US3686034A - Gravity flow sand reclamation process - Google Patents

Abstract

A GRAVITY FLOW, THERMAL RECLAMATION PROCESS FOR CONTINUOUSLY RECOVERING USED FOUNDRY SAND HAVING A CARBONACEOUS COATING THEREON WHEREIN A SUBSTANTIALLY FREE FLOWING, DOWNWARDLY GRAVITATING DISPERSION OF SAND PARTICLES IS PASSED THROUGH A VERTICALLY DISPOSED FURNACE AND SEQUENTIALLY THROUGH A SERIES OF VERTICALLY DISPLACED SAND DISPERSION GRATES TRANSVERSLY MOUNTED THEREIN WITH OPEN ZONES INTERMEDIATES THE GRATES WHEREBY THE FREE FALLING SAND PARTICLES ARE CONTINUOUSLY INTERCHANGED ALONG THE LENGTH OF THE FURNACE BETWEEN VERTICAL PLANES AND EACH SAND PARTICLE IS EXPOSED TO RADIANT HEAT AND OXYGEN IN AT LEAST ONE OF THE OPEN ZONES DURING ITS PASSAGE THROUGH THE FURNACE TO ACHIEVE RAPID AND SUBSTANTIALLY COMPLETE COMBUSTION OF THE PARTICLE COATING.

Description

Aug. 22, 1972 J. J. ROTHSCHILD 3,636,034

GRAVITY FLOW SAND RECLAMATION PROCESS Filed Oct. 14, 1970 INVENTUR /ofzm/wofhschi/d w f Hllll l' United States Patent GRAVITY FLOW SAND RECLAMATION PROCESS John J. Rothschild, Bloomfield Township, Oakland County, Mich., assignor to General Motors Corporation, Detroit, Mich.

Filed Oct. 14, 1970, Ser. No. 80,689 Int. Cl. B08b 7/00 US. Cl. 134-2 2 Claims ABSTRACT OF THE DISCLOSURE A gravity flow, thermal reclamation process for continuously recovering used foundry sand having a carbonaceous coating thereon wherein a substantially free flowing, downwardly gravitating dispersion of sand particles is passed through a vertically disposed furnace and sequentiallv through a series of vertically displaced sand dispersion grates transversely mounted therein with open zones intermediate the grates whereby the free falling sand particles are continuously interchanged along the length of the furnace between vertical planes and each sand particle is exposed to radiant heat and oxygen in at least one of the open zones during its passage through the furnace to achieve rapid and substantially complete combustion of the particle coating.

This invention relates to the reclamation of used foundry sand having a carbonaceous coating thereon and, more particularly, to a gravity flow, thermal sand reclamation process.

In many foundry operations large quantities of sand are used and to make the operation economical it is often necessary to reclaim the sand for reuse in the foundry operation. Reclamation of some sand components in the foundry operation, however, is often complicated by the presence of a carbonaceous coating on the sand grains which must be removed before the sand may be reused. For example, in foundry operations employing sand cores an oil, e.g. linseed oil, or resin binder such as furfuralurea resins and phenolic resins are added to the sand as a binder prior to forming of the cores which coats the individual sand grains to allow for forming of the cores and to increase core strength. In order to reuse the core sand, it is necessary for the sand to be thermally treated after the casting operation to burn off from the individual sand grains the carbonaceous coating which was not burned off by the heat of the castings. In addition, a number of scrap cores unsuitable for use in the casting operation are normally present in any foundry operation and, in order to reuse the core sand, it is again necessary to burn off the binder coating.

Numerous sand reclamation systems which function to burn 01f the carbonaceous coating from the sand grains are known in the art. One type involves passing a horizontal bed of sand through a furnace and exposing the bed to heat and oxygen to burn off the carbonaceous coating. The horizontal bed is often agitated or fluidized by an air updraft to provide increased exposure of the sand grains to the heat and oxygen. A second type is the packed column system wherein a vertical column of sand 3,686,034 Patented Aug. 22, 1972 is maintained and is heated along its length to burn off the resin coating. A third type is a horizontal rotary kiln comprising a refractory lined cylinder sloped toward the discharge end and a burner mounted at either the feed or discharge ends which heats the kiln to about 2000" F. wherein the sand is tumbled through the kiln thereby being exposed to heat and air. A fourth type is the vertical multiple hearth furnace wherein the sand is plowed from shelf to shelf within the gas or oil fired refractory linen shell of the furnace. Each of these systems, however, has the disadvantage of requiring relatively long sand retention times in the combustion zone, i.e. 15 to 30 minutes, to achieve combustion of the carbonaceous coating which results in a system of relatively low efficiency in terms of the quantity of sand that can be reclamied per unit time and, in addition, often results in over-heating of the sand with a resulting change in its chemical characteristics. Furthermore, themore effective systems are usually complex and expensive.

Accordingly, it is among the principal objects of this invention to provide a relatively efficient, low cost, continuous thermal sand reclamation system wherein each sand grain undergoes substantially complete combustion of its coating material with a relatively short retention time in the combustion zone.

This object and others are achieved by an improved process for thermally reclaiming used foundry sand having a carbonaceous coating thereon wherein the used sand is gravity fed as a substantially free-flowing, downwardly gravitating dispersion of sand particles into a vertically disposed furnace having a combustion zone with a plurality of sand dispersion grates positioned therein with open zones therebetween such that the freely falling sand sequentially strikes the dispersion grates whereby the sand dispersion is further dispersed and each sand particle is continuously interchanged one with another along the length of the furnace between vertical planes. Each sand particle is thereby exposed to a combustion supporting atmosphere in at least one of the open Zones. The sand dispersion is exposed to radiant heat and oxygen in each open zone resulting in substantially instantaneous combustion of the carbonaceous coating and a relatively short retention time in the furnace.

Other objects and advantages will be apparent from the following detailed description of the invention, reference being had to the accompanying drawings of which:

FIG. 1 is a cross sectional view of a gravity flow sand reclamation furnace,

FIG. 2 is a cross sectional view taken along lines 22 of FIG. 1, and

FIG. 3 is a cross sectional view taken along lines 3-3 of FIG. 1.

Referring to FIGS. 1, 2 and 3, the gravity fiow sand reclamation apparatus 10 consists generally of an elongated vertically disposed furnace 12 of square or rectangular cross section having heat resistant refractory side walls 14 defining a confined combustion chamber 16 therein. The furnace 12 is mounted on a structural fixture 18 including upwardly extending structural steel walls 20 which support the refractory side walls '14 of the furnace. Centrally above the combustion chamber 16 is an inlet opening 22 having a refractory plug 24 positioned therein and vertically movable by means of a screw 26 to vary the size of the inlet opening 22. Below the combustion chamber 16 is an unobstructed outlet opening 28 below which is positioned means for collecting and removing the treated sand after its passage through the furnace. Such means may include a bin 30, as shown in FIGS. 1 and 2, or a continuous belt (not shown) for continuously transporting the treated sand from the furnace. Positioned above the furnace 12 adjacent the inlet opening 22 are exhaust vents 32 communicating with the combustion chamber 16 through openings 34 located below the inlet opening 22 to remove the effiuent gases resulting from combustion of the sand grain coating and a portion of the heat liberated on combustion in the form of increased sensible heat in the effiuent gases.

Mounted within and across the combustion chamber 16 transverse to the vertical axis of the furnace 12 are a plurality of sand dispersion grates 36 vertically spaced from one another along the length of the chamber defining a series of open zones 38 intermediate the grates 36. In the preferred embodiment, shown in FIG. 1, the grates 36 are formed of spaced segments of 90 angle iron positioned at a 45 angle to the horizontal plane of the furnace to form rows of chevron-shaped dispersion elements. The grates 36 are supported by upwardly extending corner supports 37 and are so mounted about the vertical axis of the furnace to direct the sand particles away from the vertical axis in the upper portion of the grates and then back toward the vertical axis in the lower portion directing the sand particles angularly through the open zones toward the center of the furnace. The dispersion grates 36 function to further disperse the sand particles as they pass through the chamber sequentially onto lower dispersion grates thereby continuously interchanging the particles between vertical planes parallel to the vertical axis as the sand dispersion passes along the length of the furnace and secondly to regulate the rate of fall of the particles through the furnace.

In opposing side walls 14 of the furnace between the sand dispersion grates 36 are a plurality of radiant heating burners 40 so mounted as to direct heat toward the center of the combustion chamber 16 across the open zones 38. Radiant heating burners are localized gas heaters having variable temperature controls and the capability of providing through the burners 100%, by volume, air in excess of that needed for combustion. The burners are connected through tubes 42, 43 and 44 to a source of gas and air (not shown).

OPERATION Used core sand is first crushed and then screened through 21 screen to form a free flowing sand dispersion. The sand dispersion is then fed into the furnace 12 through the inlet opening 22 the size of which may be varied to maximize the furnace capacity for a given heat input. As the sand is fed into the furnace it falls under the force of gravity in the form of substantially freefiowing, downwardly gravitating dispersions of sand particles through the first open zone 38a where it is immediately exposed to radiant heat. The sand dispersion then falls freely striking the first set of dispersion grates 36a wherein the dispersion is further dispersed as it falls therethrough and is projected angularly toward the vertical axis through the adjacent open zone 38b and onto the next lower set of dispersion grates 36b. As the sand falls through the open zone 38b, it is again subjected to radiant heating. Thereafter, the dispersion continues to fall freely sequentially from the upper dispersion grates to lower dispersion grates located between burner rows 40 thereby being continuously further dispersed and projected angularly through the intermediate open zones 38 wherein the dispersion is again subjected to radiant heating. In this manner the sand particles are continuously interchanged between planes parallel to the vertical axis of the furnace along the length of the furnace whereby each sand grain is exposed to radiant heat and a combustion supporting atmosphere in at least one of the open zones.

The radiant burners directing heat into the open zones are operated at a temperature of about 1750* to 1800 F. During the thermal reclamation process, however, it is critically important to keep the sand below its phase change temperature which for lake sand typically used in foundry operations is on the order of 1200 F. If the sand is heated above its phase change temperature, the chemical characteristics of the sand are detrimentally altered. The problem of obtaining substantially complete combustion of the grain coating without over-heating of the sand has been overcome by the use of the radiant burners which results in rapid heat transfer to the grain coating. This rapid heat transfer causes almost instant ignition of the grain coating which supplies 25-50% of the heat for maintaining the heat of combustion.

After passing through the combustion chamber, the treated sand falls through the outlet and is collected. The temperature of the sand exiting the furnace when operated at about 1750 F. to 1800 F. is about 1000 F. At the completion of the thermal reclamation process the sand has a grain distribution equal to that of unused sand.

Thus it will be seen that my invention provides an improved process for thermally reclaiming used foundry sand achieving substantially complete combustion of the sand grain coating with short sand retention times in the combustion chamber. A furnace of the type shown in the accompanying drawings was constructed having a combustion chamber with a vertical height of about 5 feet and a cross section of 16 inches x 16 inches. Twenty-four radiant burners were mounted in the furnace side walls as shown in FIGS. 1 and 2. Used foundry sand having a carbonaceous coating thereon was passed through the furnace at a rate of two tons per hour. A given sand grain took about 5 seconds to pass completely through the furnace. It was found that three passes through the furnace were necessary to obtain complete burning off of coating. Accordingly, it is apparent that a furnace built in accordance with my invention having a vertical combustion chamber 15 feet long could process 6 tons per hour of sand and attain substantially complete combustion of the grain coating of a given sand grain in a 15 second fall through the furnace.

Although my invention has been described in terms of a certain specific embodiment, it will be recognized that other forms may be adopted by those skilled in the art within the scope of my invention.

I claim:

1. A process for continuously reclaiming used foundry sand having a carbonaceous coating thereon by burning off said coating in a furnace comprising an elongated vertically disposed combustion chamber having an inlet opening at the top thereof and an unobstructed outlet opening at the bottom thereof and having a plurality of sand dispersion grates extending transverse to the vertical axis of the furnace across said chamber, each of said grates being disposed in a vertically spaced relation along the length of said chamber thereby defining a plurality of open zones intermediate said grates, said process comprising the steps of:

gravity feeding said sand in a substantially free-flowing conditiion into said chamber through said inlet opening in the form of a substantially free-flowing downwardly gravitating dispersion of sand particles, permitting said sand to fall freely through said chamber sequentially striking each of said sand dispersion grates, said sand being further dispersed thereby away from the vertical axis in the upper portion and then back toward the vertical axis in the lower portion of each of said grates as it falls therethrough and projected angularly toward the center of said furnace through said open zones such that said sand particles are continuously interchanged one with another and between planes parallel to said vertical axis as they fall along the length of said chamber,

each of said sand particles being exposed to a com- References Cited bustion supporting atmosphere in at least one of said UNITED STATES PATENTS open zones,

radiantly heating said particles in the presence of said tensen combustion supporting atmosphere in ea of Said 5 3 570 420 3 971 g 5 :51 110 Open zones, said coating undergoing substan ially 553 31 5 1951 Horth 134 2 instantaneous and complete combustion without heat- 3:200:04 8/1965 Kurtz X ing of said sand above its phase change temp r ure, 65, 2 7 1 7 Donner 34 z5 R and 3,208,831 7/1961 Belden 134-25 R continuously removing said particles from said com- 10 bustion zone through said unobstructed outlet MORRIS O. WOLK, Primary Examiner p J. T. ZATARGA, Assistant Examiner 2. Claim 1 wherem the radlant heating step is performed by radiant gas burners operating at a temperature of FOREIGN PATENTS about 1800 F. 13419, R

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