GB1585483A

GB1585483A - Lime treatment

Info

Publication number: GB1585483A
Application number: GB735077A
Authority: GB
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1977-02-22
Filing date: 1977-02-22
Publication date: 1981-03-04
Also published as: DE2807598A1

Description

(54) LIME TREATMENT (71) We, IMPERIAL CHEMICAL INDUS TRIES LIMITED, of Imperial Chemical House, Millbank, London SWIP 3JF, a British Company 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: This invention relates to lime treatment and more particularly to a process for modifying the water-reactivity of lime.

Lime (Calcium oxide) is used in a wide variety of processes which depend on the exothermic reaction of the lime with water. For example lime is used in the manufacture of lightweight building materials, and as a source of slaked lime (hydrated lime) or milk of lime (a suspension of hydrated lime in water). The process characteristics and the nature of the final product of such reactions depend critically on the way in which the lime reacts with the water, which in turn depends on the history of the manufacture of the lime.

Lime is manufactured by roasting limestone (calcium carbonate) in a kiln, and the final product obtained depends on the particular type of kiln used, the composition of the limestone and the operating conditions. In particular the reactivity of the lime to water (measured for example in terms of the rate of evolution of heat when the lime is contacted with water) may vary considerably from lime of low reactivity to lime of high or very high reactivity.

Certain modern lime kilns tend to produce lime of high or very high reactivity, and difficulties may be encountered in the use of such limes. For example when lime is used for the manufacture of milk oflime, a highly reactive lime may give an undesirably fine precipitate.

Similarly when lightweight, cellular building materials are manufactured by the reaction of quicklime with a siliceous material in the presence of water and a gassing agent, use of a highly reactive lime may lead to particle agglomeration and to a premature set causing cracking of the product.

It has been proposed to modify the reactivity of lime by the addition of certain compounds such as sugar and calcium sulphate.

However the presence of such additives may be undesirable in the final product, and excessive amounts of additive may be necessary effectively to modify the reactivity of a lime of high or very high reactivity.

According to the present invention there is provided a process for modifying the waterreactivity of lime which comprises treating the lime with water to achieve partial hydration and simultaneously or subsequently treating the lime with carbon dioxide such that the carbon dioxide taken up by the lime is from 0.1 to 10 parts by weight per 100 parts of weight of lime and taken-up water and carbon dioxide.

The water may be added as a liquid or vapour.

The quantity of water used will depend on the degree to which it is desired to modify the water-reactivity of the lime and will naturally be less than that required to fully slake the lime (i.e. to achieve full hydration and totally remove the water-reactivity). It will be appreciated that not all the water added may actually be taken up by the lime; for example when liquid water is added to a highly reactive lime as much as half may be lost as water vapour.

The proportion of the water actually taken up by the lime for any given mode of addition may readily be determined.

Thus water will generally be added to the lime such that the water taken up by the lime is from 0.5 to 15 parts by weight of water per 100 parts by weight of lime and taken-up water, and more commonly is within the range 2 to 12 parts by weight of water per 100 parts by weight of lime and taken-up water, for example about 5 parts by weight of water per 100 parts by weight of lime and taken-up water.

The amount of carbon dioxide taken up by the lime will similarly depend on the particular way in which the lime is treated with the carbon dioxide, but may be expected to vary between 0.1 to 10 parts by weight of carbon dioxide per 100 parts by weight of lime and taken-up water and carbon dioxide.

It is to be understood that the lime naturally absorbs a small lssroportion of carbon dioxide and/or water during its manufacture or from contact with the atmosphere. The treatment of the present invention provides for take up of water and carbon dioxide in addition to any which may already be present in the lime. Thus amount of water or carbon dioxide, taken up by the lime (as that expression is used herein) means the amount taken up in addition to any water or carbon dioxide which may already be present, or which may have been absorbed by exposure to the atmosphere.

The lime is treated with water and simultaneously or subsequently with carbon dioxide.

For example the treatment may take place in any of the following ways: (a) The lime is treated with water (liquid or vapour) and the hydrated product is subsequently treated with carbon dioxide; or (b) the lime is treated with a gas comprising carbon dioxide and water vapour; or (c) The lime is treated with liquid water in the presence of carbon dioxide.

The lime is preferably stirred or agitated during the treatment. Preferably the lime is finely divided. If the lime is to be ground to a desired particle size, the water (or water and carbon dioxide) may be added during or after the grinding.

The carbon dioxide may be contacted with the lime in the form of a substantially undiluted gas (optionally containing water vapour), or the carbon dioxide (and optionally water vapour) may be present in a diluent gas.

If a diluent gas is used, the concentration of carbon dioxide should be in excess 1 part by volume of carbon dioxide per 100 parts by volume of total gas and is preferably greater than 10 parts by volume of carbon dioxide per 100 parts by volume of total gas. The carbon dioxide used to treat the lime may for example be present in the form of kiln exhaust gas which commonly contains from 12 to 33 parts by volume of carbon dioxide per 100 parts by volume of total gas comprising nitrogen and a minor proportion of water vapour and oxygen.

Atmospheric pressure is conveniently used, although higher or lower pressures may be used if desired. The contact time of the lime with the carbon dioxide will depend on factors such as the partial pressure of carbon dioxide but is typically of the order of 10 minutes.

The treatment of the lime conveniently takes place at ambient temperature, although exothermic nature of the reaction with water will raise the temperature of the reaction mixture above that of the surroundings.

Reactivity-modifying agents may additionally be added to the lime. Examples of such reactivity-modifying agents which reduce the water-reactivity of lime include sugar and calcium sulphate. The reactivity-modifying agent may be mixed as a solid with the lime or may be in solution in the added water. Alternatively the reactivity-modifying agent may be present in the water with which the lime of modified water-reactivity is subsequently reacted. It will be appreciated that very much less of the reactivity-modifying agent is required than would be necessary in the absence of the treatment of the present invention. Indeed the addition of as little as 0.1 parts by weight of reactivity-modifying agent per 100 parts by weight of total lime, water, carbon dioxide and reactivity-modifying agent may significantly reduce the water-reactivity of the lime.

The water-reactivity of the lime may also be modified by the blending of limes of different water-reactivity. For example a highly reactive lime whose water-reactivity has been modified according to the present invention may give a satisfactory water-reactivity and heat evolution over a period of 10 minutes from the addition of the water (in the manufacture of lightweight building materials for example), but it may be desirable to increase the heat evolution over the first few minutes. This may be achieved by the blending of an appropriate proportion of the original highly reactive lime with the lime of modified water-reactivity.

The process of the present invention is especially suitable for modifying the waterreactivity of limes of high and very high reactivity (as herein defined). The process of the present invention may for example be used to render such limes of high and very high reactivity more suitable for the manufacture of lightweight building materials or for the manufacture of milk of lime of an acceptable particle size.

The water-reactivity of the lime is best measured in terms of the evolution of heat, as a function of time, on the addition of water.

The water-reactivity of the lime may thus be measured in the following manner: TEST TO DETERMINE THE "WA TER REACTIVITY" OF LIME 150g of lime (or lime of modified waterreactivity) is ground if necessary to a particle diameter of less than 5 mm. The lime is added to 600 ml of distilled water at 200C contained in a 1.7 1 capacity cylindrical Dewar flask of 105 mm diameter. The Dewar flask is fitted with a 25 mm thick cork lid and a two-bladed flat paddle stirrer of 70 mm diameter. The water/lime mixture is stirred at 250 rpm and the water temperature is recorded as a function of time.

The results may be presented graphically or may be characterised in terms of: (a) the temperature after 2 minutes (2 min temperature): (b) the temperature after 5 minutes (5 min temperature): (c) the temperature after 10 minutes (10 min temperature); and (d) the time in minutes taken to reach the maximum temperature (time to maximum reactivity temperature).

The water-reactivity of the lime may conveniently be classified according to the following table:- "time to maximum reactivity tempera ture" low reactivity greater than 20 min utes moderate reactivity 10 to 20 minutes high reactivity 2 to 10 minutes very high reactivity less than 2 minutes The invention is illustrated by the following Examples: EXAMPLE I 121b of highly reactive lime was ground to a particle size of 90% less than 200 British standard mesh, and liquid water was added during the grinding such that 4.6 parts by weight of water per 100 parts by weight of lime and water was taken up by the lime.

The dry, partially hydrated lime was transferred to a mixer and was agitated in an atmosphere of substantially pure carbon dioxide for 10 minutes. The take-up of carbon dioxide was 0.9 parts by weight carbon dioxide per 100 parts by weight of total, lime, carbon dioxide and water.

A portion of the lime of modified waterreactivity was removed and tested according to the "test to determine the water-reactivity of lime", and the results are presented graphically as curve C in Figure 1. The "time to maximum reactivity temperature" of the original highly reactive lime (curve A in Figure 1) was 1.5 minutes. After the treatment of the present invention, the "time to maximum reactivity temperature" of the lime was 8.5 minutes, the "2 minute temperature" was 22.50C, the "5 minute temperature" was 320C and the "10 minutes temperature" was 60"C.

EXAMPLE 2 The procedure of Example 1 was repeated except that 1.6 parts by weight of calcium sulphate per 100 parts by weight of total lime, water, carbon dioxide and calcium sulphate was added as a powder of particle size less than 52 British Standard Mesh to the lime produced in Example 1.

The "time to maximum reactivity temperature" of the lime of modified water-reactivity was 14.5 minutes, the "2 minute temperature" was 21"C, the "5 minute temperature" was 220C and the "10 minute temperature" was 30"C (Curve D of Figure 1).

EXAMPLE 3 The procedure of Example 1 was repeated except that 0.1 parts by weight of sugar per 100 parts by weight of total lime, water, carbon dioxide and sugar was added as a powder of particle size 90% less than 52 British Standard mesh to the lime produced in Example 1.

The "time to maximum reactivity temperature" of the lime of modified water-reactivity was 18 minutes, the "2 minute temperature" was 22"C, the "5 minute temperature" was 23 C and the "10 minute temperature" was 29"C (Curve E of Figure 1).

COMPARISON I By way of comparison, the procedure of Example 1 was repeated except that the lime was not treated with carbon dioxide.

The results of the "test to determine the water-reactivity of lime" are presented graphically as curve B of Figure 1. The "time to maximum reactivity temperature" was 3 minutes, and the "2 minute temperature" was 620 C.

COMPARISON 2 By way of further comparison, sugar was added to the highly reactive lime used in Example 1 in the proportion 1.4 parts by weight of sugar per 100 parts by weight of total lime and sugar. The lime was ground to a particle size of 90% less than 200 British Standard Mesh and the sugar was incorporated therein as a powder of particle size 90% less than 52 British Standard Mesh. The lime was not treated with water or carbon dioxide.

The results of the "test to determine the water-reactivity of lime" are presented graphically as curve F of Figure 2 (which also reproduces the curves of Figure 1 for comparison).

The "time to maximum reactivity temperature" was 7.5 minutes and the "2 minute temperature" was 36 C. It can be seen that, even at the undesirably high level of sugar addition shown in the Comparison, the initial temperature rise is excessively high.

COMPARISON 3 By way of further Comparison, calcium sulphate was added to the highly reactive lime used in Example 1 in the proportion 1.6 parts by weight of calcium sulphate per 100 parts by weight of total lime and calcium sulphate. The lime was ground to a particle size of 90% less than 200 British Standard Mesh and the calcium sulphate was incorporated therein as powdered gypsum of particle size 90% less than 52 British Standard Mesh. The lime was treated with liquid water such that 4.6 parts by weight of water per 100 parts by weight of total lime, water, and calcium sulphate were taken up. However the lime was not treated with carbon dioxide.

The results of the "test to determine the water reactivity of lime" are presented graphically as curve G of Figure 2. The "time to maximum reactivity temperature was 5 minutes and the "2 minute temperature was 520C. It can be seen that the lime remains excessively reactive after this treatment.

EXAMPLE 4 The lime of modified water-reactivity produced by the process described in Example 1 (reactivity curve C in Figure 1), was used to manufacture a lightweight cellular building block by the following method: 40 Ibs of water at 200C was added to a 90 litre mild steel vessel fitter with a stirrer operating at 310 rpm. The stirrer was started and the following solid ingredients were added to the vessel.

12 Ibs of the lime produced in Example 1 (90% passing 120 British Standard Mesh) 8 Ibs cement 30 Ibs sand 50 Ibs pulversied fuel ash The mixture was stirred for 2 minutes. 20g of aluminium powder and then added as a suspension of 1 lb of water containing a wetting agent.

After mixing for a further Y2 minute, the mix was discharged into a mould. The mould was placed in a chamber heated to 500C. After 2 hours 40 minutes, the mould was dismantled, the block was cut, and placed in an autoclave.

The pressure in the autoclave was progressively raised over 9 hours to 150 psig the pressure was slowly released.

Cylindrical samples (21/2 inch diamter and 6 inch length) were cut from the autoclaved block. The strength of the product was determined after immersing some of the samples in water at ambient temperature for 24 hours by subjecting the samples, while still in the wet state, to a compression test, which had been calibrated against the test given in British Standard Specification No. 2028,1364:1968.

The bulk density of the remaining samples was determined by drying in an oven at 110 C to constant weight.

The following results were obtained:- Strength 650 psi Bulk density 49.8 Ib/ft 3 EXAMPLE 5 The procedure of Example 4 was repeated using lime of modified water-reactivity produced by the process described in Example 2 (reactivity curve D in Figure 1).

38 Ibs of water at 40 C were added to the mild steel vessel and the following solid ingredients were added: 9 Ibs of lime produced in Example 2 12.8 Ibs cement 30 Ibs sand 50 Ibs pulverised fuel ash The procedure of Example 4 was followed except that the mould was placed in the heated chamber for 2 hours 45 minutes before being dismantled to cut the block.

The average strength of the block obtained was 791 psi at a bulk density of 49.9 Ib/ft3.

EXAMPLE 6 The procedure of Example 4 was repeated using lime of modified water reactivity produced by the process described in Example 3 (reactivity curve E in Figure 1).

. 37 Ibs of water of 40 C were added to the mild steel vessel, and the following solid ingredients were added: 9 Ibs of lime produced in Example 3 12.8 Ibs cement 30 lbs sand 50 Ibs pulversied fuel ash The procedure of Example 4 was followed except that 22 g of aluminium powder were used as gassing agent, and the mould was placed in the heated chamber for 2 hours 50 minutes before being dismantled to cut the block.

The strength of the block obtained was 793 psi at a bulk density of 49.8 Ib/ft3.

COMPARISON 5 The procedure of Example 4 was repeated using respectively unmodified highly reactive lime (reactivity curve A in Figure 1) and highly reactive lime treated with water only (reactivity curve B in Figure 1). In each case blocks were produced which crumbled before meaningful strength measurements could be made.

WHAT WE CLAIM IS: 1. A process for modifying the waterreactivity of lime which comprises treating the lime with water to achieve partial hydration and simultaneously or subsequently treating the lime with carbon dioxide such that the carbon dioxide taken up by the lime is from 0.1 to 10 parts by weigh per 100 parts by weight of lime and taken-up water and carbon dioxide.

2. A process according to Claim 1 wherein the water taken up by the lime is from 0.5 to 15 parts by weight of water per 100 parts by weight of lime and taken-up water.

3. A process according to Claim 2 wherein the water taken-up by the lime is from 2 to 12 parts by weight of water per 100 parts by weight of lime and taken-up water.

4. A process according to any of the preceding claims wherein the lime is treated with substantially undiluted carbon dioxide, optionally containing water vapour.

5. A process according to any one of claims 1, 2 and 3 wherein the lime is treated with diluted carbon dioxide at a concentration in excess of 1 part by volume of carbon dioxide per 100 parts by volume of carbon dioxide and diluent gas.

6. A process according to Claim 5 wherein the lime is treated with carbon dioxide present in the form of kiln exhaust gas.

7. A process according to any of the preceding claims wherein a water-reactivity-modifying agent is added to the lime.

8. A process according to Claim 7 wherein the water-reactivity-modifying agent is sugar or calcium sulphate.

9. A process for modifying the waterreactivity of lime substantially as described with reference to any one of Examples 1 to 3.

10. A process for the manufacture of lightweight building materials which comprise incorporating therein lime whose water-reactivity has been modified by a process according to any of the preceding claims.

11. A process according to Claim 10 substantially as described with reference to any one of Examples 4 to 6.

12. Lime of modified water-reactivity whenever produced by a process according to any of

Claims

Claims 1 to 9.