Method for increasing the brightness of limestone

Method for improving the brightness of limestone utilizing a flocculating agent and a bleaching agent where the pH of the slurry is between about 7 and 9.

Most limestone deposits contain small amounts of iron oxides or other forms 
of iron. The iron imparts a yellow to pink or red tint to the ground 
product rendering it unacceptable for the most lucrative filler and 
pigment applications which require a bright white color with little or no 
hue or tint. Increased brightness has been obtained by physically removing 
iron bearing segments of the limestone by washing or comminution followed 
by classification, magnetic separation, flotation and chemically bleaching 
or leaching with solvents for iron oxides followed by filtering. 
Dithionite type bleaching agents perform most efficiently under acidic 
conditions of 2 to 5 pH, which cannot be obtained due to buffering by 
calcium carbonate in the limestone as it reacts with the acid. 
Calcium oxide has been tested since it is well known that any calcium oxide 
generated in drying limestone causes flocculation of the limestone 
particles. However, calcium oxide added stepwise up to about 3 lbs per ton 
of limestone failed to produce a satisfactory separation by centrifuging 
or filtration. The best results achieved at a pH of 11.0 yielded a cake of 
about 68% solids with a supernate containing about 16% solids, which is 
equivalent to a 23% product loss to the supernate. 
Attempts were made to treat the limestone with hydrochloric acid. However, 
the results were negative because the hydrochloric acid was immediately 
neutralized by the calcium carbonate. 
Accordingly, it is an object of the present invention to increase the 
brightness of limestone. 
It is another object of the invention to remove iron from the limestone. 
It is still another object of the invention to floccuate dispersed 
limestone particles sufficiently so that water can be removed by 
centrifuging or filtering. 
In accordance with the present invention, there is provided a method for 
improving the brightness of limestone. The limestone is first ground to 
the desired size and then is formed into a slurry, preferably with water. 
An acidic flocculating agent, in amounts sufficient to provide a pH 
between about 7 and 9 is added. A bleaching agent, in at least an amount 
sufficient to react with any iron present in the limestone, is added. 
After an adequate reaction time, the flocculate is filtered. 
If the pH of the limestone is low, i.e. 9.3 to 9.6 after grinding to size, 
then sufficient acid to flocculate for good filtration will lower the pH 
to about 7. On the other hand, if the pH is high after grinding, i.e. 9.9 
to 10.2, a higher addition of acid is required and good flocculation can 
occur at a pH as high as 8.7 to 9. However, most effective bleaching 
appears to occur at a pH of 8 or less. Accordingly, it is preferred that 
pH be maintained between about 7 and 8. 
The solids content of the slurry may be between 15 and 50%. The preferred 
solids for bleaching and filtering is between about 35 and 40% solids. 
This is a compromise of low solids 15 to 20%, which would maximize the 
removal of soluble contaminates with the high solids range of 45 to 50% 
which would increase the filter rate but not remove enough of the 
contaminates. 
The preferred flocculating material is acetic acid of 10% solution strength 
and in amounts between about 1 and 10 lbs/ton of limestone. However, other 
flocculating agents such as aluminum chloride, aluminum sulphate, citric 
acid, oxalic acid, etc., may be employed. Typically, the amount need only 
be between about 1 and 5 lbs per ton. 
As for the bleaching agents, the hydrosulphites, particularly sodium and 
zinc are preferred. Other hydrosulfites, K-Brite-3E and K-Brite-3K which 
are proprietary products of the Virginia Chemical Company, perform 
satisfactorily. In addition, other bleaching agents such as sulphur 
dioxide and zinc dust can also be used with varying results. The amount of 
bleaching agent may range between about 3 and 15 lbs per ton of limestone. 
The preferred amount is from about 6 to 10 lbs per ton of limestone, since 
little or no improvement will be obtained above this level. Also, higher 
amounts will add to the cost of the product. 
It is further preferred that the slurry be maintained at a temperature of 
between about 70.degree. to 120.degree. F. The 120.degree. F. temperature 
is the maximum because that level would be operable from the heat 
generated during attrition milling. Higher temperatures would add costs 
and complexities to the system. Elevated temperatures only increase the 
reaction rate which usually is optimum at 100.degree. to 120.degree. F. 
Variations in the process which may be optimized, would include residence 
time between the flocculant addition and the bleach addition and time for 
the bleach to complete reaction correlated with the temperature of the 
slurry.

The following examples are illustrative of the teachings of the invention. 
EXAMPLE I 
A sample of Central Texas limestone was subjected to crushing and impact 
milling such that approximately 86% was -100 mesh and 59% was -325 mesh. A 
200-pound sample of this ground limestone was slurried to a solid content 
of about 25%. 
The slurry was allowed to settle sufficiently to allow particles coarser 
than 3 microns to settle out and the fine silt was removed. The desliming 
process was repeated, removing a total of 14.5% by weight, of the 
limestone as a brown slime. The deslimed/settled fraction was again 
slurried and dispersed with 0.3% polyacrylate and fed to a sand mill which 
contained glass beads of -18 mesh plus 30 mesh size. The mill when filled 
with limestone slurry contained the following composition: beads 3.5 
parts, limestone 1 part and water 2.5 parts, by weight. The same mill used 
for this experiment was a conventional ore attrition scrubber as used for 
cleaning mineral surfaces before flotation. 
The machine was fitted with a 60-mesh screen across the discharge aperture 
to prevent discharge of the glass beads. After a sufficient residence time 
to produce the desired particle size distribution of the limestone, the 
feed slurry was metered to the scrubber at a rate which produced the 
desired size distribution on a continous basis. All of the material was 
sized less than 20 microns. 
A sample of the slurry was flocculated for filtration by the addition of 3 
lbs of acetic acid per ton of limestone which reduced the pH of the slurry 
from 10.1 to 8.0. The filter cake was dried, pulverized, and measured for 
brightness on a Photovolt reflectance meter equipped with blue, green and 
amber filters. 
A second sample was treated in a like manner except that 6 lbs of sodium 
hydrosulfite per ton of limestone was mixed into the slurry after addition 
of the acetic acid and allowed to stand for 60 minutes before filtering. 
A third sample was treated in the same manner as the second sample, except 
that 4 lbs of aluminum chloride per ton of limestone was used to reduce 
the pH of the slurry from 10.1 to 8.0 before addition of 6 lbs per ton of 
sodium hydrosulfite. The results of these bleaching tests are shown in 
Table I below. 
TABLE I 
______________________________________ 
Photovolt Brightness 
Lbs/ Slurry Bleach Blue Green Amber 
Flocculant 
Ton pH Lbs/Ton 
Filter 
Filter 
Filter 
______________________________________ 
Acetic Acid 
3.0 8.0 0 90.0 93.3 94.3 
Acetic Acid 
3.0 8.0 6 93.5 94.0 95.2 
Aluminum 
Chloride 4.0 8.0 6 93.8 94.8 95.8 
______________________________________ 
EXAMPLE II 
A Texas limestone, which was beneficiated by dry crushing, dry grinding and 
dry classification, yielded a coarse pigment of marginal quality having a 
particle sizing of 20% coarser than 20 microns and the balance less than 
20 microns with a brightness of 87 with the blue filter. 
After grinding this limestone in a like manner and to the same fineness as 
that in Example I, the slurry contained 40.6% solids, had a pH of 10.1 and 
a temperature of 120.degree. F. 
The slurry was divided into three representative samples for various 
treatments. The first treatment was simply dewater and dry. The second 
treatment was to flocculate with 3.3 pounds per ton acetic acid, bleach 
with 6 lbs per ton sodium hydrosulfite for 1 hour, dewater and dry. The 
third treatment was to flocculate with 4.6 pounds per ton aluminum 
chloride, bleach with 6 pounds per ton sodium hydrosulfite for 1 hour, 
dewater and dry. The first three treatments were carried out at a 
temperature of 120.degree. F. The fourth treatment was to flocculate with 
3.3 lbs per ton acetic acid, bleach, dewater and dry as in the third 
treatment, except that a temperature of 72.degree. F. was maintained. The 
results of these tests are shown in Table II below. 
TABLE II 
______________________________________ 
Photovolt Brightness 
Lbs/ Slurry Bleach Blue Green Amber 
Flocculant 
Ton pH Lbs/Ton 
Filter 
Filter 
Filter 
______________________________________ 
None (120.degree. F.) 
-- 10.1 0 92.5 95.0 96.5 
Acetic Acid 
(120.degree. F.) 
3.3 8.1 6 97.2 97.8 98.2 
Aluminum 
Chloride 
(120.degree. F.) 
4.6 8.0 6 97.8 97.5 98.2 
Acetic Acid 
(72.degree. F.) 
3.3 8.0 6 95.5 95.0 95.8 
______________________________________ 
EXAMPLE III 
The limestone feed for Example III was ground as in Example II, except that 
the feed rate was increased to yield a coarser product. After grinding, 
this material had a pH of 9.7. Two pounds per ton of acetic acid was 
required to produce a sufficient degree of flocculation to filter 
satisfactorily which brought the pH down to 7.4. Samples of this lot were 
heated at temperatures from 70.degree. F. to 105.degree. F. To three of 
the four samples, after the acetic was added, the sodium hydrosulfite was 
added and the samples were aged 15 minutes and filtered. The tests results 
are shown in Table III below. 
TABLE III 
__________________________________________________________________________ 
Bleach Photovolt Brightness 
Slurry 
Bleach 
Temp. 
Blue 
Green 
Amber 
Flocculant 
Lbs/Ton 
pH Lbs/Ton 
.degree.F. 
Filter 
Filter 
Filter 
__________________________________________________________________________ 
Acetic Acid 
2.0 7.4 0 70 93.2 
94.0 
95.0 
Acetic Acid 
2.0 7.4 10 70 94.5 
95.0 
95.0 
Acetic Acid 
2.0 7.4 10 92 96.7 
96.5 
97.1 
Acetic Acid 
2.0 7.4 10 105 97.0 
96.5 
97.0 
__________________________________________________________________________ 
EXAMPLE IV 
The effects of bleaching are shown on a coarser pigment grade, a size 
preferred by the paint industry, produced by dry grinding (Table IV) and 
wet grinding (Table V). The size distribution of the ground limestone is 
set forth below. 
______________________________________ 
% Wt. Finer 
Size Microns 
Dry Ground Wet Ground 
______________________________________ 
20 99.0 99.0 
5 75.0 82.0 
2 34.0 45.0 
1 13.0 20.0 
0.5 3.0 6.0 
______________________________________ 
The dry ground slurry contained 40% solids with 0.2% polyacrylate 
dispersant. It was flocculated with 2 lbs per ton acetic acid and bleached 
with 10 lbs per ton sodium hydrosulfite at 75.degree. F. It was then 
dewatered and dried for brightness measurement and the results are shown 
in Table IV below. 
TABLE IV 
______________________________________ 
Photovolt Brightness 
Blue Green Amber 
Product Bleach Time Filter Filter Filter 
______________________________________ 
1 No bleach 
0 89.0 92.0 93.0 
2 Bleach added 
15 min. 91.0 93.2 94.0 
3 Bleach added 
60 min. 91.2 93.2 94.0 
______________________________________ 
The wet ground slurry contained 40% solids. Two of the samples were heated 
to 105.degree. F. and 97.degree. F., respectively, flocculated with 2 lbs 
per ton of acetic acid and bleached with 10 lbs per ton of sodium 
hydrosulfite. The two samples were aged 15 minutes and all samples were 
dewatered and dried. The results are shown in Table V below. 
TABLE V 
______________________________________ 
Photovolt Brightness 
Blue Green Amber 
Product Temperature Filter Filter Filter 
______________________________________ 
1 No bleach 
105.degree. F. 
91.0 92.5 93.3 
2 Bleach added 
105.degree. F. 
94.1 95.0 95.8 
3 Bleach added 
97.degree. F. 
94.0 95.2 95.8 
______________________________________ 
EXAMPLE V 
Additional acidic flocculants were tested for compatibility with the sodium 
hydrosulfite bleach. The limestone slurry for these series of tests was 
prepared in like manner to that shown in Example I. The ground limestone 
slurry was heated to 105.degree. F., sufficient flocculant was added for 
satisfactory filtration and sufficient bleach was added. The samples were 
aged for 60 minutes, filtered and dried. The results are shown in Table VI 
below. 
TABLE VI 
______________________________________ 
Photovolt Brightness 
Lbs/ Slurry Bleach Blue Green Amber 
Flocculant 
Ton pH Lbs/Ton 
Filter 
Filter 
Filter 
______________________________________ 
None 0 10.1 0 92.5 95.0 96.5 
Acetic Acid 
2 9.4 6 94.3 96.0 97.0 
Acetic Acid 
3 8.5 6 96.7 95.3 97.0 
Aluminum 
Chloride 4 8.4 6 97.2 97.2 97.8 
Aluminum 
Sulfate 3 9.6 6 95.0 95.5 97.0 
Oxalic Acid 
2 8.8 6 95.2 97.0 97.2 
______________________________________ 
EXAMPLE VI 
Here the effect of varying amounts of acetic acid on the brightness of 
bleached limestone was tested. Limestone in this example was prepared in 
the same manner as in Example I. It was heated to 105.degree. F. in slurry 
form, treated with acetic acid, and agitated with 6 lbs per ton of sodium 
hydrosuofite in each case, except for the blank, and aged for 15 minutes 
before filtration. The results are shown in Table VII below. 
TABLE VII 
______________________________________ 
Photovolt Brightness 
Lbs/ Slurry Bleach Blue Green Amber 
Flocculant 
Ton pH Lbs/Ton 
Filter 
Filter 
Filter 
______________________________________ 
Blank 0 10.1 0 92.5 95.0 96.5 
Acetic Acid 
2 9.4 6 94.3 96.0 97.0 
Acetic Acid 
3 8.5 6 96.7 95.3 97.0 
Acetic Acid 
3.3 8.1 6 97.2 97.8 98.2 
______________________________________ 
EXAMPLE VII 
A limestone from a deposit in Jamaica was prepared in the same manner as 
Example II, and tested for brightness improvement by wet grinding and to 
coarse and fine grinds followed by bleaching. The size distribution of the 
grinder feed and ground products is set forth below. 
______________________________________ 
Particle Size 
Distribution 
Percent Finer 
Microns Grinder Feed 
Coarse Grind 
Fine Grind 
______________________________________ 
15 80.5 100 99.5 
10 70.0 98 99.0 
5 48.0 82 97.5 
2 17.0 42 95.5 
1 5.0 18 56.5 
0.5 2.0 6 19.0 
______________________________________ 
Bleaching tests were run on the ground samples by heating to 120.degree. 
F., and adding acetic acid for proper flocculation for filtering. Ten 
pounds per ton of sodium hydrosulfite was added and the slurry was cured 
for 45 minutes before filtering. Brightness test results are shown in 
Table VIII below. 
TABLE VIII 
______________________________________ 
Photovolt Brightness 
Lbs/ Slurry Bleach Blue Green Amber 
Flocculant 
Ton pH Lbs/Ton 
Filter 
Filter 
Filter 
______________________________________ 
Grinder feed 
0 -- 0 85.8 90.5 91.3 
Coarse grind 
0 9.5 0 89.2 92.0 93.2 
Acetic acid 
2 7.6 10 90.8 93.0 94.2 
Fine Grind 
0 10.3 0 92.5 95.0 95.0 
Acetic acid 
4.4 8.0 10 94.0 95.2 96.0 
______________________________________ 
EXAMPLE VIII 
A sample of Central Texas limestone was ground in a like manner and to the 
same size distribution as that described in Example II. The slurry 
contained 40.0% solids and had a pH of 9.8. 
The slurry was divided into 6 representative samples for treatment. The 
first treatment was simply remove water by evaporation, to serve as a 
control sample. The second treatment was to flocculate with 2.2 pounds per 
ton of acetic acid, filter and dry. The final four treatments were similar 
to the second except 4, 6, 10 and 15 pounds per ton of sodium hydrosulfite 
was added 45 minutes before filtration. All samples were held at 
105.degree. F. during the period of treatment. After drying, the 
brightness of each sample was measured with a Photovolt Reflectance Meter 
equipped with a blue filter. The results of these tests are shown in Table 
IX below. 
TABLE 1X 
______________________________________ 
Lbs/ Slurry Bleach Photovolt Brightness 
Flocculant 
Ton pH Lbs/Ton 
Blue Filter 
______________________________________ 
Control 0 0 0 93.5 
Acetic Acid 
2.2 7.9 0 93.8 
Acetic Acid 
2.2 7.9 4 95.5 
Acetic Acid 
2.2 7.9 6 96.3 
Acetic Acid 
2.2 7.9 10 96.3 
Acetic Acid 
2.2 7.9 15 96.3 
______________________________________ 
EXAMPLE IX 
A sample of Central Texas limestone was wet ground in a like manner to that 
shown in Example IV with the same size distribution as the wet ground 
example. The ground slurry sample contained 40% solids and required 2.2 
lbs acetic acid per ton of limestone to flocculate for filtration. The pH 
of slurry before bleach addition was 7.8. 
Five aliquots were removed and treated with 10 lbs per ton additions of 
sodium hydrosulfite, zinc hydrosulfite, K-Brite-3E and K-Brite-3K. The 
slurry temperature was maintained at 105.degree. F. for 60 minutes 
following additions of the bleaching agent, after which the liquid phase 
was removed and the solids dried for brightness measurements. Brightness 
test results are shown in Table X below. 
TABLE X 
______________________________________ 
Photovolt Brightness 
Bleach Blue Filter 
______________________________________ 
Control 93.5 
Sodium hydrosulfite 
95.8 
Zinc hydrosulfite 95.5 
K-Brite-3E 95.8 
K-Brite-3K 95.8 
______________________________________ 
It is intended that the foregoing description be construed as illustrative 
and not in limitation of the invention.