Method for the wet slaking of calcium and magnesium oxides from calcomagnesian compounds

Abstract

A method is shown for the slaking of calcium oxides and magnesium from calcomagnesian compound containing at least 10 wt. % of MgO in relation to the total weight of said calcomagnesian compound, in which a slaking aqueous phase is supplied to a slaking device, and slaking the compound containing anhydrous dolomite delivered to the slaking device, by means of the slaking aqueous phase, forming hydrated solid particles of Mg(OH).sub.2, in the presence of an additive.

Claims

1. A method for the wet slaking of calcium and magnesium oxides of a calco-magnesian compound, comprising the steps of: feeding a calco-magnesian compound containing MgO into a slaking equipment; feeding an aqueous slaking phase into the said slaking equipment; and slaking the said calcium and magnesium oxides of the said calco-magnesian compound fed into the said slaking equipment, with the said aqueous slaking phase, leading to the formation of slaked solid particles of calcium and magnesium hydroxides, wherein the said slaking is performed at ambient pressure in the presence of an additive selected from the group consisting of water-soluble metal hydroxides, water-soluble metal silicates, water-soluble aluminates, water-soluble aluminium salts, water-soluble metal halides, water-soluble metal nitrates, water-soluble metal lactates, water-soluble ammonium salts, ammonia and the mixtures thereof, the said calcium and magnesium oxides of the calco-magnesian compound having a magnesium oxide content of at least 10% by weight and less than 50% by weight relative to the weight of the said calco-magnesian compound, and having a calcium/magnesium molar ratio of between 0.8 and 1.2.

2. The slaking method according to claim 1, wherein the said calco-magnesian compound is selected from the group consisting of calcined or semi-hydrated dolomite, mixed calco-magnesian compounds and the mixtures thereof.

3. The slaking method according to claim 1, wherein the said water-soluble metal hydroxides are selected from the group consisting of alkaline hydroxides and the mixtures thereof.

4. The slaking method according to claim 1, wherein the said water-soluble metal silicates are selected from the group consisting of alkaline silicates, alkaline-earth silicates and the mixtures thereof.

5. The slaking method according to claim 1, wherein the said water-soluble aluminates are selected from the group consisting of potassium aluminate, sodium aluminate, lithium aluminate, ammonium aluminate and the mixtures thereof.

6. The slaking method according to claim 1, wherein the said water-soluble metal halides are selected from the group consisting of metal chlorides, metal bromides, metal fluorides and the mixtures thereof.

7. The slaking method according to claim 1, wherein the said water-soluble aluminium salts are selected from the group consisting of aluminium fluoride, aluminium nitrate, aluminium chloride, aluminium lactate and the mixtures thereof.

8. The slaking method according to claim 1, wherein the said water-soluble metal nitrates are selected from the group consisting of alkaline nitrates, alkaline-earth nitrates and the mixtures thereof.

9. The slaking method according to claim 1, wherein the said water-soluble metal halides are selected from the group consisting of alkaline halides, alkaline-earth halides and the mixtures thereof.

10. The slaking method according to claim 1, wherein the said metal halides, the said metal nitrates and the said metal lactates comprise at least one atom of a metal selected from the group consisting of aluminium, calcium and magnesium.

11. The slaking method according to claim 1, wherein the said metal halides, the said metal nitrates and the said metal lactates comprise at least one atom of a metal selected from the group consisting of aluminium and magnesium.

12. The slaking method according to claim 1, wherein the said additive is added to the said aqueous slaking phase prior to the said feeding of the said aqueous slaking phase to form an additive-containing aqueous slaking phase.

13. The slaking method according to claim 1, wherein the said additive is added to the said aqueous slaking phase inside the said slaking equipment or in the said feed of the said aqueous slaking phase.

14. The slaking method according to claim 1, wherein the said additive is added to the said MgO-containing compound or in the said feed of the said calco-magnesian compound.

15. The slaking method according to claim 1, wherein the said additive is supplied at a content of between 0.1 and 20% by weight relative to the total weight of MgO.

16. The slaking method according to claim 1, wherein the said calco-magnesian compound has a degree of conversion of MgO to Mg(OH).sub.2 of at least 10% as measured by a simplified determination test of the degree of conversion.

17. The slaking method according to claim 1, wherein the said slaking equipment is a wet process hydrator.

18. The slaking method according to claim 1, wherein the MgO degree of conversion to Mg(OH).sub.2 is improved by 30% relative to the degree of conversion obtained without an additive.

19. The slaking method according to claim 17 wherein the said slaking step has a reaction time less than 5 hours.

20. The slaking method according to claim 1, wherein the said calco-magnesian compound is a powder compound.

21. The slaking method according to claim 1, wherein the said slaked solid particles of calcium and magnesium hydroxides form a suspension or a paste of solid particles containing at least 30% water.

22. The slaking method according to claim 1, further comprising a step to de-agglomerate or mill the said slaked solid particles of calcium and magnesium hydroxides.

23. The slaking method according to claim 21 further comprising a step to sieve the said suspension.

24. The slaking method according to claim 1, wherein the said aqueous slaking phase has a temperature before slaking of between 35 and 90° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the influence of an additive according to the invention, compared with a comparative Example without additive, on the slaking reaction of calcined dolomite 1.

(2) FIG. 2 shows the influence of different additives (Na.sub.2Al.sub.2O.sub.4 and NaOH) according to the invention in relation to comparative examples without additive or in the presence of DEA on the slaking reaction of calcined dolomite 2.

DETAILED DESCRIPTION OF THE INVENTION

(3) The method of the invention therefore uses hydration via wet process that is controllable, rapid and simple of inter alia calcined dolomite, dolomitic or magnesium lime or of any compound containing MgO, with a high conversion rate and up until obtaining of fully hydrated products using standard hydration equipment through the use of an additive in preferred proportions of 0.1 to 20%, preferably 1 to 10% by weight relative to the total weight of MgO.

(4) The additives of the present invention are selected from the group formed by water-soluble metal hydroxides particularly alkaline hydroxides (in particular Na, Li or K hydroxides), water-soluble metal silicates particularly water-soluble alkaline or alkaline-earth silicates (in particular Na, Li, K, Ca or Mg silicates), water-soluble aluminates particularly K, Na, Li or NH.sub.4 aluminates (in particular sodium aluminate), water-soluble aluminium salts, particularly aluminium fluoride, nitrate, chloride or lactate (in particular aluminium nitrate and lactate), water-soluble metal halides, particularly chlorides, bromides or fluorides, water-soluble metal nitrates, water-soluble metal lactates, water-soluble ammonium salts, ammonia and the mixtures thereof.

Example 1

(5) The slaking reaction was performed following the typical configuration of the reactivity test as per standard ASTM C110-02: 2002 §12. Therefore, initially 120 g of dolomitic quicklime 1 were added to 400 g of slaking water at 40° C. not containing any additive (reference). Next another sample of 120 g of this dolomitic quicklime 1 was added to a solution of sodium aluminate (formula Na.sub.2Al.sub.2O.sub.4) at 40° C. previously prepared by dissolving 4 g of sodium aluminate in 396 g of demineralised water. In both cases the reaction mixture was stirred at 250 rpm. The sample of dolomitic lime 1 was a fine fraction having a particle size doe of 3 mm or less and industrially produced. It contained 39.7% MgO and 55.1% CaO (weight proportions determined by X-ray fluorescence) which corresponds to a Ca/Mg molar ratio (or x/y) of 1.00 and the reactivity thereof is described in Table 1 and in FIG. 1.

(6) These slaking reactions were conducted in a vessel of DEWAR type having a capacity of 665 cm.sup.3 equipped with a shaft with four paddles and with a temperature probe for the automatic acquisition of temperature data over time (one measurement every second). From the curve given in FIG. 1, the following characteristics were determined: a) t.sub.60 representing the time needed for the reaction medium to each 60° C. from the initial temperature set at 40° C.; b) t.sub.70 representing the time needed for the reaction medium to reach 70° C. from the initial temperature set at 40° C.; c) TTM representing the time to maximum temperature of the reaction medium i.e. when no temperature rise of more than 0.5° C. is observed over three consecutive measurements.

(7) The suspension (or milk) of dolomite obtained was held under agitation until the TTM value was reached then is screened through a 250 μm mesh, the retained fraction being dried at 110° C. and weighed. The weight percentage of these retained particles was then calculated in relation to the starting quantity of dolomitic quicklime in the form of fine particles. These retained particles are called <<Rejects>> and indicate the size of the fraction of the coarse, undesired particles obtained during slaking. This data is given in Table 1 below.

(8) The product which passed through the sieve was rapidly dried at 150° C. and analysed. The contents of Mg(OH).sub.2 and Ca(OH).sub.2 were calculated on the basis of measurement of heat loss under TGA as described above. The conversion rate of MgO was then calculated using formula 1. All the results are given in Table 1 and illustrated in FIG. 1.

(9) TABLE-US-00001 TABLE 1 Mg(OH).sub.2 MgO increase t.sub.60 t.sub.70 TTM Rejects d.sub.50 final initial e1 e2 tc.sub.MgO.sup.(1) tc.sub.MgO Example [min] (min] [min] wt. % weight % weight % weight % weight % weight % % % Dolomite 1 1.27 3.15 5.47 5.25 11.25 17.2 39.7 5.3 12.5 36.4 without additives Dolomite 1 + 0.87 1.52 3.60 1.17 6.16 39.3 39.7 12.1 12.5 90.8 149 Na.sub.2Al.sub.2O.sub.4 (Example 1) .sup.(1)Conversion rate of MgO to Mg(OH).sub.2 calculated using formula 1.

(10) The reactivity results (FIG. 1) show that not only was the reaction highly accelerated (even if a less rapid increase was initially observed in comparison with the comparative example without additive used as reference) in the presence of sodium aluminate in relation to an additive-free reference, but also the final temperature obtained was higher indicating a higher conversion rate.

(11) All the results are reproduced in Table 1.

(12) For the example according to the present invention, in the presence of sodium aluminate, practically complete conversion was observed (about 90% conversion rate) despite a very short reaction time in the order of only 5 minutes. In addition, the suspension obtained in the form of dolomite milk was much finer in the presence of sodium aluminate. The suspension had a median diameter d.sub.50 of about 6 μm and only 1% was rejected by 250 μm screening compared with the case using a product without additive (ds of about 11 to 13 μm and 5 weight % rejected after 250 μm screening).

Example 2

(13) Example 2 was similar to Example 1 in that a dolomite was hydrated under the same conditions and in the presence of sodium aluminate (3% relative to the weight of dolomitic quicklime). The differences between this Example 2 and Example 1 were: this time a dolomitic quicklime 2 was hydrated containing 39.5% MgO and 51.9% CaO (i.e. x/y mol=0.94) having the reactivity described in Table 2 and FIG. 2; the reaction time was 1 hour, the suspension of dolomite remaining under stirring 1 hour in the Dewar vessel before being filtered, dried and analysed as in Example 1; this time the dolomite suspension was not sieved and the conversion rate therefore represented the entirety of the solid contrary to Example 1, in which some coarse particles were removed by screening.

(14) As shown in FIG. 2, the addition of sodium aluminate during the hydration reaction allows an increase in hydration temperature and led to higher conversion rates (Table 2).

Example 3

(15) This example is exactly similar to Example 2, only the sodium aluminate was replaced by sodium hydroxide (NaOH) all proportions remaining equal.

(16) FIG. 2 shows that this additive led to a second increase in temperature after 30 minutes finally leading to a higher final temperature and hence to an improved hydration rate (Table 2). A reaction time of more than 1 hour under the same conditions (1 h30) for example) should lead to an even further improved hydration rate.

Comparative Example

(17) This comparative example is exactly similar to Examples 2 and 3, but this time the additive was diethanol-amine (DEA) also added in the proportion of 3% relative to the weight of the initial dolomitic quicklime. This time the additive led to a slight delay in the hydration reaction and to a slight reduction in hydration temperature (FIG. 2). As a result the hydration rate of the MgO portion of dolomite 2 was decreased (Table 2).

(18) The present invention is evidently in no way limited to the above-described embodiments and numerous modifications can be made thereto without departing from the scope of the appended claims.

(19) TABLE-US-00002 TABLE 2 Mg(OH).sub.2 MgO increase t.sub.60 t.sub.70 TTM final initial e1 e2 tcMgO.sup.(1) tc.sub.MgO Example [min] (min] [min] weight % weight % weight % weight % % % SSA .sup.(2) Dolomite 2 1.12 2.18 4.98 20.1 39.5 6.2 13.6 43.9 10.0 without additives Dolomite 2 + 0.95 1.77 60 29.5 9.1 13.3 66.4 51 15.5 Na.sub.2Al.sub.2O.sub.4 (Example 2) Dolomite 2 + 1.70 3.38 60 26.2 8.1 13.2 58.3 33 10.3 NaOH (Example 3) Dolomite 2 + 1.47 3.28 5.97 16.8 5.2 13.2 36.0 −18 10.4 DEA (comparative Example 1) .sup.(1)Conversion rate of MgO to Mg(OH).sub.2 calculated using formula 1. .sup.(2)BET specific surface area measured on the basis of nitrogen adsorption manometry analysis after a degassing time of at least 2 hours at 190° C.