IhTDUSTRIALAND ENGINEERING CHEMISTRY
118
VOl. 18, KO.2
The Hydration of Lime' By W. G. Whitman and G. H. B. Davis DEPARTMENT OB C H E M I C A L
ENGINEERING,
MASSACHUSETTS INSTITUTS OF TECHNOLOGY, CAMBRIDGE, MASS.
HE literature covering the influence of the method of the water was added a t such a rate that the temperature a t no hydration of lime upon the quality of the product is o:o water, 3o per cent in of the theesparse and somewhat inconclusive* BachtenkercherZ retical amount to take care of evaporation, was added rapidly attempted to show that hydrated lime exists in two different t o the lime. The temperature rose quickly and steam was forms-a fine amorphous form, stable a t ordinary tempera- generated. When the product was dry any unhydrated lumps tures, and a coarse crystalline one, which is produced when were screened Offa HYDRATION WITH WATERVAPOR-(a) At 30' C. The dry the temperature Of hydration rises above l8O0 lime was placed on a watch glass in a desiccator over water and his "crystalline and amorphous" theory was disproved by allowed to stand for 3 weeks at 300 C . Latham,3he did show that lime which is hydrated under con( b ) At looo C. Dry steam was passed over the lime for 2 ditions where the average hours. The powder was then screened to remove any untemperature of hydration& hydrated lime. above 180'F.yieldsapoorer (c) At 250' C. The lime This paper presents a study of the effect of various product (larger particles) hydration methods on the properties of hydrated lime. ~~p~~ ~ i $ ~ ~ ~ j $ than that h y d r a t e d a t The three most important variables are the ratio of chrome1 wire and suspended slightly lower temperatures. above water in an autoclave. water to lime, the temperature of hydration, and the He also showed that there is When the autoclave had been degree of agitation. a certain amount of flocculaThe quality of the hydrated product was evaluated by ~ ~ $ ~ o ~ ~ Of the hydrated lime testing its rate of reaction with acid and its rate of tube was shattered by heatand made an unsuccessful settling in water and by microscopic examination. ing the chrome1 wire elecattempt to find a suitable In general, the relative rating of quality by any one of trically. The lime which redeflocculating agent. these methods checks closely with the ratings by the e: ; : Kohl~chutter,~ in his work other two. hydrated by contact with on rates of settling, showed The best product was prepared by hydrating quickwater vapor. that hydroxide H Y D R A T I O NWITH FIVE lime with a large excess of water in boiling solution. TIMESTHEORETICAL WATER Hydration with the theoretical amount of water or with --(a) ~ ~ ~ ~ ~ c $ ~ ~ At 3oo c.~ Thelimewas~ water vapor gave a coarse inferior hydrate. added t o five times the theorapidly than that preretical amount of water in a pared by adding lime to beaker in a water bath kept
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HYDRATIONWXTH THEORETICAL AMOUNTOF WATER-(a) At 30' C. The lime was placed in a small evaporating dish and cold water was added gradually until the theoretical amount .was reached. The mass was stirred with a thermometer and November 18, 1925. Lime Lights, 1908, 223; 1909, 337; Trans. Not. Lime Assoc., 1910, 73; 1911, 209; Chem. Eng., 18, 189 (1914). :Concrete-Cement A g e , 6 , Cement Mill Section Q (1915). 4 Z . Elektrochem., 26, 159 (1919). 6 Chem. Met. Eng., 27, 1212 (1922). 1 Received
2
Tests on Product
The methods for determining relative particle size are based on three tests: rate of reaction, rate of settling, and microscopic examination. The rate of reaction was determined by adding 10 cc. of 1 N hydrochloric acid rapidly to a suspension of the hydrate which contained phenolphthalein and timing with a decimal stopwatch the interval before the reappearance of the pink color. The suspension contained 3.7 grams of hydrate (0.1 equivalent)
INDUSTRIAL A V D ENGISEERI.VG CHEJfISTRY
February, 1926
METHODOF PREPARATION Theoretical : 30' C. 1000
c.
Vapor : 30' C.
1000 c.
2500 c. Five times theoretical: 30' C. 100' C. with boilinn Twenty times theoretical: 30' C .
-
1000
c.
250' C. 60' C. with boiling 100' C. boilinrr Commercial product
T a b l e I-Time for Reaction w i t h Acid (in 0.01 M i n u t e ) Total Time (Cor.) to Neutralize Indicated Number Recorded Time to Neutralize Each Successive 10 Cc. of Acid of IO-Cc. Portions of Acid 1st 2nd 3rd 4th 5th 6th i t b 8th 9th 10th 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th
0 3
1 5
4 9
8 17
13 22
26
18
25 33
36 52
70 87
Over450 Over 450
1 1 0
2 3 0
4 5 1
6 9 2
8 12 6
20 20 15
29 32 30
47 55 60
70 97 183
Over450 Over450 Over450
0
0
0 0
1 0
3 1
9
12
17 9
26 15
55 82
0 0
0 0 0
1 1 1 1 0 2
2 2 2 2 0 6
7
12 9
15 14 12 14 4 50
22 25 30 21 8 81
56 40 82 77 16 212
0
0 0 0
0 0 1
2
5 3 5 1 14
o
6
9
2 28
diluted with water to 10 cc. and the 10-cc. dose of acid was repeated ten times to completely neutralize the lime.
Table I gives the original data on the times required for the reaction with each of the ten successive batrhes of acid. A second set of figures in this table gives the total corrected times for reaction, calculated on the assumption that the time is inversely proportional to the initial concentration of acid in the suspension. Thus, when 10 cc. of 1 N acid are added to 20 cc. of suspension, the initial concentration of acid in the mixture is N / 3 and the observed time is-corrected by multiplying by l/3. The figure for total corrected time is obtained by summing up the corrected times for the individual additions. Figure 1 shows a few typical curves of the total corrected time required to dissolve varying percentages of the lime. The rates of settling were determined by the method employed by Holmes, Fink, and Mathems in which 10 grams of hydrated lime were placed in a 100-cc. graduate (2.58 cm. internal diameter) filled to the 100-cc. mark, shaken well, and allowed to settle, readings being taken every 5 minutes. Table I1 gives these data as per cent of the total volume occupied by the suspension after various time intervals, and Figure 2 illustrates the type of curves obtained on a few of the hydrates. T a b l e 11-Data o n R a t e of Time in minutes 5 10 Vapor : 79 60 30' C. 1000 c. 74 50 Twenty times theo. retical: 30' C. 64 80 1000 c. 94 88 250' C. 94.5 90 60'C. with boiling 92 86 100°C. withboiling 98 97 Commercial product 83 73
Settling of L i m e S u s p e n s i o n s 15
20
30
43 32
28 26
25 23
47.5 80 84 78 95.5 62
41 29 60 72 68.6 79 51 69 9 4 . 3 91.6 54 41
60
Small and very tight Small and tight
Small, some tight, some loose Small and loose
0
1.5
0.5 0.5
5.1 12.4
7.7 16.1
1.2 2 . 2 3 . 4 1.5 2.7 4.5 0.2 0.6
4.7 6.5 1.6
7.6 9.4 3.7
11.2 13.4 7.5 7.1 2.3
0
0
Over450 Over450
0
0
0 0
0.2 0
0.8 0.2
2.3 0.5
5.0 1.2
Over450 Over450 Over450 Over450 Over450 Over450
0 0 0 0
0
0
0 . 2 0.6 0 . 2 0.6 0 . 2 0.6 0.2 0.6 0 0 0.8 1.9
1.8 1.4 1.1 1.4 0.1 4.2
3.5 2.7 2.0 2.7 0.4 8.2
0
0
10.8 14.8 20.2 25.9
1.3 2 . 9 5.4 8.8
0.3 3.2
0
0
0 0.3
21.8 34.6
16.4 23.0 19.5 29.2 14.2 32.5
10.0 4.0
15.5 12.2
5.4 7 . 8 13.4 4.5 7.2 1 1 . 2 3.5 6 . 8 15.0 4.5 6.8 14.5 1.8 3.4 0.9 14.4 23.4 44.6
... ... ... ...
... ... ...
... ... ... ... ...
samples, the depths of settling in 15 minutes, and the microscopic observations. T a b l e IV-Summary of R e s u l t s Total cor. Depth of time to dis- settling In solve 80% 15 min. SAMPLE LMm. Cm. TYPEOF CLUSTERS Theoretical: 30' C. 14.8 Large 1000 c. .. Large 25.9 Vapor: 16.4 68 Large 30' C. 19.5 57 Large 1000 c. 250' C. 14.2 .. Large Five times theoretical: 30' C. 10.0 .. Small tight 100' C. boiling 4.0 Small loose Twenty times theoretical: 52 Small tight 30' C. 7.8 20 Small tight 7.2 100' C. no boiling 250' C. 16 Small tight 6.8 60' C. boiling 22 Small loose 6.8 100° C. boiling 4 Small loose 1.8 Commercial hydrate Small very tight 38 23.4
..
..
The time required for reaction as given in Table IV is a function of the surface area of the hydrate and may be used as an "index" of the size of the individual particles. Nofe-The proportions of particles of various sizes in the sample may be calculated from the data on the rate of solution in hydrochloric acid by making the following assumptions: (1) The particles are all of uniform shape and not clustered; and (2) the number of particles remains constant during any one addition of acid. These calculations would be similar t o those of Murphrees on the rate of solution of crystals.
110 1200
2 2 . 1 22 21.5 21.4
21.9 21.0
25 23.9 23.9 38.5 34.5 31.0 47.5 41.8 30.9 31 29.6 27.5 83.5 70.2 35.0 29 28.9 28.8
The microscopic tests are difficult to show clearly. Table I11 gives a summary of the results, while Figure 3 shows sample drawings of the different types of clusters. T a b l e 111-Results TYPEOF CLUSTER Large and tight
119
of Microscopic E x a m i n a t i o n SAMPLE Theoretical a t 30' C. Theoretical a t 100' C. Vapor a t 30' C. Vapor a t 100' C. Vauor a t 250' C. Commercial Five times theoretical a t 30' C. Twenty times theoretical a t 30' C. Twenty times theoretical a t 100' C. not boiling Twenty times theoretical a t 250° C. Twenty times theoretical boilingat 60' C. Five times theoretical, boiling a t 100' C. Twenty times theoretical, boiling a t 100' C.
Discussion
T a u e IT' summarizes the results of the three tests, showing the total corrected times to dissolve 80 per cent of the various
Using this index the f o l l o w i n g comparisons may be drawn:
?
I-The use of excess water produces a hydrate which is much finer than that obtained by reaction with t h e o r e t i c a l water or with vapor. Thus the index numbers are all below 10 when excess water is employed, and are above 10 for all other samples, including the commercial hydrate. Increasing the excess water from five t o twenty times the theoretical i n c r e a s e s the fineness of the hydrate t o a small degree. 7,OF SAMPLE MSSOLVED 2-Boilinn markedlv increases fineness. 3-Increasing temperature increases fineness to a slight extent when excess water is employed. This is noticeable in the runs with twenty times the theoretical water a t 30°, loo", and 250' C. without boiling. Increasing temperature gives a poorer product, however, when theoretical water or water vapor is used. The run with vapor a t 250' C . is an exception which will be discussed later. 8
THISJOURNAL, 16, 148 (1923).
INDUSThTAL A N D ENGl NEERING CHEiVI8TRY
120
4-Vapor hydration gives about the same results as are obtained with theoretical water. The commercial hydrate corresponds in particle size t o vapor or theoretical water hydration a t 100” C.
The data on rate of settling lead to somewhat parallel con‘clusions. Thus, excess water and boiling produce hydrates which have slow settling rates. However, the effect of temperature of preparation is much more marked, as il100
80
00
40
20
I 0
I
I
30
60
I 90 TIME -MINUTES
I I20
I I50
I
180
lustrated by the rapid settling of the sample prepared at 30” C. with twenty times the theoretical water. This hydrate settles even more rapidly than the commercial product, although it has a much larger surface area as determined by its rate of reaction. The essential difference between these two tests is that the reaction rate is a function of surface area while the settling rate is determined by the size of agglomerates of particles. Thus, a hydrate which consisted of very fine particles would react rapidly with acid, but these particles might be clustered together in lumps which would settle rapidly. Such seems to be the case when lime is slowly hydrated a t low temperatures with excess water. The explanation of all these results may be deduced from a general principle that fine particles are produced by rapid hydration when there is little opportunity for the crystals to agglomerate or to grow in size. The use of excess water has three effects: (1) it separates the individual crystals and decreases the tendency for agglomeration and the growth of large crystals; (2) it prevents local overheating of the lime and its resulting disadvantages; and (3) the presence of excess water a t any definite uniform temperature increases the rate of hydration. The effect of agitation is similar in producing a fine product and, in particular, it prevents the formation of tight clusters of crystals. Increasing temperature, when excess water is employed, increases the rate of hydration without correspondingly increasing rate of crystal growth. The individual crystals are therefore somewhat smaller a t higher temperatures. The major effect, however, is a decreased tendency for small crystals to agglomerate into large particles. Temperature may have a quite opposite effect, however, when theoretical water or vapor is used. The heat of hydration is then generated in a mass of such small heat capacity that the temperature rises rapidly, causing overheating or “burning” and producing a coarse and inferior product. To illustrate this, a thermocouple was inserted in a hole drilled in a large lump of quicklime. The lime was placed in a dish partly filled with water so that the steam resulting from hydration of the lower part would hydrate the upper part. In this experiment a thermocouple temperature of 465” C. (869” F.) was recorded.
Vol. 18, No. 2
Johnston’s data’ show that lime cannot be hydrated a t atmospheric pressure a t temperatures above 547” C. (1017 O F.) and that at this temperature hydrated lime will lose its water and dehydrate unless put under pressure. “Burning” temperatures evidently approach this dehydrating point, and therefore the rate of hydration in any part of the mass which has been raised to such high temperatures will be very slow. Practically, if a particle of quicklime is brought up to the dehydrating point by the heat evolved around it, that particle cannot hydrate until enough heat has been lost to lower the temperature. As a result, the rate of further hydration will be determined by the rate of heat loss and this latter would be relatively slow in a lump of quicklime in the center of the hydrating mass. Such a slow rate of hydration would give large particles. It is believed that the deleterious results of “burning” during hydration are adequately explained by this concept. If the lime is hydrated under pressure the dehydrating temperature is increased by an amount which depends upon the pressure. Thus, quicklime initially a t 250” C. has been hydrated by vapor at 40 atmospheres pressure, and the product was even better than that obtained when lime initially at 100” C. was hydrated by vapor a t atmospheric pressure. I n the usual type of FIG.3 commercial h y d r a t o r T Y P E S OF C L U S T E R I N G OF PARTICLES the lime, with a slight excess of water, is fed into a system of cylo& $. inders a n d carried through by screw con00 veyors, being dis0 SMALL TlGnT CLUSTER 20 TIMES THEORETIUL charged as a hot, dry, SMALL LOOY CLUSTER 30‘ 20 T I M E THECRLTIW WATER powdered hydrate, 100. WlTn EOlLlNG Thus, although the lime is a t first partially hyd r a t e d w i t h excess water, this water rapidly changes to steam and the remainder is hydrated by vapor. As has been shown, hydration with only a slight excess of water or with steam yields a relatively coarse product. The superiority of slaked lime properly prepared in a mortar box over commercial hydrate is due primarily to the large excess of water. It is probable that the average slaking operation employs an excess corresponding to five times the theoretical amount of water. Conclusions
High-grade hydrate contains many fine particles which are produced when the rate of hydration is rapid compared with the rate of growth of the particles. Conversely, inferior coarse hydrate is formed when the rate of hydration is slow and time is allowed for crystal growth. Excess water, reasonably high temperatures, and agitation all favor rapid hydration and a fine product. Excessively high temperatures, generated locally in the lime from the heat of reaction, are very detrimental because the lime approaches its “dehydrating” temperature while still only partially hydrated. Completion of the reaction is then very slow since the lime must lose heat before hydration can continue. During this time the particles have a chance to grow in size and the final product is very coarse. An elimination of conditions which produce these effects would undoubtedly result in an improvement in the quality of most commercial hydrates. 7
2. p k 4 5 Chem , 62, 330 (1906)
