624
INDUSTRIAL AND ENGINEERING CHEMISTRY
EUROPEAN DEVELOPMENTS SUBSEQUENT TO WORK REPORTED HERE Recent work in Europe merits discussion here, as preliming and predefecation have been used very extensively and numerous systems have been devised and patented. Dkdek and Vasatko’s method (6) is used very extensively. The original system is a progressive cold wet preliming which takes place by a number of small additions of lime. Use of DBdek and Vasatko’s procedure, followed by addition of balance of the lime hot and dry or cold and wet with continuous first carbonation, has been reported by Wiklund and Lindblad (10). The above method was compared to the Dorr method of continuous defecation and carbonation using a Benning carbonator ( I O ) . Settling rates showed that the carbonation sludges from the preliming and carbonation would be hard to handle in a thickener such as a Dorr, while the sludge from the Dorr carbonation would be very satisfactory. Data are given showing that as the filtration time increases, the sedimentation velocity decreases, the volume of the precipitate increases, and the color of the thick juice decreases. Desirable physical properties of the sludge mean poor thick juice and vice versa. By the use of the Dorr continuous carbonation method a carbonation sludge with desirable physical properties was obtained, but poor, dark, thick juices were also the result. With the DBdek-T’asatko preliming and separate dry or wet main liming and continuous carbonation, good purified juices were obtained, but the sludge was such that it was impossible to handle in a thickener and continuous filters of normal size (10). The experiments by Wiklund showed that it was not practical to combine the preliming method directly with the Dorr carbonation. After considerable experimental work, both in Sweden and in Belgium, a system of preliming and defecation was worked out that gave sludges with excellent settling rates and juices light in color that were relatively stable as to color during evaporation (10). Raw juice from a preheater is mixed in the first compartment of a preliming tank with overcarbonated juice in the ratio of 1t o 1 in the next seven compartments of the preliming tank, and milk of lime is added by the DBdelr and S’asatko method, so that the alkalinity gradually increases t o about 0.25 gram of calcium oxide per 100 ml.; the juice is heated to 80” to 85” C. through a heater and then the balance of the lime is added in a liming tank, either
Vol. 43, No. 3
as dry lime or milk of lime. The alkalinity is then about 0.7 gram of calcium oxide per 100 ml.; after liming the juice is carbonated to 0.08 gram of calcium oxide per 100 ml. From this carbonation half of the juice (corresponding to total quantity of raw juice) is passed to a thickener or filters. The other half of the juice is overcarbonated t o a n alkalinity of 0.02 to 0.03 gram of calcium oxide per 100 ml. and is then recirculated to the first compartment of the preliming t,ank because it is believed t,his supplies nuclei for flocculation (10). Dkdek, of Belgium, recently (4, 5 ) gave an excellent summation of the principles of defecation on which he, Brueghel-Muller of Copenhagen, and Wiklund of Sweden, are working. BrueghelR!Iuller’s work is along the line of eliminating completely all the colloidal precipitation of nonsugars in the raw juice by t>heaction of calcium oxide. He invented a process of so-called stabilization of the colloids during preliming of the raw juice by the very slight addition of lime (about 0.02%). This stabilizat,ion is believed to be due to combination of calcium ion, pectin, and protein. Through discussions and common experiments DBdek, Brueghel-Muller, and Wiklund became a.ware that both BrueghelMuller’s and Wiklund’s methods of purification could be improved by incorporating features of the other method. Briefly, this method would combine predefecation with continuous carbonation.
ACKNOWLEDGMENT The writers wish to acknowledge the work of F. W,Kopplin for his part in these studies and also the help J. DBdek and Olaf Wiklund gave in supplying information on the recent developments and trends of purification in Europe.
LITERATURE CITED (1) Bull, A. V., U. S. Patent 1,755,165 (1930). (2) Ihid., 1,860,321 (1932). (3) Dsdek, J . , J . .fabr. sucre, 80, 63 (1939). (4) DPdek, J., personal communication, March 9, 1950. (5) DPdek, J., Socker H a n d . , 6, 50 (1950).
(6) Dsdek, J., and Vasatko, J., U. S. Patent 2,007,424 (1936). (7) McGinnis, R. A,, P r o c . Am. SOC.S u g a r Beet Technol., 5, 573 (1948). ( 8 ) Ramsey, E. R., and Bull, A. V., U. S. Patent 1,868,472 (1932). (9) Teatini, D., I b i d . , 1,988,923 (1935). (10) Wiklund, O., and Lindblad, L., Socker Handl., 9, 157-96 (1949). RECEIVED April 6 , 1950.
Reburning of Defecation Lime
Cake R. M. DANIELS AND ROBERT H. COTTON Holly Sugar Corp., Colorado S p r i n g s , Colo.
B
ECAUSE 30 t o 100 pounds of lime are used per ton of beets
in beet sugar manufacture, and good Steffen lime rock has been increasingly difficult t o obtain, especially in California, reburning of lime has been intensively studied by this company. These studies resulted in a large scale installation for reburning waste lime cake a t the -4lvarad0, Calif., plant. I n America, beet sugar factories burn lime rock in vertical kilns with coke as the fuel. The lime is used in defecation of beet diffusion juice, and carbon dioxide is employed in the carbonation process t o precipitate calcium carbonate which carries with it numerous impurities in the juice; finally, carbon dioxide is used to reduce the alkalinity of the juice before it goes to the evapora-
tors. Factories using the Steffen process use much greater amounts of lime; this is used t o recover additional sugar from molasses by precipitation of calcium saccharate. Thus, a limereburning system had t o provide adequate carbon dioxide recovery, and the objective was a carbon dioxide concentration approximating that available from a lime kiln, 30 to 35%. This was accomplished in early work by burning the lime partly by indirect heat in a muffle furnace ( I ) , thus avoiding dilution with combustion gases. Natural gas was the fuel. Figure 1 shows diagrammatically the multiple hearth furnace used in this work, referred t o as a Skinner roaster.
March 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
PILOT PLANT STUDIES I n the pilot plant studies, the roaster was 5 feet in internal diameter and 18 feet 3 inches high, divided into ten compartments. There was a total of 174 square feet of hearth area. Compartments 7 , 8, and 9 were muffles, heated indirectly. Combustion gas from No. 10 plus carbon dioxide from dissociating talcium carbonate in compartments 7 , 8, 9, and 10 provided a gas
Figure 1.
625
No. 10 hearth t o 425" C. in No. 3 hearth t o 255" C. in the combustion stack. One of the principal objects of the pilot plant experiments was t o determine the number of times, or cycles, through which the lime cake could be reburned and made usable.
Schematic Diagram of Roaster
Section on rixht pertains to both pilot plant and commercial installation. applies to commercial installation.
relatively rich in carbon dioxide (approximately 32%), which could serve as carbonating gas, while the combustion gases from the upper hearths served t o dry the wet limecake feed coming from the rotary filters a t the carbonation station. Rabble arms conveyed the lime cake from the top of the roaster successively through each hearth from Nos. 1 t o 10. Table I shows the capacity of the pilot plant roaster. The fact that the capacity when wet feed was used was lower than on dry lime rock suggested a predryer, which was incorporated later in the full scale roaster. T h e carbon dioxide a t 32 to 35% concentration amounted t o approximately 150% of the carbon dioxide combined with the calcium oxide in the feed; the excess arose from burning organic matter in the lime cake plus burning of the natural gas in hearth 10. Temperatures ranged from 1100" C. in
Other equipment
A small carbonation tank with a capacity of 1 ton of raw juice was set up. Equipment for making milk of lime and a plate and frame filter press were also provided. Lime cake from the first carbonation Oliver filters was burned in the roaster. From t h e burned lime milk of lime was made; first-carbonation juice was used for slacking the lime. This milk of lime was used to defecate raw beet juice, which was carbonated in the small experimental carbonation tank. The carbonated juice was then filtered in the plate and frame press. T h e lime cake obtained from this process, called first-cycle cake, was returned t o the roaster and again burned. T h e experiment was carried through six cycles and the lime cake from the sixth cycle was calcined in the roaster and used to make a large cooler test. Approximately 4500 pounds of firstburned lime cake were used t o start the cycle tests, and only 800
I n the beet sugar industry many thousands of tons of lime cake, precipitated calcium carbonate plus organic impurities from beet juice, are discarded each year, after being used t o defecate diffusion juice by precipitation, coagulation or flocculation, and occlusion. A t the same time good lime rock for defecation is becoming increasingly difficult to obtain. In pilot plant work a t Torrington, Wyo., and full scale operation a t Alvarado, Calif., the cake is burned in a multiple hearth furnace to produce calcium oxide of exceptionally good quality for Steffen house operation. The resultant reburned lime is equal or superior to first-burned lime in Steffen house operation, where lime is first employed to recover sucrose from molasses as a n insoluble saccharate. The calcium saccharate is then used as a source of lime in defecation of diffusion juice.
INDUSTRIAL AND ENGINEERING CHEMISTRY
626 Table I. Feed Wet filter cake, 46% H2.0 Wet cake plus crushed lime rnpk
@rL"s&d lime rock only
Small Roaster Capacity Feed, Lb./Hour 700 655 fin "" 500
CaO Burned, Lb./Sq. Foot Hearth/Day 27.7 29.9
Recovery of CaO Charged,
38.5
100
% 88 88
pounds were left a t the finish. The difference was accounted for as dust losses from the roaster and as a certain amount of discard t o keep the cycle lime separated from lime produced from other lime cake burned between cycle tests. I n all the cycle tests, except the original first-carbonation Oliver cake, the lime addition was a t the rate of 2.5% calcium oxide on beets. The cooler test is conventionally used to determine pounds of calcium oxide required per 100 pounds of sugar in molasses worked in the Steffen process for recovering sugar from molasses. I t duplicates factory concentrations and temperatures. It W B S desirable to study the value of the lime under Steffen house conditions because lime quality is more critical in St'effens than in straight house operation. Fundamental requirements of Steffen house work are precipitation of the maximum quantity of sugar with a minimum quantity of lime, and the greatest possible separation of the sugar from nonsugars. Briefly, a t about 6% sugar and 20" C., lime forms an insoluble cold saccharate from sugar in molasses. This is caught by filtration and washed. The filtrate still contains up to 1% sugar and considerable lime. 11ost of this is recovered by heating to 80" to 90" C., a t which point the sugar and lime precipitate out in the form of a hot saccharate, which is settled, filtered and washed, added t o the cold saccharate cake, and used as a source of lime in defecation while sugar is introduced into the process. Table I1 compares fresh lime from the vertical kiln with the same lime after it has been used t o defecate beet juice, follo~ved by reburning in the pilot size Skinner roaster. The reburned lime (first-cycle lime) was more effective in precipitating sucrose as the saccharate than was the original lime, because less was needed and sugar losses in the filtrate (cold waste) were less. It is believed t h a t the explanation lies in smaller particle size and greater surface activity.
Vol. 43, No. 3
even after six cycles a pound of lime from the roaster 'rws a t least as effective as the initial lime from the kiln. The titratable lime shown in Tables I1 and I11 is only an approximation of the lime determined as the oxalate, but is often used as a control figure, even though it includes calcium as carbonate plus magnesia, etc. Table I11 indicates that impurities gradually build up as the lime is progressively reburned. I n practice this is corrected by using some fresh lime to make up for lime lost as dust and in Steffen waste water, etc.
Table 111.
Analyses of Lime on Dry Basis through Six Burning Cycles
Cycle No. CaOG LIgO
Fz& E;Wo +
+?g
Con
d:gCaO bya ftitration t e r subtracting
1 2 3 4 5 6 87.27 8 3 . 3 4 8 0 . 9 2 78 85 78.09 7 5 . 3 2 2.83 4.06 5.05 5.91 5.73 7.16 2.47 2.50 2.90 2.81 2.82 3.00 2.76 3.50 4 , 3 1 5.36 5.21 6.33 0.30 0.26 0.16 0.14 0.23 0.26 0.12 0.13 0.12 0.09 0.08 0.10 1.14 2.14 1.44 1.33 2.38 1.60 0.98 1.02 1.11 1 . 1 2 1.20 1.17 0.02 0.02 0.02 0.02 0.02 0.02 2.15 2.99 3.87 4.35 4.33 5.07 91.46 89.24 8 8 . 2 8 87.24 86.24 8 4 . 8 0
C O Xa n d combined $03 'r.ith 85.12 a Determined as oxalate.
79.86
78.38
7 6 . 3 6 7 4 . 2 1 72.46
__ After the small cooler tests were completed, t,wo factory scale coolers were run, using 1200 pounds of molasses per cooler iyith regular factory equipment. Table IV gives data Lvhich closely check the small teats shown in Table 11; however, the percentage of sugar in the filtrate for cycle 1 was higher in the factory trst; in cycle 6 they were approximat,ely cqual.
Table 1V. Large Cooler Tests Type of Lime
Lime Addition
Sugar in Cooler Filtrate,
70
Amarent Purity of Cold Cake,
%
Table 11. Small Cooler Tests Lime Addition, Lb. Ca0/100 Lb. Sugar in R'lolasses Original lime from vertical 130 kiln 140 First-cycle lime 100 105 Second-cycle lime 100 105 110 Third-cycle lime 105 110 115 Fourth-cycle lime 110 120 125 Fifth-cycle lime 110 120 125 Sixth-cycle lime 120 125 130
Sugar in Cold Waste.
CaO py Titration,
0.83
0.47 0.21 0.16
93.7 91.46
0.28
89,24
%
0.21 0.18 0.47 0.31 0.21 0.47 0.21 0.16 0.94 0.52
0.36 0.68 0.42 0.32
%
88.28 87.24 88.24 84.80
Table I11 (for which the authors are indebted t o the late Fred A I . Bachler) gives analyses of the lime after reburning one t o six times. The final row of figures shows calcium oxide determined by subtracting from calcium oxide precipitated as the oxalate an amount of calcium oxide equivalent t o the sulfur trioxide and carbon dioxide found in each sample. Unfortunately, the authors know of no absolute method of determining the exact amount of calcium oxide available.to precipitate sucrose as the saccharate. The cooler tests in Table 11 are a good indication, however, and
The drop in apparent purity of the cold cake is considered significant. It indicates that sixth-cycle lime produces a saccharate cake which has more impurities than first-cycle cake. These will be introduced into the regular process and result in a loner yield of crystalline sucrose. As shown in Table V, this difficulty is overcome in commercial practice by introduction of fresh lime rock in each cycle t o replace lime lost by dust formation and in the Steffen waste water. In addition t o the cooler tests on the lime, observations were made on its behavior in defecating raw juice during each cycle. T i t h i n experimental error, no differences were observed in purity of the juice after defecation or in lime salts in the juice between the first and sixth cycle. In each cycle 2.5% lime (as titratable calcium oxide) on beets was used in the defecation. The installation of the commercial roaster at Alvarado, Calif., is shown schematically in Figure 1. A predryer was added; provision was made for automatic feed and temperature control in the hearths, and a cooling device for the hot lime was installed. It was found that an ade uatc carbonation could be accomplished with low carbon dioxidik concentration, if an adequate blower and recirculation system were supplied. The use of muffles was therefore discontinued, and the lime was heated by direct flame in all hearths ( 2 ) . During the last campaign (1949) the carbon dioxide concentration was 17.3%. No difficulties in carbonation were experienced when a Benning carbonator was used. The roaster is 20 feet in diameter and 70 feet high. Lime from
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1951
the roaster is pulverized in a Raymond mill, because some of the material forms small balls. Approximately 95% passes through a 200-mesh screen.
*
At Alvarado, as a t most plants, there was a large deposit of waste lime cake from previous years. With this as feed the roaster was able t o supply the total lime requirements of the plant for a 6-week period when handling 1412 tons of beets and 103.2 tons of molasses a day. While operations were successful on 100% lime-cake feed when batch carbonation was practiced, there was difficulty when Alvarado was equipped with a Dorr clarifier rather than Borden sock filters. A haze was evident in the juice from the Dorr clarifier; this was eliminated when 30% of the lime was supplied from fresh rock. The problem is worthy of further study. With wet lime cake the capacity of the roaster is inadequate for the plant operating a t 1800 tons' slicing capacity and working from 135 t o 145 tons of molasses per day. The roaster now supplies 60% of the lime requirements, while 40% is furnished by the old vertical kiln. The capacity of the roaster is 41 tons of calcium oxide per day or 27.3 pounds per square foot per day. The 41-ton figure applies t o wet lime cake, approximately 46% moisture. The capacity in the roaster when the feed was one half lime rock and one half partly dried defecation mud (38% moisture) is 64 tons of calcium oxide per day or 43 pounds per square foot per day. The kiln, of course, operates only on lime rock.
Table V.
Operations With a n d Without Roaster
Molasses worked, tons per day Total sugar in molasses lost in Steffen waste, % Lime addition, lb. CaO per 100 lb. sugar in molasses Bags of su a r (100 lb.) produced per ton molasses worked Lime rock, To on molasses True purity saccharate milk
02
L
x
Lime Supply Vertical kiln 40%, Vertical kiln 100% roaster 60% 120 145 5.28
4.34
139
110
5.9 121.9 85.8
6 44.3 85.5
The 1949 campaign a t Alvarado presented a n opportunity t o compare results with and without the roaster while operating the Steffen house (molasses campaign only, no beets). After 7 days (during which operations were normal) the Skinner roaster was shut down because of a shortage of natural gas fuel. Oil-burning equipment has not been installed as a substitute. The plant continued t o operate without interruption for 4.5 days more, but all the lime was supplied by the vertical kiln using lime rock. Table V summarizes the salient differences. The plant worked a t maximum capacity during both periods. Table V indicates no lower yield of crystalline sucrose from molasses on use of reburned lime. There was no significant difference in the purity of the saccharate milk produced. Saccharate milk includes both hot and cold cake. E m p i r i c a l l y , it has been observed t h a t if fresh rock is added t o keep the p e r c e n t a g e c a I c i u m oxide by t i t r a t i o n between 86 and 88, Steffen operation is s a t i s f a c t o r y . Below 86 purities of cold saccharate cake fall off, and less sugar is produced per ton of molasses.
Table VI.
627
Operations at Tracy and Alvarado
Average tons molasses worked per day Average tons lime rock per day Tons lime rock, yo on molasses worked Cost of coke and lime rock per bag sugar Cost of gas for roaster Der baz sugar Total cgst of fuel and-rock fbr lrme burning er bag su a r Afditional la%or on roaster per bag sugar Total costs of fuel, labor, rock f o r lime per bag of sugar
Table VII. Hearth NO. 1 3
Temperature,
7
1000
6
c.
375 550-600 975
Tracy
Alvarado
93.8 118.7 126.5 80.2610
134.2 72.8 54.2 $0.1131 $0 0254
$0.2610
SO 1385 $0 0108
$0.2610
SO 1493
....
.. .
Temperatures Hearth NO.
8 9 10
Temperature, O
c.
1000
1000-1026 1075-1100
.......
Table VI presents a comparison of Alvarado with the nearby Tracy plant for the whole 1949 campaign. Both plants worked similar molasses and used identical lime rock in 1949. It cannot be argued that conditions in any two factories are identical in every respect; Tracy and Alvarado, however, were nearly the same as t o lime costs before the roaster was installed a t Alvarado. Table VI shows that the present costs of lime per bag of sugar a t Tracy and Alvarado are 26.1 and 14.9 cents, a decrease of 39%. T%ble VI1 shows the temperatures maintained in the various hearths in 1949. No data are available for hearths 2, 4, and 5. Temperatures were maintained by Brown electronic controls. A temperature indicator on hearth 3 is a useful check on uniformity of feed. Thus, if the feed is uneven the temperature varies widely. As would be expected, maximum capacity and lime quality are associated with uniform feed. I n a n y new installations the figures in Table VI1 will help in selection of the proper refractory material. It has been observed t h a t the above temperatures must be reduced 50" t o 100" during a straight Steffen campaign. The principal difference in the feed in this case is a lower percentage of organic matter in the feed (lime cake from the Oliver filters) caused by absence of organic impurities from the beets. Until the temperature was lowered, difficulty was experienced with clinkering. When defecation lime cake is the feed for the roaster, the use of 40% lime from the kiln and 60% from the roaster results in a product having 86 t o 88% calcium oxide by titration. When Steffen lime cake is the feed (60% with 40% calcium oxide from the kiln), the resulting lime has 92 t o 93% calcium oxide by titration.
ACKNOWLEDGMENT The authors wish to acknowledge the able assistance of Ira Croghan and J. E. Browning in this work.
LITERATURE CITED (1) Daniels, R.M., U. S.Patent 2,194,164
( M a r c h 19, 1940). (2) Ibid., 2 , 2 7 3 , -
253 (Feb. 17, 1942).
RECEIVEDAgril 1950.
6,
