WO1999030855A1

WO1999030855A1 - A method of treatment of moulding sand

Info

Publication number: WO1999030855A1
Application number: PCT/GB1998/003641
Authority: WO
Inventors: Martin John Luxton; Brian Lynas
Original assignee: Perstorp Chemitec Ltd
Current assignee: Perstorp Chemitec Ltd
Priority date: 1997-12-12
Filing date: 1998-12-07
Publication date: 1999-06-24
Anticipated expiration: 2000-06-12
Also published as: AU1442499A; GB9726392D0

Abstract

Sand which is to be used as a sand for making a mould or core in a foundry, which can be 'new' sand, or reclaimed sand, is pre-treated by treating the sand with an organic hydroxy acid such as an α-hydroxy acid, for example lactic acid or glycolic acid. Subsequently, the pre-treatment sand my be subjected to thermal processing at a temperature in excess of 580 °C and may then be used to make a mould or core by coating the sand particles with a pottassium based alkali phenolic resin and a mixed ester catalyst. It has been found that the moulds and cores present excellent compression strength.

Description

"A METHOD OF TREATMENT OF MOULDING SAND"

THE PRESENT INVENTION relates to a method of treatment of moulding

sand intended for use in making foundry cores and moulds and sand treated by

the method.

Sand and other particulate aggregates (which will all be termed "sand"

in this Specification) are utilised to form foundry moulds and cores which are

used when making castings from molten metal. The sand must be bonded when

making the moulds and cores and it has been proposed to use an ester-cured

alkaline phenolic resin binder as a bonding material.

It is conventional to recycle the foundry sand that has been used to make

a mould or core after the casting process is finished. This minimises the cost of

obtaining fresh sand, and also minimises the problems associated with the

disposal of the used sand. An initial step in the recycling process comprises the attrition of the

bonded sand, to break up the sand into individual particles. Some of the resin

utilised as a binding material may be removed in this process, by being ground

into particles of a very small size, but typically the sand particles remain at

least partially coated with resin. The sand particles may be heat-treated in a

fluidised bed to remove more of the resin material. However, the individual

sand grains may agglomerate in the fluidised bed if the fluidised bed is operated

at a high temperature. If the fluidised bed is operated at a lower temperature,

there is a risk that undesirable emissions may be generated and also, at lower

temperatures, it is more difficult to remove the resin.

It is possible to re-use sand which has been processed in the manner

described above if the processed sand is admixed with some new sand, with the

resultant mixture comprising approximately 80% processed sand and

approximately 20% new sand.

It has been suggested that the agglomeration in the fluidised bed is

caused by the presence of alkali metal, typically sodium or potassium, in the

resin material. It is thought that the alkali metal is generally present in the form

of its hydroxide, or in the form of an alkali metal ester salt. It is thought that the de-composition of these sodium or potassium compounds in the thermal bed

initiates the agglomeration of the sand particles.

It has been suggested in WO 94/05448 that the sand particles should be

treated after attrition and before thermal processing with an inorganic acid, or

an ammonium salt of an acid to convert the alkali metal into high melting point

alkali metal compounds. It is thought that this course of action can lead to

corrosion problems for the plant used, and also can, in some cases, lead to the

creation of toxic gases, such as chlorine.

A second suggestion that has been made in WO 94/26439 involves

adding a clay additive to the sand, usually as a water-based slurry, before the

sand is subjected to thermal processing. This process, however, gives rise to

problems with pumping the clay slurry which requires continuous agitation to

remain homogeneous, and also leads to problems with dust generation.

The present invention seeks to provide an improved method for the

treatment of sand intended for use when making a mould or core as a moulding

sand, and in particular seeks to provide a process which can be used in the

reclamation of pre-used sand. However, the invention also relates to the pre-

treatment of fresh sand before it is used in making a mould or core. According to this invention there is provided a method of pre-treating a

sand to be used as a sand for use in making a mould or core comprising the step

of treating the sand with an organic hydroxy acid.

The hydroxy acid may be an α-hydroxy acid and in particular, may be

an α-hydroxy acid having a terminal carboxyl group. The acid may be lactic

acid or glycolic acid, and may be present in the form of a solution.

In one embodiment the solution is an 80% lactic acid solution, and the

sand may be pre-treated with 0.25-5 percent by weight of this solution.

Alternatively, a solution may be a 40% lactic acid solution, and the sand may

be pre-treated with 0.25-5 percent by weight of the solution. Alternatively

again, the acid may be a 49% glycolic acid solution, and the sand may be

treated with 0.25-5 percent by weight of such a solution.

A refractory material may be added to the same to assist in ensuring free

movement of the sand. The refractory material may be of paniculate form,

having a particle size of less than 0.5 mm diameter. The refractory material

may be china clay. The sand may be sand which, prior to said pre-treatment, is un-treated

and un-used sand such as Chelford 50 grade sand. Following the pre-treatment

the sand may be thermally processed at a temperature of above 580°C.

Alternatively the sand that is treated is an attrition re-claimed sand from

a foundry using alkaline phenolic resins.

Preferably the thermal reclamation is effected at a temperature of

approximately 580°C-620°C, in a commercially available fluidised bed furnace

or a rotating kiln type furnace. For laboratory tests a muffle furnace may be

used at approximately 620°C for 5 hours.

In a preferred embodiment of the invention the sand is subsequently

used to make a mould by coating the sand particles with a sodium or potassium

based alkaline phenolic resin and mixing the coated particles with a mixed ester

catalyst.

The invention relates to sand when pre-treated by the method of the

invention, and also relates to a foundiy mould or core fabricated from sand pre-

treated by the method of the invention. In order that the invention may be more readily understood, and so that

further features thereof may be appreciated, the invention will now be

described, by way of example, with reference to the accompanying drawing

which is a chart illustrating re-bond strength results.

In embodiments of the invention, sand which is to be utilised as a sand

for use in making a mould or core is pre-treated with an organic hydroxy acid,

and in particular an organic α-hydroxy acid having a terminal carboxyl group.

The term "α-hydroxy" means that the carbon atom which is adjacent the

carboxyl group is provided with a hydroxy 1 group. Preferred acids are lactic

acid (which is α-hydroxypropionic acid ) and glycolic acid (which is α-

hydroxy acetic acid).

It has been found, as will be explained in greater detail hereinafter, that

when new sand is tieated with a hydroxy acid, such as lactic acid or glycolic

acid, the sand presents enhanced compression strength characteristics as

compared with untreated sand, when used to make a casting mould.

Additionally, it has been discovered that if sand which has been used for

moulding purposes with a potassium based alkaline phenolic resin binder is

subjected to attrition and is then pre-treated with lactic acid or glycolic acid before being subjected to thermal reclamation, the reclaimed sand presents very

good compression strength characteristics, when used to make a casting mould.

In certain types of commercially available thermal reclamation furnace,

there are areas within the bed of fluidised sand in which the sand is relatively

still compared with other areas where the sand is being fluidised vigorously. It

may be necessaiy in some cases to add a small amount of a refractory material

with the alpha-hydroxy acid to ensure free movement of the sand grains. The

refractory material should preferably be in a form with particle size of less than

0.5 mm diameter, such that it can be removed during fluidisation and

calcination of the sand and collected in the dust collection equipment. This

material can be one of a wide range of products well known in the refractory

and related industries, including but not limited to: graphite, alumina, other

metal oxides or carbonates, zeolites and clays. As an example, an amount of

0.048% (on sand weight) of china clay has been found to be beneficial, and can

be added separately to the sand or in a suspension with the alpha-hydroxy acid

in water. Both base exchanging and non-base exchanging clays can be used, as

the prime function is the refractoriness of the particles.

EXAMPLE 1

New sand, which was Chelford 50 grade new sand, intended for a mould

for casting purposes was pre-treated with an 80% lactic acid solution (CH₃CH₂OH COOH). The lactic acid solution was applied to the sand at a rate

of 3 percent by weight. The sand was then treated at 620°C for five hours in a

muffle furnace.

A sample was sieved to determine particles size. 50g of sand was placed

in a stack of sieves of reducing mesh sizes. The stack of sieves was shaken

with a sieve shaker for 20 minutes and the amount of sand on each sieve was

weighed to determine the percentage weight of the sand on each sieve.

The percentage weight loss on ignition was determined by heating a ten

gram sample of the sand for approximately 2 hours at a temperature of

approximately 925°C. The percentage weight loss was measured. The pH of

the sand, following attrition, was measured by boiling 5g of sand with lOOg of

de-ionised water using a magnetic stirrer until the pH as measured with an

electronic pH meter was constant. In order to determine the percentage of

elutable alkali, in particular sodium and potassium, 5g of sand was boiled in

100 ml of de-ionised water for 10 minutes and, after cooling and filtering, the

water was diluted back to 100 ml. The alkali content of the water was

measured using a flame photometer.

A sample of the sand was subsequently mixed with 1.3 percent by

weight (based on sand weight) of APR 3000 (potassium based alkaline phenolic resin), (APR 3000 is a resin which is commercially available from Perstorp

Chemicals UK) and 22 percent by weight (based on the resin weight) of a

mixed ester catalyst comprising 5.3 percent butyrolactone, 20.1 percent

propylene carbonate and 74.4 percent tiiacetin. The sand was used to mould

5.08cm (2") cylindrical test specimens which were stored at 20°C and at 60

percent relative humidity. Compression strength of the specimens was

measured at intervals over 24 hours using a + GF + (Georg Fischer) universal

strength machine. The strength was measured in terms of kN/m . The set time

of the specimens was determined by pressing the specimens at frequent

intervals by hand until it was no longer possible to make an impression on the

sand. The results are shown in column 1 of Table 1.

EXAMPLE 2

The procedures earned out with the sand of Example 1 were repeated

but without the pre-treatment and heat treating steps. Thus, the test specimens

were made with un-treated sand and then tested. The results are shown in

column 2 of Table 1.

It is to be noted from the results of Examples 1 and 2 that the new sand

which was pre-treated with the lactic acid solution had a superior compression

strength development, had very comparable sieve grading, but had a reduced loss on ignition, and a reduced proportion of elutable sodium as compared with

the un-treated sand.

EXAMPLE 3

Re-claimed sand, initially Chelford 50 grade raw sand, from a foundry

using alkaline phenolic resins as a binder, was subjected to attrition and was

then pre-treated with a 65 percent sucrose solution, the sucrose solution being 3

percent by weight of the sand.

The sand was then subjected to thermal reclamation at 620°C for five

hours in a muffle furnace.

Samples of the sand were then taken and sieved as described in

Example 1, and also processed as described in Example 1 to determine loss on

ignition, pH and percentage of elutable alkali.

Subsequently, a sample of the sand was re-bonded with 1.3% APR 3000

(a potassium based alkaline phenolic resin) based on sand weight, together with

22% (based on resin weight) of a mixed ester catalyst comprising 5.3%

butyrolactone, 20.1% propylene carbonate and 74.7% triacetin. The sand was

moulded into 5.08 cm (2 inch) cylindrical test specimen, and the test specimens

were stored at 20°C and at 60% relative humidity for a period of 24 hours. Compression strength was measured at regular intervals during the 24 hour

period and the set time was determined. The results are indicated in column 1

of Table 2.

EXAMPLE 4

The procedures of Example 3 were repeated, but the sand was pre¬

heated with 3% by weight of an 80% lactic acid solution (α-hydroxypropionic

acid). The results are indicated in column 2 of Table 2.

EXAMPLE 5

The procedures of Example 3 were repeated, but the sand was as

received from the foundiy following attrition and was not pre-treated. The

results are indicated in column 3 of Table 2.

EXAMPLE 6

The procedures of Example 3 were repeated, but the sand was pre-

treated with 2% by weight of a 40% lactic acid solution. The results are

indicated in column 4 of Table 2.

EXAMPLE 7

The procedures of Example 3 were repeated using sand received from

the foundiy, but the pre-treating step and the thermal reclamation step were omitted. The sand was re-bonded using, (instead of 1.3% APR 3000), 1.4% by

weight APR 3000 and 22% (based on the weight of resin) of the mixed ester

catalyst. The results are indicated in column 5 of Table 2.

EXAMPLE 8

The procedures of Example 3 were repeated using sand pre-treated with

2% by weight of a 50% acetic acid solution. The results are indicated in

column 6 of Table 2.

EXAMPLE 9

Procedures of Example 3 were repeated using sand pre-treated with 2%

by weight of a 35% sorbitol solution. The results are indicated in column 7 of

Table 2.

EXAMPLE 10

The procedures of Example 3 were repeated, but the sand was pre-

treated with 1% by weight of graphite powder and 2% by weight water as a

slurry. The results are indicated in column 8 of Table 2. EXAMPLE 11

The procedures of Example 3 were repeated, but the sand was pre-

treated with 3% of an 80% propionic acid solution. The results are indicated in

column 9 of Table 2.

EXAMPLE 12

This was a repeat of Example 6, and the results are shown in column 10

of Table 2.

EXAMPLE 13

The procedures of Example 3 were repeated, but the sand was pre¬

heated with 2% by weight of a 49% glycolic acid (α-hydroxyacetic acid)

solution. The results are shown in column 1 1 of Table 2.

It is to be observed, from Table 2, that columns 2, 4, 10 and 1 1 comprise

examples of the invention. It is to be noted that these columns present the

highest compression strengths, whilst presenting very adequate loss on ignition

figures and veiy adequate elutable alkali figures.

Whilst, in the Examples, reference is made to pre-treated sand being

thermally processed at a temperature of 620°C for 5 hours in a muffle furnace

(which was the experimental procedure used by the inventors) it is believed that the same effect will be achieved on a commercial scale by thermally processing

the sand in a fluidised bed at a temperature of from 580 to 620°C for a period

of between 10 and 20 minutes.

Reference is now made to the accompanying drawing which is a

graphical figure illustrating re-bond strength results for various samples. The

samples utilised in the preparation of the chart that constitutes the

accompanying drawing were made from reclaimed sand from an aluminium

foundiy. The reclaimed sand was tieated in five different ways to produce five

different samples.

In making the first sample, the sand was subject to attrition, and was

then re-treated with an 80% lactic acid solution, with the lactic acid solution

being applied to the sand at a rate of 1% by weight. The sand was then treated

in a commercially available fluidised bed furnace at 580-590°C.

In producing the second sample, the re-claimed sand, following attrition,

was tieated with solution comprising 62% lactic acid and 23% clay. The clay

was china clay having a particle size of less than 0.5 mm diameter. The

solution was added at a rate of 1.1% by weight of the sand. The sand was then

treated in a commercially available fluidised bed furnace at 580-590°C. In making the third sample the sand was subject to attrition, and then a

25% clay slurry was prepared, using china clay having a particle size of less

than 0.5 mm, and the slurry was applied to the sand at a rate of 1% by weight.

The sand was then treated as in samples 1 and 2.

In fabricating the fourth sample, a solution was made of 62% lactic acid

and 19% calcium bentonite. This solution was applied to the sand at a rate of

1.5% by weight. The sand was then tieated as in samples 1 and 2.

In making the fifth sample, the sand was only subjected to attrition

reclamation with no thermal treatment.

Separate test specimens were moulded using sand from the five samples

of sand. Each test specimen was made by mixing the sand with 1.5% by

weight (based on sand weight) of APR 3500 (potassium/sodium based alkaline

phenolic resin), (APR 3500 is a resin which is commercially available from

Perstoip Chemicals UK) and 25% by weight (based on the resin weight) of a

mixed ester catalyst comprising 5.3% butyrolactone, 20.1% propylene

carbonate and 74.4% tiiacetin. The resultant sand was used to mould the test

specimens which were conventional 5.08 cm (2") cylindrical test specimens.

The moulds were removed from the specimens after 19 to 20 minutes and the specimens were then stored at 20°C and at a relative humidity of 50%. The

compression strength of the test specimens was measured after ^l/ι hr., 1 hr., 2

The accompanying drawing shows quite clearly that whenever the test

specimens were tested for compression strength, the samples made from sand

treated with lactic acid showed a much higher compression strength than the

samples of sand which had not been treated with lactic acid. There was no

evidence of fritting in the furnace with any of the thermally tieated samples.

Thus, the present inventors have made the surprising discovery that if

sand which is to be used as a sand for making moulds and cores, is pre-treated

with an organic hydroxy acid, and in particular, an organic α-hydroxy acid,

especially acids having terminal carboxyl groups such as lactic acid (α-

hydroxypropionic acid) and glycolic acid (α-hydroxyacetic acid) especial

benefits are provided. The pre-treatment can be the pre-treatment of new sand,

or a pre-treatment step of reclaimed sand that has been subjected to attrition.

The sand may be subjected to thermal processing or thermal reclamation before

the sand is used to make moulds or cores. Compression strength development:

1 2

1/2 hour 1 ,040 1 ,000

1 hour 1,900 1,600

2 hours 2,600 2,200

4 hours 3,200 2,600

24 hours 4,600 3.400

Set time (mins) 19 20

Sieve grading (% retained)

710 micron 0 1 0 1

500 6 7 5 8

355 20 3 20 4

250 43 44 6

180 27 6 26 2

125 2 1 2 4

90 0 2 0 3

63 0 1 0 1

Pan 0 0

AGS (ave grain size) 319 316

Loss on Ό ig¹nition % 0 06 0 2 pH 6 5 5 3

Elutable alkali %

Na 0 01 0 02

K 0 0

Total 0 01 0 02

TABLE 1 CO

TABLE 2

Claims

CLAIMS:

1. A method of pre-treating a sand to be used as a sand for making a mould

or core comprising the step of treating the sand with an organic hydroxy acid.

2. A method according to Claim 1 wherein the hydroxy acid is an α-

hydroxy acid.

3. A method according to Claim 2 wherein the α-hydroxy acid has a

terminal carboxyl group.

4. A method according to Claim 3 wherein the acid is lactic acid.

5. A method according to Claim 3 wherein the acid is glycolic acid.

6. A method according to any one of the preceding Claims wherein the

acid is present in the form of a solution.

7. A method according to Claim 6 wherein the solution is an 80% lactic

acid solution.

8. A method according to Claim 7 wherein the sand is pre-treated with

0.25-5 percent by weight of said 80% lactic acid solution.

9. A method according to Claim 6 wherein the solution is a 40% lactic acid

solution.

10. A method according to Claim 9 wherein the sand is pre-treated with

0.25-5 percent by weight of the 40% lactic acid solution.

11. A method according to Claim 6 wherein the acid is a 49% glycolic acid

solution.

12. A method according to Claim 1 1 wherein the sand is treated with 0.25-5

percent by weight of the 49% glycolic acid solution.

13. A method according to any one of the preceding Claims wherein a

refractory material is added to the sand.

14. A method according to Claim 13 wherein the refractory material is in

paniculate form, having a particle size of less than 0.5 mm in diameter.

15. A method according to Claim 13 or 14 wherein the refractory material is

china clay.

16. A method according to any one of the preceding Claims wherein the

sand, prior to said pre-treatment, is un-used sand.

17. A method according to any one of Claims 1 to 15 wherein the sand that

is pre-treated is an attrition re-claimed sand from a foundry using alkaline

phenolic resins.

18. A method according to Claim 16 or 17 wherein following the pre-

treatment, the sand is thermally processed at a temperature in excess of 580°C

19. A method according to Claim 18 wherein the sand is thermally

processed in a fluidised bed at a temperature of 580°-620°C with a residence

time of 10 to 20 minutes.

20. A method according to any one of the preceding Claims wherein the

sand is subsequently used to make a mould or core by coating the sand particles

with a potassium based alkali phenolic resin and a mixed-ester catalyst.

21. A method of pre-treating sand substantially as herein described with

reference to Example 1.

22. A method of pre-treating sand substantially as herein described with

reference to Example 4.

23. A method of pre-treating sand substantially as herein described with

reference to Examples 6 and 12.

24. A method of pre-treating sand substantially as herein described with

reference to Example 13.

25. Sand, when pre-treated by a method according to any one of the

preceding Claims.

26. A foundry mould fabricated from sand pre-treated by a method

according to any one of Claims 1 to 24.

27. Any novel feature or combination of features disclosed herein.