Research, planning aid selection of sand reclamation system.
Buried in the broad national effort to clean up the environment lies the foundry industry's huge problem with how to cope with skyrocketing waste sand disposal logistics and costs. Like other industries, it must comply with federal, state and local mandates to control hazardous manufacturing residues. Fall River Foundry, Fall River, Wisconsin, caught in the vise of severe regulations on spent sand disposal, attacked its problem head-on.
A special team was assigned to study emerging sand reclamation technologies that could reduce sand disposal or eliminate the problem entirely. The team ultimately selected a gas-fired thermal sand reclamation system manufactured by Milwaukee-based Allis Mineral Systems. The system satisfied the foundry's foreseeable needs by recovering nearly all of its spent sand and became operational in December 1991. The following synopsis relates how the system is saving the foundry over $1 million annually.
A producer of brass castings, Fall River Foundry produces an average of 16 tons of equal amounts of green and chemically bonded waste sand per day. Green shakeout sand, blast cabinet sand and floor sweepings are collected in hoppers and carried by a fork truck to a holding silo in the sand reclamation building. The returned sand goes through a lump breaker, where lumps are reduced to granular size and screened to remove tramp material. A low-intensity permanent magnet removes steel.
The sand is screened again prior to delivery to the kiln or calciner. It is fed at a controlled rate into the calciner by a variable-speed drive motor on the conveyor. The calcine process applies heat (above 1400F) to the sand long enough to "dead burn" the sand's clay component, breaking its adhesion so clay can be separated from the sand grains.
The controlled atmosphere calciner is a natural gas, direct-fired, rotary unit whose effectiveness is enhanced by a unique method for introducing secondary combustion air via an air lance. Lance air is preheated with recovered heat from the rotary cooler and blown into the calciner's solids bed to oxidize the organic binders. Introducing secondary combustion air aids the complete oxidation of the binder and avoids the production of fixed carbon, or "coke," by creating an oxidizing zone at the lance.
The internal heat generated by the combustion of organics within the bed allows the sand to reach calcination temperature quicker to effectively extend the calcination zone.
After calcination, the sand is processed though a rotary cooler where waste heat is recovered in two modes:
* ambient air is drawn into the interior of the cooler at its discharge end, where it is in direct contact in a counterflow interaction with the sand. The hot sand transfers heat to the air by a combination of convection and radiation.
* ambient air is drawn into the bottom of a cooling shroud surrounding the hot end of the cooler and heated by convection and radiation from the hot shell to the air moving in a cross-flow relative to the cooler.
The cooled sand is screened one more time before passing under a rare earth high-intensity magnet for further classification. To prepare calcined sand for rebonding, metallic particles and dead burned clay adhering to silica grains must be removed.
Separate tests on sand were run using a rare earth magnet and mechanical scrubber, which revealed that the magnet alone produced sand of sufficient quality for rebonding. Therefore, only the magnet was installed.
Fugitive dust is collected by the baghouse from the lump breaker, exit gas of the calciner cyclone, exit gas of the cooler cyclone, the permanent magnet, the rare earth magnet and the feed storage silos.
Both the calciner and cooler are equipped with single-cell cyclones through which dust is initially separated from the gas exiting each unit to minimize the dust rate exiting the calciner system. Dust collected by the calciner exit gas cyclone is returned to the boot of the bucket elevator carrying spent sand to the calciner to eliminate a hazardous waste collection point.
This practice is cost-effective because it reduces the amount of clean sand being carried to the baghouse. It also demonstrates that good dust collection design is important not only for environmental reasons, but for recovery of silica sand particles. About 3% of the total silica volume processed reports to the baghouse and the volume of silica particles lost in the calciner exit gas is negligible. Most of the material collected by the baghouse is composed of fugitive dust and clay from the calciner.
To maximize control and energy efficiency of the dust collecting process, the foundry uses a variable-frequency drive to control the baghouse fan speed which adjusts the speed to produce the minimum duct pressure required to achieve particulate suspension and proper air flow.
Controlling duct pressure also saves energy at the fan motor. Adjustable dampers set the appropriate draw at each collection station. Use of this drive results in an energy savings of over 100,000 kWh/year, equal to $3600. The foundry received an energy savings rebate from Wisconsin Power & Light Co. for installing this variable frequency drive.
The foundry reports that 25% of the treated sand is suitable for foundry reuse in either core or floor molding while 72%, although not reusable, is nontoxic and suitable for a standard landfill. The remaining 3% that is collected in the baghouse is classified as hazardous waste due to heavy metal content, and requires disposal in a hazardous waste landfill. Previously, under new disposal regulations, 100% of the sand would have been classified for hazardous waste landfill disposal.
TABULAR DATA OMITTED
The new system was sized to accommodate the foundry's present needs for two eight-hour shifts and allows for increased volume by adding another weekday or weekend shift. The foundry further reduced energy costs by purchasing natural gas on the spot market and by buying electricity under Wisconsin Power & Light's interruptible rate option. The net energy costs are $0.30/therm for natural gas and $0.036/kWh for electricity.
Energy use was measured for a 24-hour period of normal system operation using submeters set on the service entrances of the sand reclamation building and the quantity of sand processed was logged. Over the test period, the foundry processed sand for 12 rather than the normal 16 hours at a rate of 1834 lb/hr (0.917 tons). The process rate ranges from 1700-2200 lb/hr, depending on sand type being processed. The system was maintained at idle for the remaining 12 nonproduction hours to minimize warm-up time after testing and to extend the life of the calciner's refractory liner.
For the 12-hour period that sand was processed, 253.1 therms of gas (one therm = 100,000 Btu) and 483.4 kWh of electricity were used. The 12-hour idle period used 116.9 therms of gas and 184.5 kWh of electricity. The foundry currently idles at a higher temperature than necessary because of turndown limitations of the burner, but is modifying the burner to permit the calciner to be idled at a lower temperature to save idle period fuel cost.
At $0.30/therm, gas to process sand during production costs $6.33/hr ($6.90/ton) vs. $2.92/hr during the idle period. At $0.036/kWh, the electricity charges were $1.45/hr ($1.58/ton) during production and $0.55/hr at idle.
During a normal operating year, the foundry has 4000 hours of sand process time and 4760 hours of idle time. On an annual basis, it processes 3667 tons of sand for a net annual reclamation energy cost of $31,120 plus $16,517 to hold the system at idle or a net cost of $12.99/ton.
The day and night shifts require one attendant each to operate the system, which is unattended at idle. Attendants start, monitor and shut down the system, coordinate sand handling and perform routine maintenance. Forklift operators who move sand to and from the reclamation building also contribute time to the sand reclamation process. Forklift operators account for an additional three labor hours per shift. At a labor rate including benefits of $25/hour, the labor cost to operate the reclamation system is $34.38/hr or $37.50/ton.
The hot, abrasive environment of the sand reclamation system is a harsh test for the equipment included in the system. For instance, there are 31 electric motors used to power the system. Routine maintenance of belts, pulleys, motors, fans, electrical controls and connections is done by a full-time attendant (accounted for in the previous section). Maintenance help from other foundry employees is needed for one hour per week at $0.34/ton. Routine replacement of parts (bearings, belts, etc.) averages $1.45/ton (1% of the installed system's cost).
The major maintenance item is the replacement every 10 years of the calciner's castable refractory liner costing $22,000, or an additional $0.60/ton.
The annual maintenance cost for the new sand reclamation system nets out at $2.39/ton.
Installed, the system cost $530,000. A breakdown of the cost is summarized in Table 1. Annual depreciation over 10 years is $58,163. This adds $15.86/ton to the operating cost of the system.
Fall River's delivered cost of new sand is $30/ton and the disposal cost (including trucking to a standard landfill) is $10/ton. Trucking and disposal of contaminated sand in a hazardous waste landfill is $400/ton. The foundry generates 3667 tons of waste sand per year that, under current regulation, is classified as hazardous waste.
As reported earlier, the system is capable of recycling 25% of the sand for reuse in the foundry and 72% is detoxified for disposal in a standard landfill. The remaining 3% is captured in the baghouse for shipment to a hazardous waste landfill. The cost comparison of sand purchase and disposal with and without sand reclamation is shown in Table 2. The cost with the sand reclamation system is $41.70/ton; without the system $430/ton.
The foundry's total cost for sand with the reclamation system includes the costs to buy new sand and dispose of the waste sand, plus the sand reclamation system's capital depreciation, energy, labor and maintenance costs. The total of these costs is $110.44/ton. Without sand reclamation, the cost of sand would be cost of new sand plus disposal costs ($430/ton). The net avoided cost is $319.56/ton, for an annual savings of $1,171,827.
Tips from Experience
Fall River's sand reclamation success is the result of prudent planning, careful research and implementation. In retrospect, the foundry would make only two changes to its existing system.
Early research found the cost of a new building adjacent to, but separate from the foundry, was less expensive than adding on to the existing foundry. Based on equipment dimensions, the foundry constructed a building 50 ft wide x 60 ft long x 30 ft high. The equipment fits nicely, but more space for easier forklift movement and dry sand storage would have made the building ideal.
Using 300 |degrees~ cartridges in the baghouse rather than 150 |degrees~ cartridges would have also been preferable. The 150 |degrees~ cartridges require that dilution air be bled into the dust collecting system to cool the air before it gets to the baghouse. This extra air requirement adds operational costs to the dust collecting process because a larger baghouse fan is necessary. Experience shows that cartridges, more effective than bags for removing fine dust particles, do not hold up as well as bags when moisture is present in the gas stream.
To minimize maintenance delays and expenses, sand and air handling equipment was chosen that is common to the type used elsewhere in the foundry, minimizing the need to stock special replacement parts.
Even with the sand reclamation system, Fall River Foundry still faces serious problems with its sand disposal. Nearly 75% of its sand must be taken to a landfill and landfill space is running out. Of Wisconsin's 1046 licensed landfills in 1985, only 250 remained in 1992. The foundry continues to work on improving its reclamation process so that more sand can be reused in the foundry. Its objective is to increase the current level of TABULAR DATA OMITTED reusable sand from 25% to 60%. One way it will do this is by changing the process flow in the core molding area. Presently, sand to certain core molding machines is distributed from the main sand storage silo to the core molding floor. More recycled sand will be used when the foundry separates these core molding machines from the main sand conveying system and establishes a separate sand feed system dedicated to feeding only recycled sand to these machines.
Other opportunities may lie in alternative type of binders to make sand more suitable for thermal reclamation.
Fall River Foundry's sand recovery work exemplifies ecological awareness and its decision to invest in the system only after careful study of its options proved economically sound.
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|Date:||May 1, 1993|
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