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Thermal sand reclamation joins foundry and supplier skills.

The reclamation of spent foundry sand is a complex problem driven by economics. The cost of new sand and disposal of spent foundry sands continue to rise along with the costs of meeting environmental compliance regulations. Eventually, foundries may be unable to dispose of spent sand at any cost.

It has been said that sand reclamation is not one process but a combination of many processes. Whether reclamation is economically feasible depends on how well the foundry is able to process and utilize its various sources of spent sand. The complexity of sand reclamation was noted a number of years ago when the AFS Sand Reclamation Committee struggled to arrive at the following definition of the process.

Sand reclamation is the physical, chemical or thermal treatment of a refractory aggregate to allow its reuse without significantly lowering its original useful properties for the application involved. "

The approach to reclamation must begin by identifying all sources of spent sand in the foundry and determining the quantity and characteristics of the sand from each of those sources. The next step is to determine the least amount of processing necessary to each of these sources which will render the sand usable somewhere in the foundry. Total System Approach

In a "total system" approach to sand reclamation, full consideration must also be given to the waste streams generated by reclamation processes and the ramifications there might be to their disposal. Thus, it is imperative that dust collection systems be dedicated to reclamation equipment and not shared with other systems. This total system approach might entail taking baghouse dust containing concentrations of organics from particulators and dry attrition scrubbers, and injecting it into a thermal system to eliminate or minimize the amount of material to be taken to an approved disposal site.

The physical and chemical characteristics of the base sand being reclaimed must be understood in the total system approach. For example, does the sand contain compound grains? What is the durability of the sand when exposed to a given reclamation system? What changes take place in the surface morphology during reclamation, and what impact does this have when the grains are coated with resin for reuse in cores or molds? What changes occur chemically during reclamation, and how will they affect rebonding of the sand grains? What ramifications do any changes in these characteristics have on the quality of castings produced?

Equipment manufacturers and foundry plant engineers must be aware of these process ramifications and work closely with foundry process engineers if sand reclamation is to succeed. Thermal reclamation should be considered only when it has been demonstrated that the reclaimed sands produced by less expensive basic methods, such as particulation and dry attrition scrubbing, cannot be used somewhere in the foundry. Process Objectives

It is important to establish what is to be achieved from a thermal reclamation system. In other words, a process want list' should be developed. For simplicity, thermal reclamation of refuse sand can be divided into three basic source categories: * sand from a core or molding process

that coats the grains with an organic

binder or where the grains are coated

by the condensation of vaporized organic

material used for the patterns

(i.e. expendable pattern casting); * sand from a core or molding process

that coats the grains with an inorganic

binder; * sand from both of the above categories

where both organic and inorganic

binders must be removed from the

sand grains.

Before discussing the specific process requirements of a thermal reclaimer, several key system process issues must be addressed. The amount of binder or condensate on the sand grains will vary depending upon the degree of exposure of the grains to the molten metal and the type of metal being cast. The fuel value, amount and consistency of these combustibles must be known and controlled in the feed material entering a thermal reclaimer.

It must also be recognized that, in all of these basic categories of refuse sand, some additional organic and inorganic materials can be found. Fine mineral particles such as silica and zircon flour can be present from mold, core and pattern coating materials. The thermal reclaimer and dust collection equipment must be capable of combusting or removing these materials from the sand.

It is critically important that any metallics present, magnetic and non-magnetic, must be removed before the refuse sand enters a thermal reclamation process. An efficient thermal reclamation process, with or without a separate attrition process, must take all of these factors into consideration.

The "ideal" process objectives for a thermal reclaimer must: * completely remove all organic material

from the surface

of the sand grains

with a minimum of

energy expended in

heating the sand

grain; * avoid fracturing of

sand grains; * not produce products

of combustion

from the burning of

organics on the

sand grains, nor

should the fuel residue

remain on the

sand grains;

not alter the sand

grain morphology; * not alter the

chemical properties

of the sand (pH, ADV,

nitrogen and sulfur content); * not allow the build up unwanted elements

in the sand such as sodium

and potassium; * control temperature, particularly

clay-bonded refuse sand. Design Parameters

Several facilities design objectives are important from the perspective of those personnel who must operate and maintain the sand reclamation equipment. They are also pertinent to plant engineering personnel, to equipment manufacturers and, above all, to those foundry managers who decide how much, when, and where financial resources will be allocated for sand reclamation equipment.

It goes without saying that the individual pieces of equipment should be ruggedly built to survive in the foundry environment. The equipment designer should strive to minimize the number of moving parts that require lubrication and tend to fail in an environment filled with abrasive dust, heat and smoke. Controls should be easy to understand and readily accessible by the operator. Ample viewing windows, conveniently located, should allow the operator to see what is going on.

From the plant engineering perspective, a total system approach to design makes it imperative that it be a stand-alone system. The system must be controllable and produce a consistent product. Adequate sand storage before and after the thermal reclaimer is essential to assure that the most economic operating pattern is maintained. Process Controls

Plant engineering, working in conjunction with equipment suppliers and process engineering, must design a reclamation system to control the following basic areas: * sand particle sizing and segregation, * sand flow rate, * air flow, * temperature, * emissions (solids and gases), * sand chemistry.

The control required in each of these areas can only be achieved by making sure that the following happens: * all sand preparation and feed equipment

is dedicated to the reclamation

system, and the following is done on

a controlled and consistent basis: - refuse sand is blended to provide

uniform feed to the reclaimer; - all magnetic and nonmagnetic metals,

paper, filters, etc. are removed

from the feed material; - a two-day minimum supply of sand

before and after the calciner is

provided; * a baghouse dedicated to reclamation

is provided and the following is monitored

and/or data recorded: - reclaimer exhaust; - discharge weight and/or volume is monitored at least daily.

The reclaimer exhaust stream is isolated to facilitate treatment for environmental disposal reasons and to capitalize on any potential uses for the exhausted fines.

The controls used for monitoring these facilities should be "on-line," real-time controls and should be supplemented by a laboratory statistical control program that monitors the following process variables: * screen analysis, * AFS clay, * pH and ADV, * wettability (visual), * strength tests (tensile and transverse).

it is important that on-line controls, pressure switches, air and water flowmeters, temperature sensors, etc. be easily visible to equipment operators and maintenance personnel. Manual damper and valve positions should be marked so that original settings are simple to reset after performing maintenance on the equipment.

The keys to a successful thermal reclamation system lie in shop floor operating consistency (sand in, sand out) and strict adherence to the system's production routines and maintenance schedules. In turn, management's responsibility is to provide adequate financial support for a reclamation system and the time to select the engineered system best suited to a foundry's operations.
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.

Article Details
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Title Annotation:Special Report: Sand Reclamation
Author:Good, George
Publication:Modern Casting
Date:Jul 1, 1991
Words:1354
Previous Article:Waste management is STILL a business decision.
Next Article:Tackling waste management at Globe Valve.
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