Economics, quality lead move to reclaim clay-bonded sand.

Recovery gains, improved process controls and reduction in new foundry sand and disposal costs justify capital outlays.

The reclamation (recycling) of foundry sands is of increasing importance to the foundry industry because of the rapid closing of landfill sites and the prohibitive cost of opening new ones. Limiting the generation of all types of spent (waste) sand is becoming the rule rather than the exception. Restricting and finally banning the dumping of classified wastes has increased interest in alternatives to dumping, as has the rising cost of new sand.

Thermal or thermo-mechanical sand reclamation systems are the only economical technologies available that satisfy Resource Conservation and Recovery Act (RCRA) requirements. Current environmental laws make sand reclamation essential for several reasons:

* Spent sand stream volumes can be reduced 85-95%, leaving only 5-15% of the original volume of spent sand classified as waste. Disposal costs can be reduced by nearly the same amount, even after adjusting for direct reclamation costs.

* Special permits are not required normally by local, state and federal agencies for the reclamation of a natural resource. Required permits are necessary, however, for industrial equipment installations.

* New sand costs can be reduced by 90-95%.

* Local, state and federal agencies endorse reclamation as an alternative to disposal in land fills.

Sand reclamation corroborates EPA's waste strategy that states: "Energy/material recovery is the reuse and recycling of wastes for the original or some other purpose, such as material recovery or energy production."

Recycling sand is prudent because direct costs associated with the castings are lowered as new sand purchases are reduced and disposal costs are minimized.

More significant than chemically bonded sand reclamation is the reclaiming of waste sand taken from molding lines. Foundries are reclaiming 100% clay bonded molding sands, 100% chemically bonded molds and cores (core butts) or a random mixture of both.

There are several successful U.S.-made thermal sand reclamation systems for clay-bonded sand, more than reported by any other country. They are said to be the only systems processing 100% clay-bonded or mixed sands to a condition comparable to new sand suitable for cores and as new sand replacement for molding sand additions.

An analysis of a typical thermally reclaimed sand/new sand comparison is shown in Table 1.

The reclamation system feed rates vary from 500 lb/hr to six ton/hr. Operating unattended 24 hr/day, a six ton/hr unit processes up to 144 ton/day. Thermal sand reclaimer throughput is tailored to a foundry's needs, assuring that reclaimed sand for coreroom and additions is available. A sand recovery system can eliminate up to 90% of the new sand requirements and 75-80% of disposal costs normal to green sand systems. A successful installation avoids problems with varying acid demand values and AFS grain fineness numbers that create numerous binder formulation difficulties.

Table 1. Comparison of Typical Thermally Reclaimed Sand to New
Sand
                                New      System     Reclaimed
                                Sand     Sand       Sand
Loss on Ignition (%)            0.10      4.41       0.06
Live Clay - MB Clay (%)         0.00      9.24       0.00
Total Clay - AFS Clay (%)       0.00     13.39       1.64
pH Value of the Sand(*)         7.10      8.50       6.90
ADV (pH=7)                      1.50     13.00       3.00
Screen Analysis
20 (% retained)                 T         6.53       0.22
30                              0.31      2.05       0.52
40                              3.85      8.62       4.86
50                             10.64     24.09      14.34
70                             36.87     36.49      37.17
100                            35.11     16.80      31.02
140                            10.79      3.57      10.41
200                             2.15      0.97       1.37
270                             0.22      0.32       0.08
Pan                             0.08      0.57       0.15
AFS Grain Fineness             62.90     50.92      60.26
* pH test run in deionized water with a pH = 6.9

Clay-Bonded Sands

The makeup of clay bonded sand provides a totally different set of parameters and equipment requirements compared to chemically bonded sand reclamation. For example, system experience indicates there is not a predetermined calcining temperature and each case must be considered separately. Generally, optimum operating parameters won't be known until reclamation levels have been established. Calcining temperature and time are critical in relation to furnace pressure, bed temperature, postscrubbing intensity and post-classification.

Preprocessed spent sand (100% clay bonded, 100% chemically bonded or a random mixture of both) is fed at a variable rate by a low-speed feeder into the calcining fluid bed at 1350-1650F, depending on sand characteristics. Calcining heat is controlled by a combustion system (usually natural or propane gas). Supplemental heat is supplied by a recuperator located in the exhaust flue and from heat generated by the combustion of organic materials in the feed material. All supplemental fuel is fired in a fire box just below the calcining chamber, eliminating "hot spots" or coking, and ensuring a consistent and even temperature in the calcining bed.

Silica leaves the calcining chamber through a transfer duct into a precool chamber and is discharged at 700-850F into postcooling and postscrubbing equipment to facilitate rapid heat transfer. External cooling is used to control and maintain the sand discharge temperature and cooling coils furnish additional indirect cooling. The unit is kept at a slight negative pressure by a dust collection system to provide supplemental classification by removing some fines remaining on the sand as it enters the cooler. The sand then goes through a pneumatic scrubber that provides a gentle attrition action to remove dead clay and residual organic materials by an air curtain. The final calcined, cooled and scrubbed sand product has properties similar to new sand and is finally discharged at 100-120F, ready to be returned to the new or reclaimed silos.

Computerization

Newer systems use complete graphics monitor/display/control systems to provide precise process controls. They include archiving capability, visual operating displays and documenting and profiling trends at all critical process points. Status and alarm screens and self-help diagnostic screens are part of the computer controls. A graphics control system monitors the entire reclamation system on-site or to a geographically remote monitoring location linked via a modem. Local operator and troubleshooting responsibility is reduced by the capability to anticipate problems and facilitate corrections.

Basic screens of a graphics monitor display control system include:

* thermal sand reclaimer;

* cooler for chemically bonded sand;

* cooler-scrubber for clay-bonded sand;

* fabric filter emission control system;

* preprocessing and sand handling systems;

* postprocessing and sand handling systems;

* alarms;

* diagnostic systems.

Reclamation Economics

Case Study: A sand waste stream from a Midwestern foundry that was being landfilled in 1989 shows the typical waste sand from a clay-bonded molding line recovering resin-bonded cores.

Table 2. Sand Waste Stream from Midwestern Foundry Showing
Typical Waste Sand from Clay-Bonded Molding Line with
Resin-Bonded Cores.
Base Silica (AFS #60-70)            1900 lb     85.43%
Clay-Bond (total clay)               160 lb      7.19%
Additives (carbonaceous)              54 lb      2.43%
Resin Binders (core residue)           6 lb      0.27%
Silica Fines (-140 mesh)             100 lb      4.50%
Oxide                                  4 lb      0.18%
Total                               2224 lb    100.0%

There was no silica available for reuse, leaving 2224 lb of waste material to be disposed. There was 11.2% more waste than that made up by the wastestream.

A thermal sand reclaimer and post-pneumatic scrubber system were installed to produce 1800 lb of silica for reuse in the coreroom or on the molding line as new sand addition. It yielded 90% of the base silica, leaving 10% of new sand (200 lb) to be purchased to replace removed fines. That left 364 lb of waste material, reducing disposal costs to 16.4% of the original calculation. Waste sand was reduced about 84%.

Another foundry's experience with 6000 tons of new sand purchased illustrates the benefits of sand reclamation. Its direct costs for disposal at $50/ton represent an annual cost of $333,600. Using a thermal sand reclaimer, the annual cost of disposal is reduced to $54,600, a savings of $279,000. Similarly, new sand costs are reduced by 90%. Valued at $39/ton delivered, this amounts to $210,600/yr saved, resulting in a grand total of $489,608. The direct cost of reclaiming the sand (fuel, power, labor, maintenance) ranges from $11-13 per ton of feed material, or $66,000 to $78,000. Net annual savings can reach $423,608.

Process guarantees usual to foundries operating reclamation systems include the AFS GFN, sand yield and thermal efficiency. Added information measures thermal efficiency, acid demand value, loss on ignition, rated capacity and residual clay.

Reclamation & the Environment

The exhaust from reclamation equipment meets current air emission standards. Table 3 is test data from sampling ports located in the calciner flue, before any dilution from other exhaust points and before any pollution abatement equipment.

Waste minimization and the resultant cost savings are the obvious driving force behind using thermal sand reclamation. As pressures created by the elimination of approved landfill sites increase, waste sand systems will become as important to foundrymen as their melting equipment.

Each operating system is tailored for the specific foundry it serves. Problems with varying acid demand values (ADVs) and grain fineness numbers are reduced or eliminated, and the critical properties of reclaimed sand are enhanced for each installation.

Table 3. Exhaust Data from Multiple Fluid Bed Thermal Sand
Reclaimer Processing Clay-Bonded Sand.
Stack Gas:
Temperature                                        561.130F
Velocity, Ft/sec                                     53.083
Volume Flow, ACFM                                 2,297.064
SCFM                                                118.666
SCFH                                             67,112.000
Moisture, Percent                                     4.815
Percent C|O.sub.2~                                    3.000
Percent |O.sub.2~                                    17.000
Sample:
Volume, SCF                                          41.340
Percent Isokinetic Ratio                            101.066
Particulate:
Concentration, grains/scf                               402
Emissions, lb/hr                                     32.560
S|O.sub.2~ (Sulfur Dioxide)
Concentration, grains/scf                            35.730
Emissions, lb/hour                                    0.360
CO (Carbon Monoxide)
Concentration, grains/scf                           225.900
Emissions, lb/hour                                    1.016
NO & N|O.sub.2~ (NOX)
Concentration, grains/scf                            49.550
Emissions, lb/hour                                    0.258
VOC as |C.sub.3~|H.sub.8~
Concentration, grains/scf                            10.200
Emissions, lb/hour                                    0.075
NH3 (Ammonia)
Concentration, grains/scf                             0.115
Emissions, lb/hour                                    0.003
|H.sub.2~S (Hydrogen Sulfide)
Concentration, grains/scf                             7.491
Emissions, lb/hour                                    0.044
HCN (Hydrogen Cyanide)
Concentration, grains/scf                             0.174
Emissions, lb/hour                                    0.000
HCHO (Formaldehyde)
Concentration, grains/scf                             0.010
Emissions, lb/hour                                    0.000