I
S.
D. FAUST
and
H. E. ORFORD
New Jersey Agricultural Experiment Station, New Brunswick, N. 1.
Reducing Sludge Volume with Crystal Seeding in
...
Disposal of Sulfuric Acid Waste Is deposition of calcium sulfate on seed crystals responsible for reducing volume when sludges are treated with gypsum or return
sl udge? This work proves that the answer is “yes”
IN
DISPOSAL OF industrial waste solutions of sulfuric acid, lime neutralization is the most common treatment. The resultant calcium sulfate sludge and oth2r reaction products constitute a major problem. T o concentrate these sludges, crystal seeding has been investigated (2,3), and when sludge return or addition of native gypsum is used for this purpose, the concentrating effect has been attributed to deposition of calcium sulfate on seed crystals. However, other than sludge compaction, no evidence has been available to indicate that crystal seeding is responsible. Now, microscopic examination seems to verify this theory, and for unseeded sludges, it suggests that crystalline properties of size and habit cause poor settling and compaction.
Experimental An initial dosage of native gypsum was added to 500 ml. of an agitated sulfuric acid solution, and after a 6minute reaction period, the mixture, neutralize@ with a 10% lime slurry, was transferred to a 1-liter graduated cylinder for sedimentation. The continuous sludge return process was simulated by siphoning off the supernatant and removing an aliquot of sludge for dry solids determination. The remaining portion of sludge was then returned for the next neutralization. This procedure
was repeated until a desired volumetric sludge return was obtained, usually within four to six recycles, that was held constant thereafter. The high calcium and dolomitic quicklimes, typical of commercial products, slaked with 95’ to 100’ C. distilled water, were allowed to reach room temperature before use. The acid solutions were prepared by diluting 95% sulid. Native gypsum was as comterra alba, which is 97.8% CaSOq2H20. Precipitated gypsum was prepared from lime and sulfuric acid.
Results Other reports (7, 4, 5) confirm the crystalline habit of calcium sulfate as a needle crystallizing in the monoclinic system. Another crystal habit, the “bow-ties” form, may be identical with the prigmatic hexagonal forms used by Palacios and Galloni (5) in their work on the crystalline structure of gypsum. O n the other hand, native gypsum is a small, spherical particle, without the typical acicular shape, because it is obtained by grinding the gypsum rock. That a spherical particle will settle and compact to a higher degree than a needle-shaped particle is shown by comparing the settling characteristics for various amounts of precipitated and native gypsum-e.g., 15,000 mg. per liter of precipitated gypsum settled to a dry solids concentration of 3.6%, whereas native gypsum settled to 43.3%. ed dosages of native gypsum were from a previous study (3) to imum control of sludge volumes (Table I). Sludge volume was reduced in all systems and sludge per cent dry solids increased. In Figure I, absence of needle-shaped crystals suggests that the reaction product has deposited on the seed material, , reduction in sludge volume and
2000 to 50,000 mg. per liter were added to the dolomitic quicklime system (Table I). The per cent increase of sludge solids concentration suggests that precipitated gypsum is a poor seeding material when compared with equivalent amounts of native gypsum-e.g. 12,000 mg. per liter of precipitated gypsum added to the high calcium quicklimeacid, system effected a 146% concentration of the sludge, whereas a 10,000 mg. per liter dosages of native gypsum effected a 550-fold concentration. The remaining figures show some concentration of sludge, but excessive dosages of precipitated gypsum do not necessarily effect greater sludge densities. Adding a precipitated gypsum seed permits little, if any, deposition of the reaction product (Figure 2). Predominance of the needle-shaped crystal may again account for the poor settling and compaction characteristics. In Table 11, data under the “sludge” columns are averages obtained from the indicated number of recycles of the sludge loading. Waste sludge volume was reduced and per cent of sludge dry solids were increased for a 300- to 500fold sludge concentration. For high calcium quicklime with 1% acid, 35,600 mg. of seed per liter effected a 300% concentration of sludge as compared to a 150- to 156-fold concentration with 30,000 to 40,000 mg. of precipitated gypsum per liter (Table I). Similar ob-
sition-a spherical particle is prod, which settles and compacts better Figure 1. Seeded calcium sulfate sludge after one recycle. The absence of needle-shaped crystals suggests that products. of reaction have been deposited on the seed material
Seed dosages of precipitated gypsum ranging from 4000 to 80,000 mg. per liter were added to the high calcium quicklimk system, whereas dosages of
Figure 2. When precipitated gypsum is used as.a seed, acicular crystals indicate that little, if any, of the reaction products are deposited VOL. 50, NO. 10
OCTOBER 1958
1537
-
servations from dolomitic quicklime-acid systems indicate an improvement in seeding properties of return sludge. Figure 3 shows an increase in size of the seeded calcium sulfate sludge particle after 12 recycles from the once recycled seeded sludge particles (Figure 1). This presumably is caused by continued deposition of reaction product. r h a t some of the reaction product does not deposit on the seed particle may account for a degree of concentration by return sludge, which is between that of native and precipitated gypsum.
material is added to the acid prior to lime neutralization, subsequent deposition takes place. Likewise Tables I and I1 show these seeded particles have better settling and compaction characteristics than unseeded sludges. Production of a sludge with a larger mean particle size is not, however, the only factor concerned with sedimentation and compaction as previously suggested ( 3 ) . The predominantly acicular habit of the calcium sulfate crystal also appears to be a factor. Random sedimentation of these particles produces thin sludge, whereas a spherical particle affords a dense sludge (Table I). Thus, native gypsum modifies the habit of the reaction -it deposits on the seed material to form a spherical rather than a needlelike shape, Continuous recycling of this
Discussion This study ends speculation (2, 3) about whether or not reaction products are deposited on seed crystals. Figures 1 and 3 show clearly that when seed
Table I.
Effect of Gypsum Seed Dosages (Acid sample, 500 ml.)
Sludge Vol.
Gypsum, Type
Acid, %
Lime
Native
HCQ
Seed Dosage %/I.
Temp., 25’ C. Control
1%
10-000
HCQ
%”
Control
2%
20-000 DQ
1%
Control
DQ
2%
Control
4-000 10-000
...
pH
(1 hr.),
(6 Min.)
10.4 11.1 12.1
78
10.1
76
5.3 4.0 3.9 3.8
... 89 ...
Ppt.
25.3 50.5 76.0 126.0 189.0 253.0 378.0 631.0
32.0 3.54 11.0 19.5 550 50.0 5.3 20.0 22.4 420 16.0 2.8 5.0 19.7 700 36.4 3.6 10.0 21.6 600 5.23 G. CaO.MgO/L. Acid 20.0 3.96 16.0 6.05 153 12.0 8.42 213 12.0 8.78 222 17.0 7.52 190 18.0 8.03 203 7.98 202 22.0 27.0 7.96 201 32.0 9.60 242
4.5 4.5 4.7 5.0 4.9 6.1 5.0 4.3
...
Temp., 29’ C. ; 13.2 G. Sludge/L. Acid ; Seed Age 2 Hr. ; 5.5 g. CaO/L. Acid 9.9 25.0 5.29 HCQ 1% Control 4-000 8-000 12-000 20-000 30-000 40-000 60-000
80-000
...
%c
... ... ...
Temp., 26‘ C. ; 7.92 G. Sludge/L. Acid; Seed Age, 3 Hr. ; 1% Control 4.6 Ppt. DQ 2-000 4-000 6-000 10-000 15-000 20-000 30-000 50-000
9%
%b
...
88
...
Dry solids (1 hr,), Concn.,
30.3
10.1
61.0
10.0
91.0 152.0 227.0 303.0 455.0 606.0
10.0 10.2 9.8 9.9 9.8 10.1
26.0 28.0 30.0 37.0 44.0 40.0 46.0 66.0
6.78 8.29 7.73
7.70 7.91 8.23 8.93 10.4
...
128 138 146 146 150 156 169 197
HCQ = high calcium quicklime; DQ = dolomitic quicklime. a % of dry sludge formed from ’ increase in sludge dry solids concentration. control neutralization. % of acid volume. %
Figure 3. Seeded calcium sulfate sludge after 12 recycles. Particle size is substantially increased
seeded sludge causes particle size to build up, but this advantage is offset by a decrease in amount of reaction product deposition (Figure 3). The proper choice of seed is usually governed by the system involved and the substance being crystallized. Seed materials, containing nuclei of sufficient size, are added to supersaturated systems to induce crystallization; often a rough surface suffices. When crystal growth is desired, however, a seed of the same substance with proper habit and orientation is selected. Thus, native gypsum is more effective, probably because a more perfect orientation is acquired through an infinite aging period. If true, this would account for the essentially ineffective seeding properties of precipitated gypsum where age has not perfected its crystalline structure. Conclusions
Deposition of calcium sulfate on the native gypsum seed improves sludge settling and compaction characteristics. More calcium sulfate is deposited on native gypsum than on precipitated gypsum seed particles and continuous return of sludges initially seeded with the native form is more effective than with the precipitated form. Calcium sulfate crystal habit and particle size are factors governing settling and compaction characteristics. Literature Cited
Table il.
%
%*
Effect of Recycling Native Gypsum Seeded Sludges (Acid sample, 500 ml.; temp., 25’ C.) Initial Sludge Sludge Loading Seed ReVol. Dry solids Waste Dry Dosage cycles, (lhr.), (1 hr.), vol., Concn., Mg./l. %” Mg./L.b N o . %” % %’ %d
1
C 25
35,600
256
18,400
254
...
...
Acid, Vol.
Lime HCQ DQ
1
DQ
2
c 15 c
30
C = control.
a
...
...
... ...
... 12.. ... 15.. 8-000
10-000
...
..
30.0 32.2 30.0
21.1 30.0 43.0
4.6 13.9 2.42 11.8
30.0 7.1 30.0 6.3 30.0 12.8
...
300
...
490
...
4.5 41,500 310 15-000 10 13.8 310 % of dry solids formed from control neutralization. Added native gyp-
% increase in sludge dry solids concentration.
sum.
% of acid volume.
1538
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Andrews, H., “The Production, Properties, and Uses of Calcium Sulfate Plasters,” Building Research Congress, 1951. (2) Faust, S. D., Orford, H. E., Ind. Wastes 2. NO. 2. 36 11957). (3) Faust, S . D., Orford, H.’E., Parsons, W. A,, Sewage and Znd. Wustes. 28, No. 7, 872 (1956). (4) Palacias, J., Cabrera, J., Andes SOC. esbali. J2s. y quim. 27, 535 (1929). (5) Palacios. J.. Galloni, E. E.. Zbid.,32, 779 (1934). ’ \
,
RECEIVED for review October 3, 1957 ACCEPTEDJune 23, 1958 Work supported by the National Lime Association, Washington, D. C.
