US3445251A - Molding sand - Google Patents

Description

M. J. NEVINS May 20, 1969 MODDING SAND Filed April 13 1966 www Mg@ United States Patent 3,445,251 MOLDING SAND Michael J. Nevins, Houston, Tex., assignor to National Lead Company, New York, N.Y., a corporation of New Jersey Filed Apr. 13, 1966, Ser. No. 542,320 Int. Cl. C04b 33/12; BZSb 7/34 U.S. Cl. 10G-38.3 5 Claims ABSTRACT F THE DISCLOSURE A molding sand composition comprises sand as a major constituent, the binder being bentonite 'adlmxed with about one-eighth to about three times its Weight of alkaline alkali metal humate, suflicient binder being used to obtain a green compression strength of at least four pounds per square inch. Within this range of humatezbentonite ratios, the green compression strength is very much higher than would be predicted from the individual components.

This invention relates to molding sands such as are used in the metal casting art and more particularly to an improved molding sand in which certain humates are used with clay as a binder.

As is well known, in the sand casting of metals, a mold is formed from a suitable sand which is combined with binding material which permits the sand to be molded into the desired shape, and to retain that shape during the casting process. At the same time, the nature of the bond between the sand particles must be such that it will give way subsequent to the pouring and solidifying of the metal, in order that the casting may be readily removed and freed of sand particles. This is particularly necessary where the shape of the casting includes re-entrant cavities.

As is likewise well known in the art, it is necessary for the bonding described to take place without occluding the pore channels in the sand mold, as the mold must retain permeability for gases inevitably produced at the time of pouring. Furthermore, the sand mold must presenta surface to the impinging uid lmetal which after cooling and shaking out leaves the metal surface as smooth as possible, so that cleanup operations on the casting may be minimized. Furthermore, in order to produce castings in` an economical fashion, it is highly desirable that the sand after casting and shaking out should be capable of substantially indenite re-use, by reworking with additional bonding agents as needed.

Testing methods have been developed in the foundary art which permit the assessment of some of the desirable properties of a molding sand composition; theseinclude various compression strength tests, permeability tests, and the like. Some a'dditional desirable properties, such as the ability to impart a smooth, clean surface to the casting, are necessarily judged in a more qualitative fashion, although these -are equally important.

The most Widely used and generally the most satisfactory bonding agent =for ordinary sand molds is bentonite, which may be of a highly swelling type as found in Wyoming, South Dakota, and elsewhere; or it may be of a lesser swelling variety, as found in Texas and Mississippithese two varieties being often known as western bentonite and southern bentonite respectively.

Minor amounts of other additives are commonly used besides the sand, bentonites, and tempering water of the basic molding sand composition. These generally include cereal binders, particularly those ma'de from the proteinrich fraction of the maize kernel; dextrine, powdered bituminous coal (sea coal), wood flour, rpitch, silica flour, and others. Nonbentonitic clays such as tire clay may sometimes be added in addition to the bentonite to improve strength at high temperatures. The sand itself is generally silica sand, that is, quartz, although other mmerals may be used for special compositions, such as olivine and Zircon.

In spite of the antiquity of the sand casting process and the vast amount of Idevelopmental work which has been carried on in this field, there remains substantial room for improvement in sand properties. Better peel, shakeout, and surface detail, reduction in expansion defects, higher green strengths, and other improvements are still looked for in this art.

An object of the present invention is to provide a molding sand composition containing bentonite and .a certain humate additive, which is easily prepared and maintained and has improved properties.

Further objects of the invention will appear as the description thereof proceeds.

In the drawings, FIGURLE l is a graph showing the irnproved results obtainable with my invention.

Generally speaking and in accordance with an illustrative embodiment of my invention, I provide a molding sand composition, the major constitutent of which is sand, and a minor constituent of which is bentonite together with fnom about one-eighth to about three times its weight of an alkaline, alkalized humate as defined below. It is to be understood that as is common in the molding sand art, suicient water is incorporated in the composition to provide adequate binding properties. As is well known, bone-dry bentonite has little or no binding eiect on the sand particles, and likewise, when a gross excess of water is used, the bentonite can no longer act -as an adhesive for the sand grains.

The bentonite, ias already discussed, may be any of those commonly employed in the molding sand art, and may be the highly swelling type exemplified by Wyoming bentonite or the lesser swelling types as found in Texas and elsewhere. As is usual in the art, the bentonite is used in a comminuted form, such as ground to pass a 200- mesh screen, although coarser and finer grinds may be used. As far 4as the sand and the bentonite are concerned, therefore, the procedure is conventional.

The alkaline, alkalized humate is any alkali metal salt of humic acid, such as sodium, potassium, vor lithium humate, these being water soluble, as is well known, and is most conveniently derived from lignite, particularly weathered lignite such as occurs in great abundance in North Dakota, Texas, New Mexico, northern Europe, and elsewhere.

In order to form the alkaline, alkalized humate, humic acid is reacted with an alkali-metal alkali, preferably selected from the group consisting of sodium and potassium alkalies. This group includes such well-known members as caustic soda, soda ash, potash lye, and the like. It is not necessary to use a chemically pure humic acid, but indeed, I prefer to utilize a lignite rich in alkalisoluble constituents and simply grind these together with the selected alkali, preferably with enough water to bring about a rapid neutralization reaction, if indeed, the lignite is not moist enough to provide the necessary water by itself. This grinding together can be carried out in any suitable apparatus and may even be done in the foundry muller during preparation of the molding sand. More complicated procedures may be used, such as dispersing the ground lignite in a solution of the alkali and water, settling out the insoluble constituents, and recovering the alkalized humate by drying and grinding, but this increases the cost and scarcely affects the nal results.

The humic acid will, in general, require a minimum of about one-fourteenth to about one-fourth of its dry weight of sodium hydroxide to produce a sodium humate (which is the alkalized humate in this instance) which at the same time is alkaline, viz., which has a pH in excess of 7. (Of course, if other alkalies are used, the known prnciplcs of chemical stoichiometry should be observed.) Humic acids in general, and in particular those derived from lignite have equivalent weights in the neighborhood of 320 and 360. Thus, if a weathered lignite having 80% (by dry weight) of alkalized, soluble humic acids is utilized, admixing this with about one-tenth of its weight of sodium hydroxide will, as may be readily calculated, result in the conversion of all of the humic acid to sodium humate. In view of the fact that humic acid is not as strong an acid as sodium hydroxide is a base, sodium humate will have a slightly alkaline pH, in the neighborhood of 8 to 9.

Larger amounts of alkali may be used, over and above that necessary to convert the humic acid to sodium humate; but at great excesses, the disadvantage arises of difficulty in handling the material, breathing the dust, and the like. Indeed, in accordance with a further aspect of my invention, additional alkali in the form of sodium hydroxide may be employed.

Lesser amounts of alkali than required for complete neutralization of the humic acid are also usable provided the resulting humate is alkaline, ie., over 7 pH. Thus, the alkali metal humate used in the invention may be termed an alkaline alkali metal humate.

The relative proportions of bentonite to the alkaline, alkalized humate is, in accordance with my invention, from about one-eighth to about three parts of the latter for each part of bentonite by weight. The best ratio is from about 75:25 to 50:50 parts bentonite to parts humate. The exact ratio preferred will depend upon the thermal characteristics and mass of metal being poured, and like factors. I have had somewhat better results using bentonite of the Wyoming type than of the so-called southern type, although both may be used, separately and indeed in admixture.

The results of proceeding in accordance with my invention are remarkable and unexpected in that a super-additive effect is obtained within the relative proportions disclosed and claimed. This is shown graphically in the drawing, in which the green compressive strength and the green shear strength are shown for a series of molding sand compositions made up of standard Ottawa sand, 50- to 70-grain neness number, using 94% sand, 4% total binder, and 2% moisture. The binder was varied throughout a range of 100% Wyoming bentonite and zero percent sodium humate to zero percent Wyoming bentonite and 100% sodium humate. The percentage of the sodium humate in the binder is plotted as abscissae and the strengths in pounds per square inch as ordinates. It will be noted that whereas 100% Wyoming bentonite in the binder gave a green compressive strength of 4.6 p.s.i., and 100% sodium humate in the binder gave a green cornpressive strength of about 7.5 p.s.i., quite remarkably with all intermediate ranges between about 1./s :1 sodium humate1bentonite (11% sodium humate) and 3:1 (75% sodium humate) the green compressive strength was greater than would be calculated by arithmetic means, and indeed reached values in excess of 10 p.s.i. for ratios within my preferred range. Thus, equal parts of sodium humate and Wyoming bentonite gave a green compressive strength of 11 p.s.i., almost twice as great as the figure of about 5.8 as calculated on the basis of simple additivity.

Exactly similar statements held for the green shear strengths. These were 1.4 p.s.i. for all bentonite and 1.9 for all sodium humate. For mixed binders having ratios within the disclosed and claimed inventive range, the green shear. strength in all cases is substantially greater than additive, and indeed, was about 2.6 p.s.i., for the 50:50 mixture, whereas the arithmetically expected value would have been 1.65.

These synergistic results were not found with mixtures of non-bentonitic clays and alkaline humate. Although the reason mixtures of bentonitic and alkaline humate give such super-additive sand strengths is not known, the fact that the absorption capacity and pH values of these mix- EXAMPLE l Fifty pounds of 20G-mesh Wyoming bentonite were dry blended with S0 pounds of the product obtained by grinding weathered North Dakota lignite having an alkali solubility .of 78% with one-tenth its weight (dry basis) of sodium hydroxide, and subsequently drying and grinding `the product. This material was used as a binder in making up a molding sand mix consisting of 94 parts by Weight Ottawa sand, 4 parts of this binder, and 2 parts water. The ingredients without the water were dry mulled for 3 minutes in a foundry muller, the water then added, and then mulled for -an additional 10 minutes. The molding sand composition-s thus produced were tested in accordance with standard methods of the American Found-rymens Association. The green compressive strength and the green shear strength appear in the drawing; the dry compressive strength was 19 p.s.i., and the permeability was 1-89 by the standard test method. This produced a molding sand having excellent molding properties, good shakeout, and leaving a clean surface on the casting.

EXAMPLE 2 The procedure of Example l was carried out, except :that Texas bentonite was used in place of the Wyoming bentonite. With the same proportions, the green compressive strength was 9.5 p.s.i., the green shear strength was 2.3 p.s.i., the dry compression strength was 22 p.s.i., and the permeability was 202.

EXAMPLE 3 In this example, the effect of alkaline, alkalized humate was compared with unalkalized humic material. To a foundry muller was added 94 pounds of Ottawa sand, followed by 2 pounds of Wyoming bentonite. The -muller was started, .and 2 pounds of the alkaline, alkalized humate, prepa-red as in Example l, were added and dry mulling continued vfor three minutes. Two pounds of water were .then added and mulling continued for an additional ten minutes.

The same procedure was repeated except that two pounds of ground Weathered North Dakota lignite, which had not been subjected to the alkalzing process, were substituted for the alkaline humate. These foundry sand mixes were tested by standard methods ofthe American Foundrymens Association, with the following results:

EXAMPLE 4 A foundry test was carried out using about 18,000 pounds of a silica sand known as Manley No. 410 (fineness number of 55), made up to an eventual binder content of 4.4% by weight, the sand binder consisting of by weight of Wyoming bentonite and 25% by Weight of the product obtained by mixing lignite with sodium hydroxide in a 4ratio of 6:1. Water was .added lto an eventual moisture content of about 3.2%. This was used in making a gray iron casting in the form of a hub approximately 5 inches in diameter and 5 inches high. The .sand unix previously used was a conventional one using the same base sand, approximately 5% Wyoming bentonite and from 3% to 5% of bituminous coal (sea coal). With the previously used sand, trouble had been experienced with small gas explosions in the mold during pouring. This was completely eliminated when using sand of Athe present invention, and the amount of smoke produced during the .pouring was substantially reduced.

Moreover, the inventive sand composition was successfully used with about 2% less moisture than the previously used sand, and it had two 'and onetha'lf times the permeability of the latter, even though the same sand base was used.

EXAMPLE 5 Good resul-ts obtained Ias described in Example 4 above gave encouragement to the production of a 1,300-pound batch of sand mix using a much ner sand, Manley number 805, with the additives as previously described in Example 4. The inherently vhigh permeability obtained in laccordance with the invention permitted the use of the liner sand with a consequent improvement in the finish of the castings obtained. The permeability of the mix was 55, which was higher than the average permeability ot about 45 of the coarser sand ymix formerly used, and already described in Example 4 above.

The amount of binder, -that is, the .total amount of -bentonite and the alkaline, alkalized humate used in my molding sands is subject to some variation, depending upon the personal preferences of the foundrymen, the sand grain fineness number, and the characteristics of the particular bentonite chosen. In general, suflicient binder rshould be used to give a green compression strength of at least 4 p.s.i. In practice, with 'commonly used sands and commonly used bentonites and using the alkalized humate in accordance with my invention, a total binder content of 2% to 3% is sucient to achieve this minimum. As stated Iand shown in the examples, however, approximately 4% binder is a good working average and gives green compression strengths well in excess of the minimum acceptable minimum of 4 p.s.i. The amount of water used is likewise subject to variation, depending upon the components selected, and -indeed, even the climatic conditions under which the molding sand composition is used. Here again, about 2% to 4% is a good working range.

It will be understood that while I prefer to practice my invention by using simply sand, bentonite, the alkalized humate as described, and water, it is not, in gene-ral, harmful to add any of the minor additives often used in connection with clay-sand molding sands, such as small amounts (perhaps 1% lto 2%) of silica our, iron oxide, finely powdered coal, cereal, pitch, and the like. However, :these additional additives are by no means necessary, and one of the great practical advantages of the present invention is that it :renders unnecessary the inclusion of such organic additives as coal or pitch in the molding sand composition.

Throughout the specification and claims, p.s.i. is an abbreviation for pounds per square inch.

When using weathered lignite as the sou-ree of the humate, I prefer that it have an alkali solubility of at lea-st by weight, so as .to keep the amount of unreacted lignite to a minimum when the alkaline, alkalized humate is prepared therefrom.

It will be appreciated that while I have described my invention with the aid of numerous examples, specific materials, specific proportions, and the like, considerable variation is possible in materials and proportions, within the broad scope ofthe invention as set -forth in ythe claims which follow.

Having described my invention, I claim:

1. A molding sand composition comprising as a major constituent sand, and suflcient bentonite admxed with from about one-eighth to about three times the weight of said ben-tonite of an alkaline, alkali metal humate to f impart to said composition a green compressive strength of at least four pounds per square inch.

2. A composition in accordance with claim 1 wherein said alkali metal is selected from the group consisting of sodium, potassium, .and lithium.

3. A compositi-on in accordance with claim 1 wherein said humate is sodium huma-te.

4. A composition in accord-ance with claim 1 wherein said humate is the reaction product of lignite having an alkali solubility of at least 65% by dry weight with sucient sodium hydroxide to produce an alkaline sodium humate.

S. A composition in accordance with claim 1 wherein the bentonite and alkaline, alkali met-al humate have been mixed together before .addition to said sand.

References Cited UNITED STATES PATENTS 1,420,750 6/1922 Rodman 260-515 2,237,593 4/1941 Dunbeck 106--38.9 3,210,203 10/1965- Cowan et al M16-38.35 3,212,144 10/1965 Capps 106-38.35 XR FOREIGN PATENTS 717,419 9/ 1965 Canada.

JULIUS FROME, Primary Examiner. L. HAYES, Assistant Examiner.

U.S. Cl. X.R. 10G- 38.35, 38.9, 71

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