GB1570483A - Process of obtaining hydrated dolomite lime for use in silicate brick manufacture - Google Patents

Description

(54) A PROCESS OF OBTAINING HYDRATED DOLOMITE LIME FOR USE IN SILICATE BRICK MANUFACTURE (71) We, SUTCLIFFE, SPEAKMAN & BR< COMPANY LIMITED, a British Company, of Leigh, Lancashire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a process of obtaining hydrated dolomite lime for use in silicate brick manufacture.

Calcium silicate bricks are usually made by mixing together lime, sand and water in definite proportions. The mixture is then pressed into brick form and hardened in a steam autoclave at a pressure ranging from 150 to 260 Ibs per square inch for several hours.

The resultant product is a hard, durable brick for construction purposes.

The reaction which occurs in the autoclave is between the free silica in the sand and the hydrated calcium silicate which binds the sand grains together.

The requirements for a successful brick product are that the sand grains should be free from any coating which would inhibit the calcium silicate reaction and that, prior to pressing the sand/lime mixture into brick form, the lime should be substantially fully hydrated.

If the lime is not substantially fully hydrated, then further hydration will occur in the autoclave, causing the bricks to burst.

A dolomite lime is traditionally obtained by calcining a dolomite rock, which is a calcium magnesium carbonate complex.

Magnesium carbonate calcines to form magnesium oxide at a much lower temperature (500600"C) than that at which calcium carbonate calcines to form calcium oxide, (over 950"C).

When a mixture of these two carbonates is calcined until all the calcium carbonate has been changed to calcium oxide, the magnesium oxide which forms first, becomes seriously overcalcined. In this state the necessary reaction between magnesium oxide'and water, to form magnesium hydrate, becomes extremely difficult to promote by normal commercial slaking processes.

With modern lime kilns, the control of kiln temperatures, rate of throughput of the rock, and the particle size of the rock can be control led so that the over calcining of the magnesium oxide can be kept to a minimum. This type of lime can usually be successfully slaked in a bigb pressure steam slaking drum.

There are, however, many sources of dolomite lime in which the original stone has been burnt in a very primitive manner, in which there is no control over the temperature of calcining. This occurs in many parts of the world where lime burning is still a village industry.

The stone is usually collected in pieces varying from four inches to twelve inches in diameter, these stones then being brought to a central area for burning or calcining.

The stones are placed into a pile and heated by either a wood fire or, by a natural gas burner, as is the case in some countries in the Middle East. Buming is continued for 2 days, o until the whole pile is considered to be burnt.

After the allotted time, the source of heat is extinguished and the pile allowed to cool.

Slaking of the crudely burnt lime is then achieved by spraying the pile with water. The burnt lime reacts with the water to form calcium and magnesium hydroxides and in doing so, falls to a fine powder.

The slaked lime is allowed to dry, then is usually sieved through rotatable sieves. The material which does not pass through the sieve is discarded as unslaked material and "foreign" rocks. The material which passes through the sieves is collected as hydrated lime.

This so-called hydrated lime still contains a high proportion of unslaked material and be cause of the relatively high temperatures achieved in burning, especially when using natural gas bumers, and also the duration of the calcining period, this is extremely difficult toslake.

Even conventional pressure hydrators cannot produce a consistently fully hydrated product from this type of lime, without it having been subjected to some form ofpre-treatment.

As stated above, burnt lime, when slaked with water, falls to a fine powder. This fact indicates that any coarse material present has not fully reacted with water.

It is among the objects of the invention to mitigate these disadvantages and to seek to provide a process for producing usable hydrated dolomite lime.

According to the invention there is provided a process of obtaining hydrated dolomite lime for use in calcium silicate brick manufacture, comprising: providing calcined dolomite; separating from the calcined dolomite a fraction consisting predominantly of particles of size less than 106 microns; and slaking this fraction under pressure and for a time sufficient to cause substantially complete hydration of the magnesium oxide and calcium oxide therein.

Preferably, the separation of said fraction may be carried out by a sieving process.

Altematively, the separation of said fraction may be carried out in an air classifier, whereby gritty particles are rejected.

The classified lime of the required size may be mixed with sand and water prior to the slaking.

The slaking may comprise high pressure steam slaking.

The steam pressure in the high pressure steam slaking may be greater than 100 lbs per square inch.

The steam pressure may be 140 lbs per square inch.

The slaking may be carried out for a period of 1 hour.

The calcined dolomite may be that produced from primitive village lime works.

Experiments have been carried out to confirm that any coarse material has not fully reacted with water and to decide at below what particle size the lime (i.e. calcined dolomite) becomes susceptible to pressure hydration, and result in a product which is compatible for use in calcium silicate brick manufacture.

Experiments were carried out on a dolomite lime with the following analysis after slaking: Impurities (Si02 + Rio,) % % 1.72 Total Calcium compounds (CaO) % 42.62 Total Magnesium Compounds (MgO) % 35.87 Loss on Ignition % 19.86 Carbonates (CO2) % 3.24 By conventional calculation, the above is composed as follows: Calcium Carbonate CaC03 % 7.35 Calcium Hydroxide Ca(OH)2 % 50.83 Magnesium Hydroxide Mg(OH)2 % 17.20 Magnesium Oxide MgO % 22.97 It will be seen that just over one fifth of the material is unslaked magnesium oxide.

Investigations were carried out in which the calcined dolomite sample, before slaking, was sieved through progressively finer sieves and the magnesium or oxide content of each fraction was determined by analysis.

It was noted in these experiments that the calcined dolomite fraction passing through a 150 B.S. mesh (106 micron aperture) contained only 10.09% by weight of magnesium oxide, less than half of the original amount present.

This clearly indicated that most of the overburnt unsound material was contained in the gritty parts of the lime, and that this part of the lime should be removed prior to its use in calcium silicate brick manufacture.

In order to confirm this the following specific brick making tests were carried out: Tests were carried out on the lime as received and on the fractions of lime sieved through a 16 B.S. mesh and through a 150 B.S.

mesh and also using the residual lime greater than 150 B.S. mesh and ground to standard lime fineness, i.e. lime which has a particle size such that a minimum of 95% of the total passes through a 150 B.S. mesh sieve.

The standard amount of lime used in all the series of tests was equivalent to 10% effective Ca(OH)2 (17.2% of sample).

The slaking tests were carried out with a moisture content of 12% of the sand-lime mixture, and the moisture content of the mixture at the pressing stage was 5.0%.

Test No. 1 Lime as received A sand/lime mix was slaked at 70 lbs/in2 steam pressure for 1 hour. The pressed bricks were autoclave cured at 140/150 lbs/in2 for 10 hours.

Result: All the specimens burst during autoclave curing. (Lime unsound).

Test No. 2 Lime as received Sand/lime mix slaked at 150 Ibslin2 steam pressure for 1 hour. Pressed bricks autoclave cured at 140/150 lbs/in2 for 10 hours.

Result: All the specimens burst during autoclave curing (lime unsound).

Test No. 3 The lime was sieved through 16 B.S. mesh (1.0 mm apperture).

The sand/lime mix was slaked at 150 lbs/in2 for 1 hour.

Pressed bricks autoclave cured at 140/150 lbs/in2 for 10 hours.

Result: All the specimens expanded and crazed during autoclave curing. (Lime unsound).

Test No. 4 Lime Sieved through 150 B.S. mesh In this test the sand/lime mixture was not slaked.

Pressed bricks autoclave cured at 140/150 lbs/in2 for 10 hours.

Result: All the specimens produced had expanded by approximately 10% in length. This indicates that slaking had occurred in the autoclave. The bricks were not sound.

Test No. 5 Lime Sieved through 150 B.S. mesh A sand/lime mixture was slaked at 25 Ibis/ in2 steam pressure for 1 hour. The pressed bricks were autoclave cured at 140/150 lbs/ in2 for 10 hours.

Result: All the specimens had expanded. The bricks were not sound.

Test No.6 Lime Sieved through 150 B.S. mesh Sand/lime mix slaked at 70 Ibs/in2 steam pressure for 1 hour. Pressed bricks autoclave cured at 140/150 Ibs/in2 for 10 hours.

Result: The specimens had expanded slightly (approximately 1.0% in Length) and were surface crazed (unsound) because the pressure during slaking was not sufficiently high to cause substantially complete slaking of the calcined dolomite.

Test No.7 Lime Sieved through 150 B.S. mesh Sand/lime mix slaked at 120/125 lbs/in2 steam pressure for 1 hour. Pressed bricks autoclave cured at 145/150 lbs/in2 for 10 hours.

Result: All the specimens produced were sound.

TestNo.8 Lime Sieved through 150 B.S. mesh Sand/lime mix slaked at 140/150 lbs/in2 steam pressure for 1 hour. Pressed bricks autoclave cured at 135 lbs/in2 for 10 hours.

Result: All the specimens produced were sound.

Test No. 9 Oversize Lime remaining on 150 B.S. mesh ground to pass 150 B.S. Mesh Sand/lime mix slaked at 150 lbs/in2 steam pressure for 1 hour. Pressed bricks autoclave cured at 140 lbs/in2 for 10 hours.

Result: The specimens had expanded slightly (approximately 1.0% in length) and were surface crazed. (unsound).

Analysis of Lime after slaking in Test 8 Loss on ignition % 38.15 Carbonates (CO2) % 2.49 Calcium compounds (CaO) % 37.80 Magnesium compounds (MgO) % 32.40 Impurities (SiO2 + R2 03) % 1.81 Probable Composition Calcium Carbonate CaCO3 % 5.66 Calcium Hydroxide Ca(OH)2 % 45.71 Magnesium Hydroxide Mg(OH)2 % 4698 Impurities (SiO2 + R2 03) % 1.81 These tests show that when a dolomite limestone is burnt, a certain proportion is inevitably overburnt. Subsequent normal slaking of this material results in a product which is a mixture of fully slaked, partially slaked and unslaked material.

The very fine portion of this mixture contains a considerably reduced amount of unslaked material, which when separated e.g. by an air classifier or in a sieve, is suitable for use with a high pressure slaking system.

The specific tests have shown that sound calcium silicate bricks were produced from the fraction of lime finer than 150 B.S. mesh (0.10 mm), which was then pressure slaked with the sand and water at 140 lbs/in2 steam pressure for 1 hour prior to pressing into bricks. After autoclave curing under standard conditions all the brick specimens were found to be sound.

WHAT WE CLAIM IS: 1. A process of obtaining hydrated dolomite lime for use in calcium silicate brick manufacture, comprising: providing calcined dolomite; separating from the calcined dolomite a fraction consisting predominantly of particles of size less than 106 microns; and slaking this fraction under pressure and for a time sufficient to cause substantially complete hydration of the magnesium oxide and calcium oxide therein.

2. A process according to claim 1, in which the separation of said fraction is carried out by

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. (1.0 mm apperture). The sand/lime mix was slaked at 150 lbs/in2 for 1 hour. Pressed bricks autoclave cured at 140/150 lbs/in2 for 10 hours. Result: All the specimens expanded and crazed during autoclave curing. (Lime unsound). Test No. 4 Lime Sieved through 150 B.S. mesh In this test the sand/lime mixture was not slaked. Pressed bricks autoclave cured at 140/150 lbs/in2 for 10 hours. Result: All the specimens produced had expanded by approximately 10% in length. This indicates that slaking had occurred in the autoclave. The bricks were not sound. Test No. 5 Lime Sieved through 150 B.S. mesh A sand/lime mixture was slaked at 25 Ibis/ in2 steam pressure for 1 hour. The pressed bricks were autoclave cured at 140/150 lbs/ in2 for 10 hours. Result: All the specimens had expanded. The bricks were not sound. Test No.6 Lime Sieved through 150 B.S. mesh Sand/lime mix slaked at 70 Ibs/in2 steam pressure for 1 hour. Pressed bricks autoclave cured at 140/150 Ibs/in2 for 10 hours. Result: The specimens had expanded slightly (approximately 1.0% in Length) and were surface crazed (unsound) because the pressure during slaking was not sufficiently high to cause substantially complete slaking of the calcined dolomite. Test No.7 Lime Sieved through 150 B.S. mesh Sand/lime mix slaked at 120/125 lbs/in2 steam pressure for 1 hour. Pressed bricks autoclave cured at 145/150 lbs/in2 for 10 hours. Result: All the specimens produced were sound. TestNo.8 Lime Sieved through 150 B.S. mesh Sand/lime mix slaked at 140/150 lbs/in2 steam pressure for 1 hour. Pressed bricks autoclave cured at 135 lbs/in2 for 10 hours. Result: All the specimens produced were sound. Test No. 9 Oversize Lime remaining on 150 B.S. mesh ground to pass 150 B.S. Mesh Sand/lime mix slaked at 150 lbs/in2 steam pressure for 1 hour. Pressed bricks autoclave cured at 140 lbs/in2 for 10 hours. Result: The specimens had expanded slightly (approximately 1.0% in length) and were surface crazed. (unsound). Analysis of Lime after slaking in Test 8 Loss on ignition % 38.15 Carbonates (CO2) % 2.49 Calcium compounds (CaO) % 37.80 Magnesium compounds (MgO) % 32.40 Impurities (SiO2 + R2 03) % 1.81 Probable Composition Calcium Carbonate CaCO3 % 5.66 Calcium Hydroxide Ca(OH)2 % 45.71 Magnesium Hydroxide Mg(OH)2 % 4698 Impurities (SiO2 + R2 03) % 1.81 These tests show that when a dolomite limestone is burnt, a certain proportion is inevitably overburnt. Subsequent normal slaking of this material results in a product which is a mixture of fully slaked, partially slaked and unslaked material. The very fine portion of this mixture contains a considerably reduced amount of unslaked material, which when separated e.g. by an air classifier or in a sieve, is suitable for use with a high pressure slaking system. The specific tests have shown that sound calcium silicate bricks were produced from the fraction of lime finer than 150 B.S. mesh (0.10 mm), which was then pressure slaked with the sand and water at 140 lbs/in2 steam pressure for 1 hour prior to pressing into bricks. After autoclave curing under standard conditions all the brick specimens were found to be sound. WHAT WE CLAIM IS:

1. A process of obtaining hydrated dolomite lime for use in calcium silicate brick manufacture, comprising: providing calcined dolomite; separating from the calcined dolomite a fraction consisting predominantly of particles of size less than 106 microns; and slaking this fraction under pressure and for a time sufficient to cause substantially complete hydration of the magnesium oxide and calcium oxide therein.

2. A process according to claim 1, in which the separation of said fraction is carried out by

sieving.

3. A process according to claim 1, in which the separation of said fraction is carried out in an air classifier.

4. A process according to any one of the preceding claims, in which said fraction is mixed with sand and water prior to slaking.

5. A process according to any one of the preceding claims, in which the slaking comprises high pressure steam slaking.

6. A process according to claim 5, in which the steam pressure is a pressure in the range of 100 to 150 lbs per square inch.

7. A process according to claim 6, in which the pressure is 140 ibs per square inch.

8. A process according to claim 6 or claim 7, in which slaking takes place for a period of 1 hour.

9. A process according to any one of the preceding claims, in which the previously slaked and sieved lime is that obtained from a village industry lime works.

10. Hydrated dolomite lime obtained by a process according to any preceding claim.

11. A brick making process comprising pressing hydrated dolomite lime according to claim 10 into bricks.

12. A brick made by a process according to claim 11.