Process for beneficiating Minnesota kaolin

A process for increasing the brightness of Minnesota kaolin clay containing chlorite and siderite mineral impurities comprising treating the clay with a strong mineral acid preferably in combination with magnetic separation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for treating a kaolin clay contaminated 
with iron-containing minerals such as chlorite and siderite to improve its 
brightness and, more particularly, to an improved process of bleaching a 
Minnesota primary kaolin clay with a novel combination of a strong acid 
leach and a magnetic separation. 
2. Description of Related Art 
Extensive deposits of a primary kaolin in the Minnesota River Valley region 
of the United States have been known for many years. These deposits have 
been evaluated on numerous occasions over the past fifty years in attempts 
to produce a kaolin suitable for paper coating and paper filling 
applications. Among the agencies that have been involved in unsuccessful 
attempts to commercialize Minnesota kaolin have been the Legislative 
Commission on Minnesota Resources (LCMR), the Minnesota Department of 
Natural Resources, the Minnesota Geological Survey, Natural Resources 
Research Institute, and the Mineral Resources Research Center. At least 
one of the major kaolin producers in Georgia has worked with these clays 
and holds extensive mineral rights in the area. Notwithstanding these 
efforts, thus far no one has identified a suitable process for producing 
an 80-87 brightness kaolin as measured by the standard G.E. scale (see 
TAPPI Procedure T-646-05-75 and T-649-05-75) on a meaningful portion of 
these reserves. As those skilled in this art appreciate (e.g., see U.S. 
Pat. No. 4,997,550), even brightness improvement of less than a full point 
can be very significant. 
Among the beneficiation methods previously tried on Minnesota kaolin are 
the following: degritting and fractionation followed by steps such as high 
gradient magnetic separation (HGMS), flotation, delamination and leaching 
with sodium dithionite (hydrosulphite), normally under an acid condition. 
In most cases, the degritted brightness of the Minnesota kaolin is in the 
60-70 brightness range and even magnetic separation rarely furnishes more 
than 5 brightness points improvement. Sodium dithionite leaching raises 
the brightness a maximum of 10 points, but the resulting low brightness 
kaolin remains below about 80 brightness nonetheless, and does not 
consistently meet kaolin paper filler standards. 
U.S. Pat. No. 3,193,344 describes a process for bleaching kaolin clay 
contaminated with highly colored iron impurities using the combination of 
a reducing agent and an iron binding agent generally described as a 
water-soluble polyfunctional organic compound, one active group of which 
comprises a mercaptan (--SH) radical and another active group of which 
comprises a radical, which together with the mercaptan group is capable of 
chelating iron. According to the patent, the clay first is treated with 
the reducing agent to solubilize iron as ferrous iron. Preferably, the 
reaction of the clay and the reducing agent is conducted at a slight 
degree of acidity. Water-soluble salts of hydrosulphurous acid are 
preferably employed as the reducing agent. Salts identified as 
particularly satisfactory for this purpose include sodium hydrosulphite, 
zinc hydrosulphite, and calcium hydrosulphite. Further, hydrosulphurous 
compounds, such as taught in U.S. Pat. No. 2,339,594 also are taught; 
e.g., alkali metal and zinc salts of hydrosulphurous acids and the 
sulfoxylates. 
Thereafter, the reduced clay is treated with the iron binding agent. 
Preferably such iron binding agent is a water-soluble acid containing a 
carboxyl and a mercaptan group, preferably with the --SH group in the 
alpha position relative to the carboxyl or group, such as in 
mercaptoacetic acid. Such a mercapto-carboxy acid forms in aqueous 
solution a water-soluble, or at least a water-dispersible, compound or 
complex of sufficiently slight degree of dissociation to prevent reaction 
of iron and oxygen in a reaction mixture and thus so prevents oxidation 
and reabsorption of iron on the clay from which it was previously removed. 
The complexed iron then is readily removed from the clay, as by 
filtration. Other compounds which similarly form water-soluble compounds 
with ferrous iron of a sufficiently low degree of dissociation to prevent 
reoxidation of ferrous iron compounds to ferric iron compounds in presence 
of dissolved oxygen, water, and clay include thiomalic acid and 
mercaptoethanol. For convenience, the clay can be treated with a mixture 
of the reducing agent and the iron binding agent. This admixture of such 
dibasic mercaptan acid and reducing sulfoxylate is effective at the 
relatively acid pH conditions at which such sulfoxylate is most effective 
to reduce iron compounds associated with even relatively acid reacting 
clays without producing an objectionable or strong odor. 
U.S. Pat. Nos. 5,061,461 and 5,085,707 also describe techniques used for 
processing kaolin clay, particularly clay used for paper tiller and 
coatings. In summarizing conventional processing procedures, it is noted 
that following initial particle sizing, the clay is leached to remove 
iron-based color compounds by first acidifying the clay with sulfuric acid 
or alum to a pH of 3 to 5 to solubilize the iron and flocculate the clay, 
which aids subsequent filtration, followed by treatment with a reducing 
bleach agent such as sodium hydrosulphite (dithionite) which generally is 
more effective at an acid pH. This bleaching, typically conducted at a 
kaolin slurry solids content of about 30 to 40%, reduces the iron to the 
more soluble ferrous form. The iron then is removed via a dewatering 
process (filtration). The clay is flocculated and then dewatered to a 
solids level of about 60%. The filter cake then either is dried or 
redispersed with additional clay if it is to be sold as about a 70% solids 
slurry. The '707 patent indicates that to produce high brightness 
products, the clay may be processed through flotation or magnetic 
separation. As noted in the '461 patent this normally takes place before 
dithionite leaching and filtration. See also U.S. Pat. No. 3,853,983 in 
this regard, which describes a process for improving the brightness of an 
iron pyrite-contaminated clay using a combination of magnetic separation 
and dithionite leaching. 
Other patents that relate to the beneficiation and brightening of kaolin 
clays include U.S. Pat. Nos. 3,826,365; 3,853,983; 4,055,485; 4,227,920; 
4,419,228; and 4,781,298.

DESCRIPTION OF THE INVENTION 
It has surprisingly been discovered that Minnesota primary kaolin clay 
containing chlorite and siderite mineral impurities having a substandard 
brightness such as in the 50-70 range can be raised to a commercially 
satisfactory level of brightness in the 80-90 range using a process that 
consists essentially of acid leaching at a low pH with a strong mineral 
acid preferably in conjunction with magnetic separation. Using the process 
of the present invention the brightness of a crude Minnesota primary 
kaolin clay (particle size typically less than 325 mesh) can be increased 
10 to 15 or more G.E. brightness points over conventional processing. 
The clay minerals in the Minnesota River Valley were formed from the 
weathering of felsic rock types such as granite. As is characteristic of 
primary kaolins, the Minnesota kaolin contains substantial quantities of 
quartz sand amounting to 40-60% of the primary crude kaolin. The kaolin 
also is heavily contaminated with chlorite and siderite impurities. 
In accordance with this invention, after conventional preliminary treatment 
and beneficiation, an aqueous slurry of the Minnesota kaolin generally at 
a solids content between about 20 and 60% by weight and more usually 
between 25 and 50%, is treated with a strong mineral acid in an amount 
between about 0.1 to 10.0% by weight, preferably between about 0.5 to 10% 
by weight, and most usually at about at least 1% by weight, so as to 
result in a slurry pH below about 3.0 and generally between about 1 and 3 
in the kaolin slurry. The weight percent acid reagent is based on the 
weight of active acid per unit weight of kaolin solids in the slurry. 
Conventional preliminary treatment generally involves crushing the kaolin 
crude ore, pulping the crushed ore in water, degritting, and 
fractionation, to recover one or more fractions of a desired particle size 
distribution. Flotation treatment of a fractionated portion of the crude 
also may be utilized as an additional step to remove colored impurities. 
Preferably, the kaolin slurry subjected to the acid leaching of the 
present invention has a solids content of 25 to 40%. Suitable acids are 
the strong acids having a pKa of less than 3.0 and include sulfuric acid, 
phosphoric acid, nitric acid and hydrochloric acid. 
In accordance with a preferred embodiment of the invention, either before 
or after the acid leach step, the kaolin slurry is subjected to 
conventional magnetic separation treatment. Preferably the magnetic 
separation is done before the acid leach treatment. The conditions for 
magnetic separation are not narrowly critical, and the typical conditions 
and processes used in the prior art and well known to those skilled in the 
art for beneficiating a kaolin slurry using magnetic separation (HGMS) can 
suitably be employed. For specific guidance on the magnetic separation 
step, please refer to U.S. Pat. Nos. 3,471,011,3,667,689, and 3,974,067 
for further information, the relevant disclosure of which are hereby 
incorporated by reference. The magnetic separation is conducted on a 
kaolin slurry having a solids density of at least 15 wt. %. 
Following acid leaching, the kaolin slurry is dewatered, possibly with 
washing, using conventional kaolin filtration (solid separation) 
procedures, to produce a solids content cake generally at least 10 to 40% 
higher than the starting slurry. Normally, the solids content of the 
dewatered cake is above 60-70%. 
The following examples are given as specific illustrations of the present 
invention, and not for the purpose of limiting the invention. Reference 
should be made to the appended claims to determine the invention's scope. 
EXAMPLE 1 
A sample of crude primary Minnesota kaolin from the Minnesota River Valley 
was screened through a 325 mesh screen and a slurry containing 26% solids 
(-325 mesh) kaolin by weight was mixed with 1% by weight active 
hydrochloric acid from a 37% solution based on the weight of the kaolin, 
stored for 16 hours and then filtered. The initial GE brightness of the 
kaolin was 68.5. After the acid leaching process, the brightness was 
raised to 79.4. 
EXAMPLE 2 
Example I was repeated using 1% by weight hydrochloric acid based on the 
weight of the kaolin and the slurry was rapidly agitated for 20 minutes at 
a temperature of 120.degree. F. After filtering, washing and drying, the 
treated kaolin had a brightness of 81.3, an increase of 12.8 points. 
EXAMPLE 3 
Example 2 was repeated except that 1% by weight (based on the weight of 
kaolin solids) concentrated (85%) phosphoric acid was used in lieu of 
hydrochloric acid. The finished brightness of the acid leached clay was 
84.0, a significant increase of 15.5 points. 
EXAMPLE 4 
The procedure of Example 3 was repeated using a different source of 
Minnesota kaolin which after delamination had an initial brightness of 
70.0. This clay first was treated by two passes through a magnetic 
separator, giving a clay having a brightness of 74.7. After treating the 
magnetically separated clay with hydrochloric acid in an amount of 1% by 
weight active basis as described in Example 3, the clay brightness was 
raised to 82.2. 
EXAMPLE 5 
In this example, the magnetically separated clay of Example 4 was acid 
leached with concentrated phosphoric acid (85%). The clay brightness after 
magnetic separation, as above, was 74.7. After treating the clay with 1 
wt. % phosphoric acid active basis, the brightness of the clay was raised 
to 83.6. 
Other strong mineral acids such as nitric and sulfuric acid produce results 
nearly equivalent to hydrochloric acid. 
Mineral acid leaching on these clays is significantly more effective than 
leaching with sodium dithionite at an acid pH which is the industry 
standard in kaolin production. A comparison of the acid leaching agent of 
the present invention with the prior art sodium dithionite leaching, the 
industry standard, is shown in Example 6. 
EXAMPLE 6 
A crude Minnesota primary kaolin from the Minnesota River Valley was 
dispersed in water to produce a 50% solids slurry. After screening the 
slurry on a 100 mesh screen, the kaolin (-100 mesh) had a brightness of 
60.8. Delamination raised the brightness to 70.9 and after magnetic 
separation this was raised to 72.8. Reduction leaching of this clay using 
0.5% by weight sodium dithionite at a pH of 3.0 to 3.5 raised the 
brightness to 78.0. Further leaching of this 78.0 brightness clay with 
phosphoric acid as described in Example 2 raised the brightness of the 
clay further to 87.7, an increase of nearly 9 points. 
Use of high shear with the phosphoric acid leach produced a brightness of 
87 to 90 starting with chide clay having a brightness in the range of 60 
to 70. 
EXAMPLE 7 
A crude Minnesota kaolin from the Minnesota River Valley was dispersed in 
water to produce 50% solids slurry. After degritting on a 325 mesh (44 
micron) screen the brightness of the less than 325 sieve fraction was 
69.5. The sample was treated with 1% hydrochloric acid in accordance with 
Example 2. The brightness of the acid leached product was 74.0. The 
leached clay was then redispersed in water and passed through the magnetic 
separator operating at 20 kilogauss with a one minute retention time. The 
brightness of the recovered nonmagnetic fraction was 76. 
EXAMPLE 8 
The same crude Minnesota kaolin as used in Example 7 was degritted to 
produce a minus 44 micron fraction having a brightness of 69.5. This 
sample was treated on the magnetic separator operating at 20 kilogauss and 
using one minute retention time. The resulting non-magnetic fraction had a 
brightness of 71.6. This product then was leached with 1% hydrochloric 
acid as described in Example 7 and produced a kaolin clay having a 
brightness of 81.1. 
A comparison of the results in Examples 7 and 8 shows that magnetic 
separation prior to acid leaching gives superior results. Another 
advantage of conducting the acid leaching after magnetic separation is 
that the clay can be directly filtered, redispersed, and dried. It has 
been found that the acid leach treatment eliminates the necessity for a 
dithionite leaching. 
Dithionite leaching of mineral acid-treated Minnesota kaolin resulted in 
negligible brightness increases, thereby rendering the dithionite leaching 
unnecessary. Therefore, the mineral acid leaching of these clays is a more 
effective substitute for dithionite leaching. The mineral acid can be 
added before magnetic separation and after the fractionation where it 
becomes the flocculating agent for the clay prior to filtration. 
By means of the process of this invention, it now is possible to convert a 
significant portion of the up to present nearly useless Minnesota kaolins 
into excellent paper filling clays and high quality paper coating clays. 
The application of acid leaching to these clays produces a totally 
unexpected beneficial result which far exceeds the results of applying 
these acids to normal coating clays from the Georgia area. 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since they are to 
be regarded as illustrative rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.