the bottom of a 4-sided bin with one or two rows of ingot and filling the rest with returns or scrap castings. This creates the “pillow effect” on each charge into the furnace.

In this scenario, the over/under conveyor is not utilized, rather a more extensive conveyor system which indexes bins in a loop from material loading to charging the furnace to a queue of empty bins, and so on.

Automatically Guided Vehicles

One of the more recent developments in the automation of melting furnaces is the use of AGV’s, which are of course not inherently new, just new to us. In the standard scenario of placing charging bins underneath trim presses to collect trimmings and loading/unloading either a standard charging unit or one with an over/under conveyor, the AGV take the place of the fork lift operator and is always taking care that the furnace is loaded with a full bin of material. Furthermore, the AGV is responsible for placing empty bins either under trim presses or in staging areas, loading bins in alternate fashion between ingot and returns/scrap material, and removing empty bins from the melting furnace after they have been used to charge material into the shaft.

AGV’s are already being used in this fashion in a handful of metal casting facilities in Europe, and have more recently found their way into the North America die casting industry, by way of a European parent company.

Summary

Shaft melting furnaces continue to gain acceptance in North America. While they have not enjoyed the same status of “standard technology” as they do in Europe, more and more metal casters are taking advantage of the efficiencies in gas consumption and metal loss reduction that shaft melters have to offer. The installed base in both OEM automotive companies and their tier 1 suppliers continues to grow, especially as the demand for higher integrity die castings continues to rise.

Shaft melting furnaces offer further advantages in terms of overall metal quality, metal temperature consistency, reduced hydrogen content, cleaning, and automation.

Finally, suppliers of shaft melting furnaces can provide interested metal casters with ROI calculations that are based off of customer data and are therefore individualized for each project.

References

Online Document

J.F. Schifo, J.T. Radia (2004) ITP Metal Casting: Theoretical/Best Practice Energy Use in Metalcasting Operations.Kerimada Environmental, Inc. p 39. https://energy.gov/sites/prod/files/2013/11/f4/doebestpractice_052804.pdf Accessed 25 September 2017