Should Your Foundry Be Reclaiming Its Sand?
Is your foundry a likely candidate for the installation of a sand reclamation unit? If so, what do you need to know to select the proper equipment?
As with any capital equipment purchase, many questions arise and specifications must be considered to ensure a good fit between the foundry operation and the new reclamation system. This article explores some of those questions as well as alerts foundries to common pitfalls in the reclamation system selection process.
Questions to Ask
How much am I currently spending on sand? This question must be asked to determine if sand reclamation will be a cost-effective alternative for your foundry. When compiling information on the cost of the sand used, include the total tonnage of sand used in the foundry, the price of the sand and its delivery cost, the disposal cost for spent sand, tipping fees, and all labor, handling and storage costs associated with spent sand. This figure then will need to be compared to the total of all the costs involved in operating a reclamation system to measure potential economic benefits.
Is my foundry currently isolating the spent sand from other waste streams? To effectively reclaim foundry sand, the sand entering the reclamation unit must be free of metallics, dust collection system waste, contaminants and other materials that may be combined and melted onto the surface of the reclaimed sand as it exits the reclamation unit.
Mechanical, pneumatic, thermal and wet reclamation systems have advantages and disadvantages. The selection of the proper equipment is predicated on the foundry's metal, casting size and design, binder system, and degree of processing required to provide a suitable reclaimed sand.
Mechanical and Pneumatic Systems--Mechanical and pneumatic scrubbers are the least expensive units and typically require the least floor space for their installation. Mechanical reclamation uses abrasion to break up sand and remove the binders, and pneumatic reclamation uses air to impact the sand against other sand grains and hardened surfaces to eliminate the residual binders. These units can remove the residual binders from the sand grain's surface, but they will not extract the binder imbedded in the cracks and interstices of the grains.
The grain structure of the sand affects the efficiency and yield of all reclamation units. Angular sand grains degrade much faster than a rounded grain structure, increasing dust for disposal and lowering system efficiency. Due to the aggressive scrubbing action of the mechanical and pneumatic scrubbers, these units are more sensitive to the physical structure of the sand.
Mechanical and pneumatic scrubbers have an advantage--they can be operated on an "as needed" basis. Most sand reclamation manufacturers will recommend that the foundry purchase a lower capacity unit that can be operated on a 24-hr schedule rather than a larger unit that can produce the required sand during a one-shift operation. In addition, a foundry can purchase two smaller units rather than one large unit to minimize the impact of lost time due to maintenance and the effects of production downsizing.
Thermal Systems--Thermal reclamation uses heat to burn away binders and other materials in the sand. The thermal reclamation unit can effectively remove 90-100% of the residual binder from sand, but it requires a greater quantity of energy to heat and cool the sand, and the exhaust gases must be permitted and monitored. Excessive temperatures [more than 1400F (760C)] can affect the physical performance of some foundry sands.
Lake sands (used extensively in the automotive foundries in the U.S.) contain minerals in the form of carbonates. These carbonates (calcium, sodium, magnesium and potassium) undergo a transition when heated. The carbonates lose carbon dioxide, making them more soluble and reactive with the binders and hardeners during any subsequent use. This same phenomenon occurs at the mold-metal interface when the casting process heats the sand, but the majority of the sand in the mold does not achieve the temperature required to calcine or thermally alter the sand.
Manufacturers of thermal reclamation units suggest that the foundry utilize the minimum temperature that will remove the residual binder system effectively. Minimizing the temperature in the reclamation process will provide energy savings and longer refractory life while reducing the damaging effect of altering the sand's reactivity.
Changing the solubility of these minerals by exposing them to high temperatures will increase the pH value. This change in reactivity may either depress or accelerate the hardening rate for the binder system depending on the type of binder used. With certain sands, changing the type of sand, the type of binder, or the ratio of hardener to binder may be necessary to achieve satisfactory results with the reclaimed sand.
Wet Systems--Several large wet reclamation units are operating in the U.S., but they are limited to processing sand from green sand or sodium silicate bonded operations. Wet reclamation uses water to scrub and remove undesirable materials from the surface of the sand particles. The water must be processed and treated prior to disposal. Since both the green sand and sodium silicate binders are basic (more than 7 pH), the disposal of this water can be problematic. After the sand has been washed and passed through a vacuum filter to remove the majority of the water, the sand must be dried and cooled prior to its reuse.
Beyond the purchase of a sand reclamation unit, additional equipment may be required to ensure an efficient operation.
* Most foundries have a shakeout unit for removing the castings from the molds, but these units do not effectively reduce the lumps from sand molds to a usable size. A reclamation unit may require the sand to be crushed by a vibrating crusher to a size suitable for processing.
* The sand must be transported from shakeout to reclamation by oscillating conveyor, hot belt or a vehicle loader.
* Since the reclamation unit does not function well with metal flash, gaggers, rebar, castings, etc., the foundry should install a scalping screen or magnetic separator to remove these materials prior to processing.
* Reclamation units are efficient when the quantity of feed sand is consistent. Since most shakeout units do not provide a constant and consistent discharge rate to feed the reclaimer, the foundry must install a hopper or bin to feed the sand.
* To place the sand into the hopper or bin, the foundry may require the installation of an inclined belt, bucket elevator or receiving hopper.
* The discharge from the hopper or storage bin must provide a consistent feed to the reclaimer. This feed rate can be controlled with a gravity-fed gate or a vibratory feeder.
* Regardless of the type of reclamation unit, the sand will have to be cooled prior to reuse as almost all chemical binder systems are sensitive to sand temperatures and moisture content. Therefore, a cooler, water or water/air fluidized bed will be required.
* After discharge from the cooler unit, the sand should be passed over a scalping screen to remove the agglomerated grains and undesirable materials that may have entered the system.
* After cooling and scalping, the reclaimed sand must be transported by conveyor belt and bucket elevator or by a pneumatic transport system to a storage silo so it can be fed to the mixers or mullers. Since the reclamation unit will return most (70-95%) of the reclaimed sand into the storage silo, the current sand silo may be suitable for this purpose. However, the reclaimer will not return 100% of the feed sand into the storage silo, so an additional silo will be required to store the makeup sand needed to maintain the quality and quantity of sand required by the foundry. This additional silo also may be required when maintenance of the reclamation unit forces the foundry to revert back to purchasing all new sand.
* A foundry using reclaimed sand will blend it with new sand at varying ratios to provide the characteristics demanded by the mold or core applications. To provide this level of control, a system for blending the reclaimed and new sand at the desired ratios will be required.
* In addition to the ancillary equipment discussed, the foundry must have sufficient dry or wet dust collection capacity to handle the added requirements of the reclaimer, cooler and transport system.
If the decision has been made to purchase a reclamation system, the following questions must be asked to ensure a successful, efficient and cost-effective installation and start-up.
* Prior to deciding to install a reclamation unit in your foundry, contact your binder supplier for its recommendations regarding the effect that reclamation may have on its product's performance. A foundry using a mechanical or pneumatic reclamation scrubber for an acid catalyzed binder may have to change to a catalyst with a lower concentration due to the residual hardener left on the sand grain surface.
* Contact the reclaimer manufacturers and ask for a list of their customers with molding and casting processes similar to your foundry. Contact these and ask about the overall cost of the equipment and all associated costs for the ancillary equipment, fuel, maintenance, spare parts and labor. Find out if the equipment has excessive downtime and if a large quantity of spare parts must be maintained in inventory.
* Contact your local gas and electric providers to determine if they have any research funding for investigating energy conservation projects that may apply to your installation. Contact local, state and federal agencies to obtain any matching funds or energy conservation funding that may be available.
* Investigate all process and exhaust permits required for your installation.
Once a reclamation system is installed, foundries must remember that they are their own sand supplier. The specifications that have been established for new sand will have to be monitored and controlled on the reclaimed sand.
This article is not intended to scare anyone away from implementing a sand reclamation system but rather to start the thinking and planning process prior to making a decision. Sand reclamation is in your future. Governmental, environmental and economic forces will make it happen in your foundry.
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|Comment:||Should Your Foundry Be Reclaiming Its Sand?|
|Date:||Aug 1, 2001|
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