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Selecting a thermal reclaimer for green sand.

Selecting a reclamation system capable of producing core-quality sand from green sand can be difficult due largely to the clay and carbon used in the molding process. This difficulty is compounded further by the overall complexity and wide fluctuations in the chemistry of system sand. It is one thing to thermally reclaim a chemically bonded sand for reuse in cores and molds with only minor variations of the basic binder content. It is an entirely different matter to reclaim a sand with these same variations combined with a clay-carbon binder and, in some cases, a base sand of lower than desirable purity, and ultimately trying to use the reclaimed sand with a sensitive core binder system.

Further complicating this selection process is the fact that, in many applications, these systems require higher than average tonnage throughput and the ability to accumulate large quantities of sand. This, together with plant building and space considerations, can dramatically affect the cost of the system.

Defining Thermal Reclamation

Green sand thermal reclamation is basically a two-stage process that includes calcining and post treatment. Calcining is the physical and chemical breakdown of volatile materials and clay by heating the material to a temperature just short of fusion for sufficient time to affect the breakdown. in the post-treatment stage, residual ash and dead clay are removed from the sand to eliminate their detrimental effect on the pH and acid demand value. Post treatment also removes unwanted surface area on the resultant core sand that requires extra resin to coat. Post treatment also includes cooling and classifying of the sand.

There are other system components to be covered later in this article, but some variation of the calciner and post treatment is common to all green sand reclaimers.

Rotary Kiln Reclaimers

Basically, there are two major types of thermal reclaimers: the rotary kiln and fluid bed. There are also two distinct concepts in post treatment: the cooler-classifier with physical scrubbing; and the cooler-classifier and/or scrubbing with chemical treatment.

Rotary kiln reclaimers consist of an indirectly fired rotary kiln, which is fed by an automatically metered sealed screw feeder. The temperature in the kiln can be preset within the range of 1400-1600F. The sand is advanced in a cylindrical kiln by longitudinal stirring and lifting flights that mechanically fluidize the sand. This causes intimate contact between the sand and the oxidizing air as the material advances through the kiln. The sealed variable-speed screw feeder has a hollow shaft that allows process air to be introduced into the kiln which, in turn, is used to burn off all volatile materials. This air is supplied by a combustion air blower with special valving that meters the correct amount of air to provide complete combustion without the need for an after-burner. The drum and lifting flights are indirectly heated by a fuel-air mixer that is burned in a firebox surrounding the outside of the drum. An optional exhaust heat recuperator can be fitted to the unit, which affects a 20% savings in fuel and at the same time reduces the gas exhaust temperature.

After about 40 minutes residence time, the sand cascades down a sealed chute into a second rotary drum. The sand is cooled from its 1400-1600F temperature range down to ambient [+ or -] 20F by cascading the sand through a slow-moving stream of ambient air together with direct heat transfer into a relatively cool drum. The reverse flow cooling air is mixed with the hot gases from the calciner drum and a small quantity of ambient air to form a combined exhaust gas of about 4000 cfm per ton per hour at approximately 160F. The discharge end of the cooling drum is fitted with a screen that will pass 20 mesh and smaller material and scalp off the unwanted large particles. The calcined and cooled sand is then processed through scrubbing, classifying and cooling to remove the unwanted fines. Nearly all green sand reclaimers employ a similar second-stage process. The rotary kiln units are available in six models ranging from one-half to seven tons per hour. Advantages of this type of system include:

* Indirect firing that permits a clean discharge without an afterburner.

* Positive through-feed that can process sands having widely varying densities, such as chromite sands.

* A relatively squat arrangement requiring less headroom than some of the fluid bed units.

* Eliminating the need for refractory lining because the calciner is constructed of high-temperature steel.

Fluid Bed Reclaimers

Fluid bed reclamation systems employ of a variety of fluid bed reactors designed and built as separate components or integrated into one common structure. These can be either directly or indirectly fired. This type of reclaimer usually employs fluid bed devices to perform preheating, drying, calcining and primary cooling operations.

One type of fluid bed reclamation unit combines the preheater, calciner and primary cooler as a simple, single vessel. Incoming spent sand is fed into the top preheat chamber with a variable-speed screw feeder fitted to a surge hopper that provides for at least two hours running time capacity.

The preheat section of this unit uses gases from the heat recuperator to preheat sand to 350-450F to remove residual moisture. The sand cascades over an adjustable weir and passes down into an externally fired fluid bed reactor that introduces 550F secondary air from the recuperator. Additional heat is supplied as needed to supplement the heat generated by the combustion of the organics in the sand.

The sand is retained in the calcining zone for about one hour at 1350-1650F. It then flows over a weir into the pre-cooling section in the lower portion of the unit where it is cooled in a fluid bed to 600-750F. The gases passing through the recuperator pass through a cyclone unit to remove the large particulate from the gas stream. Usually this type of reclaimer is fitted with a secondary cooler, which is a combination fluid bed unit with internal water-cooled coils to reduce the sand temperature to 80-110F, depending upon the prevailing ambient conditions. This after-cooler is also fitted with a plenum chamber above the fluid bed that permits a slight negative pressure exhaust to help remove unwanted fines.

Sand at this point is ready to proceed to the second stage where additional scrubbing and classification occurs. This type of reclaimer is available in 10 models ranging in capacity from 500 lb per hour to 15 tons per hour.

A second type of fluid bed reclaimer is available in which the calciner and the primary and secondary cooler-classifiers are separate units. These calciners are internally fired and employ a unique fluidizing plate. The plate employs raised fluid bed tubes that permit the sand to form a natural insulation bed between the hot sand in the fluidized zone and the gas-air mixture plenum below. These units are fitted with sand preheaters and recuperators to increase thermal efficiency.

In operation, the incoming sand is preheated to about 570F in the preheater and then conveyed into the calciner where it is held for 30-40 minutes at 1400-1600F, depending upon the binder types in the sand. The calcined sand then cascades over a weir plate and drops down into a fluid bed cooler-classifier that employs a water-cooled tube bundle in the fluid bed media for direct air and indirect water cooling. This separate unit is capable of cooling the sand down to 100F.

This second type of reclamation system uses a unique system to further process the sand following calcining: chemical pH correction. The technique employs a group of measuring probes, a continuous mixer and an acid-solution tank and metering system. it measures the pH of the calcined sand and adds the acid solution in a continuous mixer in amounts necessary to correct to the desired pH value. After mixing, the pH-corrected sand is stored in a small silo for about one hour and then is dried, cooled and classified down to the desired temperature as required for the specific core process. In the case of high impurity lake sands, the calcined sand can be processed in a mechanical attrition reclaimer to remove more of the dead clay and calcium oxide prior to the pH correction.

This type of reclaim system has the ability to modify tile pH of the calcined sand to more closely match the desired set point for a specific core binder requirement. it is available in five models with capacities ranging from 1000 lb. to 7.5 ton per hour.

Fluid bed reclaimers have the following general advantages:

* fewer moving parts, particularly in the calciner itself;

* less floor space requirements, but usually at the expense of more overhead requirements;

* pH adjustment capabilities with certain units.

System Components

Although there are considerable design differences in the calciner and precooler units, virtually all thermal reclaimers require many of the same components. Since reclaimers, coolers and post scrubbers have already been discussed, the following section will discuss other elements necessary for an effective thermal reclamation system.

Driers--If incoming sand has a high moisture content, a separate, dedicated drier is recommended. It relieves the thermal unit of drying duties and dedicates its energy to the reclaiming task.

Metallic Separator--A basic must for all systems is some combination of magnetic separation and/or particulate screening device located ahead of the calciner infeed conveyor to remove tramp metal prior to entering storage. Larger metallic particles can plug the calciners and other downstream components and can lead to undue maintenance and inconsistent feed rates to the calciner screws, belts or vibrating feeder,.

Storage Bins and Silos--Since most green sand reclaim systems will run more than one shift per day and, in some cases, have to handle large instantaneous quantities of core scrap, floor sweepings and other refuse sand, it makes sense to provide sufficient storage ahead of and behind the reclaimer. This will allow the system to accumulate sand during operating shifts and permit the reclaimer to run 24 hours a day (and, if possible, more than five days a week). if the sand is being reclaimed from the outside where the moisture content can reach 8-10%, it is better to use an existing sand drier, if available, or purchase a dedicated drier able to handle this high moisture content. 11 the moisture content is less than 2.0-3.0%, the calciner's preheater may be used.

Lump Reduction and Granulation-- Although most thermal reclaimers can handle fairly large lump sizes, it becomes difficult to achieve a constant infeed rate if the lump size is greater than 5% - 1/4 in. x 0. Therefore, if a system has a fair percentage of scrap cores and core butts or large lumps of dried out molding sand, a conservatively sized lump reduction unit is mandatory. Because some of the larger scrap cores, core butts and sand lumps may require a protracted time in the granulator, it is recommended that the unit be at least 33-50% greater than its system's capacity.

Final Classification--In many cases, the input sand to the reclaim system is consistent enough in physical and chemical makeup that the classifying equipment built into the coolers, pneumatic scrubbers and separators will be sufficient to provide grain fineness control equal to or better than new sand. if the reclaimed sand is for all purpose use (molding sand makeup and core making) and the physical nature of the infeed sand varies widely, supplemental classifying equipment is available and may be appropriate.

Control System--Thermal reclaim systems, by their very nature, employ a variety of controls for temperature, combustion, fuel flow, air flow, material levels and logic switching. These controls make thermal reclaimers natural candidates for programmable controllers, which, if properly designed, can provide all logic and motor sequencing, diagnostics and process control. Three primary goals for the control system are continuous operation, consistent temperature in the calciner and consistency in the entire system.

Dust Collection--All systems require some form of dust collection, both for housekeeping and process control. The success of a thermal reclaim system often is no better than its dust collection system because of its importance to the system in removing unwanted fines; retaining usable sand grains; removal of some heat from the process; good housekeeping; and component reliability.

Robert J. Smith SandMold Systems, Inc. Newaygo, Michigan Diagram Omitted
COPYRIGHT 1991 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:Special Report: Sand Reclamation; part
Author:Smith, Robert J.
Publication:Modern Casting
Date:Aug 1, 1991
Words:2035
Previous Article:Group acts on regulatory issues.
Next Article:Sand reclamation cost-savings worksheet.
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