Reduction of Alkalies in
Portland Cement USE OF CALCIUM CHLORIDE *
a
E. R. HOLDEN', Missouri Portland Cement Company, St. Louis, Mo. The total carbonate content of the kiln feed was determined by an acid-alkali titration and was calculated as calcium carbonate. Silica, iron oxide, alumina, calcium oxide, and magnesium oxide were determined by standard methods.
The reduction in the amount of sodium oxide and potassium oxide in portland cement clinker when calcium chloride is added to the kiln feed was quantitatively evaluated with the object of producing low-alkali cement. Commercial scale studies were carried out in both wet and dry process plants. In the wet process plant each alkali was decreased in amounts proportional to the quantity of calcium chloride added in the range investigated. In the dry process plant the potassium oxide removal was much greater than the sodium oxide removal per unit increase in the amount of calcium chloride at the lower rates of addition; the reverse of this was found to take place at the higher rates. However, in both plants the molecular sum of the alkalies removed was found to be approximately proportional to the quantity of calcium chloride added. The effect of returned dust, collected from the stack gases, on the alkali content of the clinker was also investigated.
wEr
PROCESS
EXPERIMENTAL PROCEDURE. Two 240-foot rotary kilns with diameters of 11feet 3 inches a t the front, tapering to 10 feet a t the rear, were used: No. 1, the control kiln, and No. 2, the experimental kiln to which calcium chloride was added. Both kilns were operated under the same conditions as far as possible. The kiln feed was the same in composition a t any given time because it is pumped from basins through a pipe which supplies both kilns. Likewise, the design of the plant is such that the coal used was of the same composition and physical state. The calcium chloride, technical grade flake assaying 79% calcium chloride, was dissolved in water t o form a 28% solution which was then fed by gravity to kiln 2 from a constant level feed tank. The rate of addition was controlled by means of a rotameter a t the two lower rates of addition and by a manometer a t the highest rate. The various rates were based on the amount of anhydrous calcium chloride t o be added to the dry weight of the kiln feed. The rate of flow was checked frequently by measuring the time required to fill a 1-gallon container. The flow rate of the kiln feed slurry, which was periodically checked in a similar manner, varied so slightly that it could be considered as constant throughout the plant study. Clinker samples were taken hourly and composited during periods of 12 or more hours. Each composite was crushed in a hammer mill, put through a disk mill, and split. The final grinding was then completed by hand with a porcelain mortar and pestle. In the first period of the plant study, samples were taken under normal conditions. The precipitator dust, collected in kiln exit chambers by Cottrell precipitators, was returned to each kiln in an amount equal to that produced by a kiln (about 12.5 tons per day). This was followed by a period in which the dust was returned only to kiln 1. During the remainder of the study the dust was not returned to either kiln and various amounts of calcium chloride were added to kiln 2. EXPERIMENTAL RESULTS.The sodium oxide, potassium oxide, and the total alkali expressed as sodium oxide in the composite clinker samples taken in the various periods are given in Table I. During the first 12 hours of each rate of calcium chloride addition (samples 5, 7, and 9) the kiln was permitted to approach a steady state. The alkali removal attributable to the use of calcium chloride was calculated from samples 6, 8, and 10, with calcium chloride rates of 0.25, 0.50, and 0.75%, respectively, by subtracting the alkali contents for the clinker of kiln 2 from those of kiln 1. The results, shown graphically in Figure 1, are somewhat less pronounced than those obtained in some laboratory burns but are of a magnitude great enough t o ensure the practicability of the method. The average alkali contents of the clinker to be expected a t the different rates of calcium chloride addition, also shown in Figure 1, were estimated by subtracting the alkali removal in
F
OR many years it has been known that the addition of calcium chloride to the kiln feed in the production of portland cement brings about a reduction in the alkali content of the resulting clinker. Investigations have been carried out on a laboraatory scale (6, 15), and in some instances calcium chloride has been added to a kiln. The material in a kiln reaches a temperature of approximately 1350" * 75" C. for a period of about 20 minutes (4). During this treatment considerable amounts of the alkalies are lost by volatilization. Thus when calcium chloride is added to the kiln feed it, in effect, causes the removal of residual alkalies. The recent greatly increased interest in the role of the alkalies (1,9,8, 10,21, IS, 24) makes it desirable to evaluate the aforementioned process on a commercial scale. Presented in this paper are the findings of experimental studies in which various amounts of calcium chloride were added to the kiln feed of both wet and dry process plants. Information was also obtained concerning the recycling of the alkalies because of returned dust collected from the stack gases. ANALYTICAL METHODS
For the determination of the sodium oxide and potassium oxide in clinker, kiln feed, and dust samples, the ASTM, C114-46 method (2) was used. This method utilizes the more important features of the Smith method for the determination of alkalies in siliceous materials (12). The method was checked several times by running Bureau of Standards samples: Cement Reference Laboratory cement No. 4, 1940, and argillaceous limestone No. 1A. The average deviation from the reference values was found t o be O.O20J,. In the course of the afialytical work it was observed that the values obtained in the andysis of unknown clinker samples were reproducible; the average difference between duplicate determinations was less than 0.01% and the largest difference was 0.03%. 1 Present address, Naval Civil Engineering Research and Evalution Laboratory, Port Hueneme, Calif.
337
INDUSTRIAL AND ENGINEERING CHEMISTRY
338
4
36
i)
12
'
0.00
0.24
0.45
0.54
0.25
0.46
0.25
0.26
0.44
0.55
0.24
0.32
0.45
9.43 0.42
0.54 0.53
0.25 0.25
0.36 0.35
0.49 0.48
0.36
0.52
0.24
0.21
0.38
{!:::
0.44 0.44
0.59 0.59
0.26 0.25
0.29 0.28
0.4,: 0.43
0.37
0.49
0.21
0.22
0.35
0.40 0.40
0.51 0.51
0.19 0.19
0.15 0.15
0.29 0.29
0.43
0.55
0.26
0.33
0.18
0.43
0.58
0.28
0.42
O.5R
6
24
0.26
7
12
0.50
8
24
9
12
Discarded
Discarded
Vol. 42, No. 2
0.50 0.75
E;:{ 0.28
10
14
0.75
11
24
0.00
0.25 '0 25 \,o:25 0.27
12
24,
0.00
0.30
each case from the average allrali content with no calcium chloride addition. The initial alkali content of the raw material on a loss-free basis is normally approximately 0.4% sodium oxide and 1.0% potassium oxide. The total alkali loss, computed from the above values, is in each case considerably greater than the molecular equivalent of the chloride added. Hoivever, the alkali lossrs have been based on the normal alkali contents of the clinker since the removal of the residual alkalies is the primary purpose. This procedure also partially eliminates differences due to raw materials and kiln design to facilitate comparisons. I n general, the alkali removal was approximately proportional to the amount of calcium chloride used. Considering the alkalies on a molecular basis, the decrease in the potassium oxide content was about 2.8 times that of the sodium oxide in the range investigated. This result is explained by the facts that: initially there was, in each case, more potassium present; and the vapor pres-
0.55
sure of potassium chloride is somewhat higher than that of sodium chloride throughout this temperature region ( 9 , 1 6 ) . PRECIPITATOR DUST. Judging from the alkali contents of the clinker from kiln 1, the discarding of the dust has no measurable effect on the sodium oxide content of the clinker. It did, however, result in an average decrease of 0.19% potassium oxide in the clinker, equivalent to 0.13% total alkali as sodium oxide. DRY PROCESS
EXPERIAIESTAL PROCEDURE. Basically the experimental procedure used a t the dry process plant was the same as described above for the wet process plant. Two 160-foot rotary kilns, 10 feet in diameter except for 40-foot mid-sections with a diameter of 17 feet, were used: No. 1, the control kiln, and Yo. 2, the experimental kiln. The kilns do not have precipitators but do have dust chambers in nhich a substantial quantity of dust is collected. The dust is returned to the kiln feed tank by means of a common screw conveyer, and thence to the kilns m-ith the kiln feed; the dust is divided equally between the t x o kilns. It was not feasible TABLE11. ALKALIES hTORXALLY IK CLINKER O F DRYPROCESS to discard the chamber dust. However, variations in the alkali PLANT content of the dust, as will be shown, did not appreciably altc.1 the (Each composite sample was compofied of even-hour samples taken over a alkali content of the materials entering the kiln. 12-hour period from both kilns) Prior to the addition of the calcium chloride, even-hour samples Alkalies Normally in Clinker, 70 Composite Total from both lrilns nere composited during 12-hour periods. These Sample NazO KzO (as NmO) composite samples were analyzed to determine the average allrali 0.58 0.46 1A 0.28 0.62 0.49 2A 0.30 content of the clinker under normal conditions. 0.44 0.58 0.29 3A The calcium chloride was added dry, the rate of addition again 0.45 0.60 0.30 4A 0.37 0.54 0.30 5A being based on the amount of anhydrous calcium chloride to be 0.62 0.32 0.46 6-4 0.46 0.82 0.32 ?A added to the dry weight of the kiln feed. h large quantity of caldverage 0.301 0.446 0.59 cium chloiide was made available in this plant study, so that it was possible to inOF CALCIUM CHLORIDE ADDEDTO KILNFEEDO F DRYPROCESS vestigate w.hat effect it would have when TABLE 111. EFFECT PLAST added in higher percentages. The calcium Alkalies in Clinkers, 70r chloride was hoisted to the kiln feed platExptl., Kiln 2 Control, Kiln 1 Consecutive CaClz Composite Time InterAdded t o Total Total form and put into the kiln feed conveyer Sample Val, Hours Kiln 2, % Nan0 KzO (as NazO) SazO KzO (as NazO) by hand. The correct number of bags 1 24 1.5 was added per hour to obtain addition 2 12 1.5 0:31 0:kO O:k7 0:OZ 0:OO 0:OZ 3 12 1.5 0.27 0.33 0.49 0.00 0.00 0.00 rates of 1.5, 1.0, and 0.5%. Each rate was 4 24 1.0 5 12 1.0 0:34 o:i9 o:& o:is o:o6 0:22 maintained for 48 hours. During thc 6 12 1.0 0.33 0.48 0.65 0.20 0.06 0.24 first 24 hours the kiln was permitted 7 24 0.5 8 12 0.5 0139 0:62 0180 0129 0:Zl 0:43 The to approach a steady state. 9 12 0.5 0.36 0.59 0.75 0.31 0.24 0.47 10 24 0.0 0.32 0.46 0.62 .. .. .. composite samples of the next two 1211 24 0.0 0.32 0.53 0.67 .. .. .. hour periods were analyzed and the results 12 24 0.0 0.31 0.46 0.61 .. .. averaged. ~~
-
. I
February 1950
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
rate, were determined. The re6ults are given in Table IV. From these data the alkalies contributed to the materials entering the kilns-by alkalies initially in the raw mix and by alkalies in the dust-were calculated and are given in Table V. The amounts that would be contributed if calcium chloride were added to both kilns were also calculated. The data show conclusively that the high values for the alkali content of the control kiln were not due to an alkali build-up in the dust. At the 0.5% calcium chloride rate the alkalies contributed by the dust were not perceptibly greater than when no calcium chloride was being added. Even in the extreme case, where 1.5% calcium chloride would be added to both kilns, the calculated increase is only 0.19% total alkali. Considering that about 50% total alkali of the raw material is normally lost in, the kilns because of volatilization, and also that the increase of the alkalies of the dust, in all probabiliti, would be due to alkalies in the form of chlorides, which would be easily revolatilized, it is unlikely that this build-up could be of any consequence under any circumstances. The major constituents of the clinker were determined to establish what effect variations in composition may have had on the burning temperature. I n each case there were two clinker composite samples taken under the same conditions and corresponding to the dust and kiln feed samples considered above. The average analyses, silica-sesquioxide ratios, and lime factors under each condition are given in Table VI. The total carbonate titrations of the kiln feed, also given in Table VI, were considerably lower than normal during the 1.0 and 0.5% calcium chloride addition periods. This was reflected in the composition of the clinker, particularly in the silica and calcium oxide contents and in the lime factor. The lowering of the calcium carbonate content of the kiln feed would lower the burning temperature of the kilns. Consequently, the amounts of alkalies retained in the clinker were greater than they u-ould have been had the calcium carbonate content of the kiln feed been maintained constant. The alkali content of the clinker from the kilns was affected significantly by two factors: calcium chloride in the dust returned to the kilns, and changes in the total carbonate content of the kiln feed, which in turn altered the burning temperature. These factors mould affect the composition of the kiln feed entering each kiln equally but were not of the same magnitude a t the different rates of calcium chloride addition. To obtain a comparison of the various levels of calcium chloride, i t was necessary to correct for these other variables. This was accomplished by multiplying each alkali content of the clinker from the experimental kiln 2 by a ratio R:
TABLEIV. ALKALICONTENTOF RAWMIX AND DUSTAT VARIOUS RATESOF CALCIUM CHLORIDEADDITIONIN DRY PROCESS PLANT (All d a t a on loss-free basis; samples taken for 24 hours starting 3 hours prior to clinker samples designated by the same numbers in Tables I1 a n d 111) Alkali Content 7’ Raw Mix b u s t from Kiln 2 ’ CaClz ComAdded to Total Total posite Kiln 2, (as (as % NazO KzO NazO) NazO KzO NazO) Sample 3.21 3.37 1.09 1.26 0.43 1.01 2-3 1.5 2.08 2.05 1.19 0.68 0.49 1.06 5-6 1.0 1.81 1.72 1.19 0.53 0.47 1.09 0.5 8-9 6A-7A 0.0 0.43 1.00 1.09 0.47 1.87 1.70
U
339
Each composite sample of clinker consisted of 12 samples, which were crushed in a hammer mill, put through a disk mill, and split. The final grinding was then completed by hand ALKALI R E M A I N I N G with a porcelain mortar and pestle. Dust and kiln feed samples were taken on the even hours and composited to correspond to certain clinker samples, as indicated in the anal0.4 0.6 0.8 yses presented. I n O O 02 % CALCIUM CHLORIDE every case the samFigure 1. Effect of Calcium Chlopling was set up to ride on Alkali Content of Clinker i n obtain an average reWet Process Plant sult over a period of 12 or more hours. This procedure was used to eliminate as much as possible the unavoidable variations in the burning conditions of the kilns. Even with this procedure the errors due to such variations are, admittedly, in all probability somewhat greater than the precision of the analytical methods used. EXPERIMENTAL RESULTS.The alkali contents of the clinker composite samples, taken during the period in which calcium R = An/Ac chloride was not being added to the kiln feed, are given in Table 11. The data indicate that under normal conditions the alkali where A , = normal average alkali content, and A , = alkali concontent of the clinker is quite uniform. tent of control kiln 1. The alkali contents of the clinker composite samples, taken The application of R is based on the assumption that variations when various percentages of calcium chloride were added t o the occurring in the clinker of the experimental kiln, resulting from kiln feed, are given in Table 111. Calcium chloride was very efthese factors, were proportional to the variations observed in the fective in reducing the alkali content. However, the alkali conclinker of the control kiln. Although this relation would not be expected to be strictly linear, the corrections are justified in that tent of the control kiln clinker was not constant. The lower than they are comparatively small. The corrected effect of the calnormal values a t the 1.5% calcium chloride rate were, without cium chloride on the alkali content of the clinker is shown graphdoubt, due to calcium chloride being carried into the chamber dust ically in Figure 2. The alkali removal, Figure 2, was obtained by from kiln 2 and returning to each kiln in equal amounts. The higher than normal values at the 0.5% calcium chloride rate would be the resultant effect of any build-up in the alkali conTABLE v. ALKALIESCONTRIBUTED T O KILNFEEDB Y RAWbfIX AND DUSTI N DRY tent of the dust and any factors affecting PROCESS PLANT the burning temperature. (Calculated from d a t a i n Table IV on basis of 65 tons dust and 500 tons raw mix per kiln per day) order to ascertainthe extentto which ,-Alkalies Contributed t o Kiln Feed, any alkali build-up in the dust may have Dust with CaClz D u s t with CaC1z CaClz Added t o Kiln 2 O& Added to Both Kilns Raw Mix been reflected in the composition of the Corn- Added t o Total Total clinker, it was necessary to obtain further g$:$e 2’ N ~ ~ KO~ O (as NazO) NazO K20 (as NazO) NazO KzO (asTotal NazO) analytical information. The alkali con2 -3 1.5 0.381 0.894 0.97 0.100 0,292 0.29 0.145 0.369 0.39 5-6 1.0 0.434 0.938 1.05 0.036 0.227 0.22 0.078 0.239 0.24 tents of dust and raw mix (kiln feed prior 8-9 0.5 0.416 0.965 1.05 0.058 0.212 0.20 0.061 0.208 0.20 to dust return) samples, corresponding GA-7~ 0.0 0 . 3 8 1 0.885 0.96 0.054 0.215 0.20 0.054 0.215 0.20 to clinker samples at each calcium chloride
uu
~~
Kiz
INDUSTRIAL AND ENGINEERING CHEMISTRY
340 TABLEVI. Composite Sample
Vol. 42, No. 2
KILNFEEDTOTAL CARBOSATE TITRATIOKS AND MAJORCONSTITUENTS OF CLIXSBER IN DRYPROCESS PMXT
CaClp Kiln Feed Total Added bo Carbonate Kiln 2, Titration as % CaCOs, %
-
Si01
Fe20a
2-3 5-6 8-9
1.5 1.0 0.5
78.2 77.2 76.8
21.82 22.54 22.47
3.15 3.26 3.22
2-3 5-6 8-9
1.5 1.0 0.5
78.2 77.2 76.8
21.13 22.50 22.58
2.97 3.14 3.22
Clinker Constituents, % Theoretical Theoretical Total 3Ca0.Si0za 2Ca0.Si0za
CaO hlgo Loss KILN 1, COXTROL KILN 2.25 0.46 6.55 64.80 63.39 2.39 0.33 6.76 2.24 7.01 63.21 0.39 KILN2, EXPERIMENTAL KILN 2.22 6.23 66.36 0.10 2.26 0.06 6.74 64.61 6.68 64.07 2.29 0.23 BOTHKILNS 6.61 64.67 2.42 0.37
.&In03
6A-7h Sone 78.0 21.75 3.25 a Calculated according t o Bogue (6) and Dah1 (7). b Determined by equation: lime factor = ( 3 C a 0 . SiOn -!r 2Ca0. sioz Si09 ratio.
SiOn/ RnOa
Lime Factorb
99.03 98.67 98.54
49.4 36.6 34.8
25.3 37.1 38.2
2.35 2.25 2.20
2.42 2.27 2.25
99.01 99.31 99.04
63.4 42.3 39.9
12.8 32.7 34.5
2.30 2.28 2.28
2.61 2.33 2.30
99.07
48.9
25.5
2.21
- SiOz)/SiOz; lime factor i8 equivalent to: c a o available for formation
subtracting the corrected alkali contents from the average content of the clinker when no calcium chloride was added. At the lower levels of calcium chloride addition the potassium oxide removal is much greater than that of the sodium oxide. However, as the calcium chloride is increased, the increment of potassium oxide removal becomes smaller and that of the sodium oxide becomes larger. As a result, the decrease in total alkali is approximately proportional to the amount of calcium chloride used. The following theoretical considerations are offered in explanation of the relative quantities of alkalies lost with successive increases in the calcium chloride. It seems probable that the alkalies normally lost in some other form are volatilized largely as the chlorides within the limits of the amounts of chloride added. If this assumption is correct, the conditions given below, relating the quantities of calcium chloride to the molecular equivalent amounts of initial alkalies lost from the raw materials, would have significant bearing: 1. Up to 0.7% calcium chloride addition, the equivalent chloride is less than the potassium oxide loss 2 . Between 0.7 and 1.1% calcium chloride addition, the equivalent chloride exceeds the potassium oxide loss but is less than the total alkali loss 3. Over 1.1% calcium chloride addition, the equivalent chloride exceeds the total alkali loss. In conditions (1) and ( 2 ) the volatilization of both alkalies would be limited by the quantities of chloride present. However, in condition (3), as the chloride is present in excess, the volatilixation of the alkalies would not be limited except when low concentrations of the chloride develop because of the loss of alkali chlorides. The sequence of these conditions results in a progressive elimination of competition between potassium and sodium for the available chloride, The potassium chloride formed would evaporate a t a higher rate by virtue of the fact that it has a higher vapor pressure. As larger amounts of chloride are added, the slope of the potassium oxide curve decreases because of the diminishing amounts of potassium remaining in the clinker. The increasing increments of chloride not used by potassium remain accessible for sodium. It is suggested that the apparent absence of an asymptotic limit for sodium oxide is occasioned by the predominance of the potassium chloride volatilization until the potassium drops to such levels that it no longer competes with sodium for chloride. The slope in this region is also affected by the relatively large residual sodium content. Interpreted in this manner the findings of this investigation are in very good agreement with the known properties of the alkali chlorides involved. The calcium oxide content of the clinker was increased somewhat as the result of the addition of calcium chloride. This would, theoretically, cause the burning temperature in the experimental kiln to be higher than i t would have been if the clinker calcium oxide content could have been maintained a t a constant level. Under plant conditions this error was unavoidable. It is doubtful that this would make an appreciable difference in the alkali
2.42 of silicates/
volatilization a t the 0.570 calcium chloride rate. However, at the higher rates the volatilization may have been increased slightly by this effect. BURNING CHARACTERISTICS AND PLANT OPERATIOXS
The experimental kiln burned with less difficulty during the period that calcium chloride was being added. Less coal was required, even though the kiln feed of the last 4 days was increased about 4% over the normal rate in an effort to maintain the kiln coating. The kiln atmosphere was much clearer, apparently because of less dusting in the reaction zone. The fluxing action might be expected as calcium chloride is analogous to calcium fluoride; the fluxing characteristics of calcium fluoride are fairly well established in the cement industry. The fact that calcium chloride acted as a flux at comparatively small percentages indicates strong fluxing properties. The low melting point of calcium chloride (772" C.) would make it readily available for chemical reaction. Whatever water of hydration of the calcium chloride existv would be lost at a temperature of 200" C. As the raw mix approaches the calcining temperature of 825 C., the calcium chloride would become a mobile liquid. It seems likely that the liquid calcium chloride is responsible for the clearing of the kiln atmosphere by physical retention of the dust particles which are normally carried into the gases by agitation owing to the rotation of the kiln and carbon dioxide escaping from the raw mix. The increase of the calcium oxide content of the experimental kiln clinker was approximately 0.3% greater than the increase that the calcium chloride would theoretically produce. This map be associated with the physical effect mentioned.
Figure 2.
Effect of Calcium Chloride on Alkali Content of Clinker in Dry Process Plant
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 19.50
. .
Because of its deliquescent nature, calcium chloride cannot be interground with the raw materials in a dry process plant. However, this is not necessary as it will melt and be mixed adequately by the rotation of a kiln before reaching the reaction zone. The considerable increase of chlorides in the stack gases may cause, in some plants, accumulative deposits on force-draft fans, precipitator electrodes, and similar equipment that comes in contact with these gases. Although no difficulties were encountered in this respect, unfortunately the brevity of the tests and the design of the plants were such that definite information could not be obtained concerning this possible action. I n general the results of the study indicate that the method affords a means of reducing the alkali content to any desired level. It facilitates changes to and from the manufacture of other types of cements without bringing in special low alkali raw materials in large tonnages, often prohibitive in cost, and causing undue material flow complexity. The full scale tests also indicate that the method can probably be used in many of the existing plants with little or no change in plant design.
EFFICIENCY OF CALCIUM CHLORIDEAT TABLE VII. APPARENT VARIOUSRATESOF ADDITION
CaCL Added t o Kiln Feed (as Naz0 on Loss-Free Basis), % 0.22 0.44 0.65
Total Alkalies Removed (as NazO), % WET PROCESS 0.06 0.15 0.23
0.43 0.86 1.29
DRYPROCESS 0.24 0.35 0.55
A,
=
0.559 K , [CaClZ] 100 - L
0
[CaC12] S 1.5%
where A , = total alkali removal expressed as sodium oxide; K , = apparent efficiency of CaC12; L = ignition loss of raw material; and [CaClz] = percentage of calcium chloride added on dry weight basis. The actual amounts of total alkali removed are compared with the calcuated values, using the above equation, in Table VIII. As the largest deviation is 0.04% the equation appears to give a close approximation of the alkali removal. This relation should be useful in plants producing clinker with alkali contents of the same order of magnitude as those studied. SUMMARY AND CONCLUSIONS
The reduction in the alkali content of the clinker in a wet process lant has been determined when 0 t o 0.75% calcium chloride is ad&d to the kiln feed and, in a dry process, when 0 to 1.5% calcium chloride is added. The efficiency of the calcium chloride for total alkali removal has been found to be approximately constant in each type of plant, regardless of the quantity of calcium chloride added in the ranges investigated. The average calcium chloride apparently utilized in the wet process plant was 3270, whereas in the dry process i t was 4870. It is concluded that in plants producing clinker with alkali contents of an order of magnitude the same as those studied, which is very often the case, the alkali removal may be related t o the calcium chloride efficiency b y an empirical equation. At the lower levels of calcium chloride addition the potassium oxide removal in both plants was much greater than that of the sodium oxide. Higher levels of calcium chloride addition were investigated in the dry process plant. The rate of potassium oxide removal was found to decrease when larger amounts of the chloride were added. The rate of sodium oxide removal was observed to be approximately constant u p t o t h e point where the chloride added exceeded the molecular equivalent quantity of the potassium oxide removal. Above this value the rate of sodium oxide removal increased markedly apparently because of the increasing quanti-
parent Efficiency
%
Average
27 34 34 32
Average
56 44 45 45
TABLEVIII. TOTALALKALIESREMOVEDEXPERIMENTALLY COMPARED TO CALCULATED VALVES
Alkalies Removed [as NapO), Yo Actual value Calcd. value WET PROCESS 0.07 0.06 0.14 0.15 0.22 0.21 DRYPROCESS 0.21 0.24 0.41 0.35 0.62 0.55
CaClz Added t o Kiln Feed, % 0.25 0.50 0.75
EFFICIENCY OF CALCIUM CHLORIDE
To establish the efficiency of the calcium chloride on a molecular basis, the sodium oxide equivalents were computed as given in Table VII. The total alkali removed, attributable to the use of calcium chloride, also given in Table VII, is equal to the calcium chloride apparently utilized in volatilizing the alkalies. On this basis the percentage of the total calcium chloride apparently utilized in each case was calculated and is given in the same table as the apparent efficiency. These values can be considered to be approximately constant in view of the experimental difficulties in analytically measuring the values from which they were calculated. This being the case the alkali removal may be related to the amount of calcium chloride used in a given plant by the empirical equation:
341
0.5 1.0 1.5
ties of the chloride present in excess of that utilized by the potassium oxide. I n the wet process plant the effect of returning the Cottrell precipitator dust t o the system was determined. The sodium oxide content of the clinker was not increased by a measurable amount, but the potassium oxide was increased 0.19%, which is e uivalent to 0.13% increase in total alkali, ex ressed as sodium oxi2e. I n the dry process plant the e&ct of returned chamber dust on the alkali content of the materials entering the kiln was determined. Normally the dust was found to increase the alkali content of the kiln feed 0.20%in total alkali. No measurable buildup was found when 0.570 calcium chloride was added to the kiln. The increases in alkali content of the feed were relatively small when larger amounts of calcium chloride were added. It was roncluded that under these conditions the return of the dust would have only a negligible effect on the alkali content of the clinker produced. Calcium chloride was observed to have strong fluxing properties. ACKNOWLEDGMENT
The author wishes to thank Earl H. Gray for his very willing cooperation and valuable suggestions, especially in the planning of the plant operations. LITERATURE CITED
(1) Am. SOC. Testing iMateriaZs, Proc. 48, 1055-127 (1948) (reprint). (2) Am. SOC.Testing Materials, Standards, Part II, 26-8 (1946). (3) Blanks, R. F., et aZ., Am. SOC. Testing Materials, Bull. 142,
28-34 (1946). Bogue, R. H., ”Chemistry of Portland Cement,” pp. 94-102, New York, Reinhold Publishing Corp., 1947. (5) Bogue, R. H., IND.ENG.CHEM.,ANAL.ED.,1, 192-7 (1929). (6) Dahl, L. A., presented at meeting of Portland Cement Association, St. Louis, Mo. (May 15, 1941). (7) Dahl, L. A., Rock Products, 32, No. 23, 50-2 (1929) (8) I-Iansen, W. C., J . Am. Concrete Inst., Proc., 40, 213-27 (1944). (9) “International Critical Tables,” Vol. 3, p. 214, New York, MoGraw-Hill Book Co., 1928. (10) Kalousek, G. L., et al., J . Research Natl. Bur. Standards, 30, 215-55 (1943). (11) Lerch, William, Portland Cement Association, Series 2856-7-8 (November 1946). (12) Smith, J. L., Am. J.Sci., 2, 50, 269 (1871). (13) Stanton, T. E., Am. Soc. Civil Engrs., Proc., 66, 1781-811 (1940). (14) U. S. Dept. Interior, Bur. Reclamation, Conf. on Alkalies in Cement and Their Effect on Aggregates and Concretes, Denver, Colo. (February 1941). (15) Woods, Hubert, Rock Products, 45, No. 2, 66-8 (1942). (16) Zimm, B. H., and Mayer, J. E., J . Chem. Phys., 12,362-9 (1944). (4)
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RECEIVED March 31, 1949.
