Boric acid having improved handling properties

The lubricity of particulate boric acid is reduced by treating the surface of the particles with gaseous ammonia. Fine particles of boric acid treated with a small amount of gaseous ammonia are readily fed to a continuous compactor system.

This invention relates to particulate boric acid having improved physical 
properties, and especially to finely divided boric acid having improved 
handling properties. 
BACKGROUND OF THE INVENTION 
The lubricity of particulate boric acid is well-known and, in fact, this 
property was used to advantage several years ago by sprinkling large 
crystals of boric acid on dance floors. However, this lubricity presents 
problems in handling particulate boric acid, especially fine particles of 
boric acid such as those of less than about 200 mesh (75 microns) in size. 
In the manufacture of boric acid, crystals of such fine particle size are 
generally separated from the product. These fines are unsuitable for most 
customers due to their dustiness and difficult handling properties such as 
in conveyance of products as in bins or on moving belts. Therefore, the 
manufacturer would like to compact these fine particles so as to convert 
them into a product more acceptable to the market place. However, the 
difficult handling characteristics of the fine material, due to its 
lubricity as well as low bulk density, make it extremely difficult to feed 
into a continuous compactor system. 
SUMMARY OF INVENTION 
This invention provides a method for substantially reducing the lubricity 
of particulate boric acid and provides a novel product having improved 
handling properties. According to this invention, particulate boric acid 
is treated with a small amount of gaseous ammonia, thereby substantially 
reducing the lubricity so that it may be readily handled with industrial 
equipment such as conveyor belts, feed bins and compactor systems. It is 
postulated that the ammonia molecule is chemically bonded by an acid-base 
reaction to the surface atoms of the boric acid particle, thereby reducing 
the lubricity between the boric acid crystals, although the invention is 
not limited to any particular mechanism or explanation. 
DESCRIPTION OF THE INVENTION 
The treatment with gaseous ammonia, such as anhydrous ammonia, preferably 
takes place at about ambient temperatures, although temperatures of from 
about 0.degree. to about 50.degree. C. may be employed. The treatment 
consists of merely exposing the surface of the boric acid particles to an 
atmosphere containing gaseous ammonia, such as an air stream, into which 
ammonia gas is injected, or an atmosphere of substantially pure gaseous 
ammonia. 
The amount of ammonia fed to the air stream may be adjusted to provide a 
product having the desired ammonia content, which suitably is in the range 
of from about 5 to 5000 parts per million ammonia on the boric acid 
product. Alternatively, an atmosphere of substantially pure gaseous 
ammonia may be provided, such as by injection into a screw conveyor, 
pugmill or other equipment in which the boric acid is conveyed, thereby 
providing good mixing and contact of the boric acid with the ammonia. The 
boric acid reacts rapidly with the ammonia and the amount of ammonia fed 
to the system may be readily adjusted so that there is no odor of free 
ammonia. 
A boric acid particle having an ammonia content of from about 25 to 150 
parts per million is especially preferred. Such a product has a 
substantially reduced lubricity, thus providing boric acid with improved 
handling characteristics. For example, such a product can be readily drawn 
into the bite of a continuous compactor system, and therefore produce a 
compactor cake with good physical integrity which can be comminuted to 
provide a boric acid product with low dusting tendencies and having a 
particle size distribution which is acceptable to the market place. Such 
low levels of ammonia in the product have little or no effect on the 
behavior of the boric acid in most end uses. For example, if the boric 
acid is used in glass manufacture, the small amount of ammonia on the 
particle will be quantitatively converted to gaseous nitrogen and water in 
the glass furnace. 
The following examples are presented to illustrate the method and novel 
product of this invention.

EXAMPLE 1 
In a laboratory study, a sample of boric acid dust was divided into two 
portions of about one kg. each. One sample was placed in a laboratory 
fluidized bed and dry air was passed through the dust for 10 minutes at a 
rate to just fluidize the solid boric acid. The second sample was treated 
in an identical manner except that ammonia gas was injected into the feed 
air stream at a rate of about one percent of the total air stream. After 
this treatment the two samples of boric acid were compared. In a large 
beaker, the sample treated with ammonia showed remarkably different flow 
characteristics to the sample without ammonia treatment. When the flat 
part of a spatula was moved through the untreated sample, the boric acid 
dust flowed around the spatula in a manner resembling a fluid, whereas the 
treated sample did not flow readily, but moved ahead of the spatula and 
tended to clump when compressed between the spatula and the beaker wall. 
EXAMPLE 2 
Boric acid fines, collected in a dust collector and containing 60 to 70 
percent -200 mesh (75 microns) solid boric acid, was fed to the force 
feeder cone of a Komareck-Greaves compactor (10 inches diameter, 5.5 
inches wide, corrugated rolls) under hydraulic pressure of 1,500 lbs. The 
force feeder was ineffective in this system and the boric acid could not 
be put through the compactor. Ammonia gas was then injected at the base of 
the force feeder cone at a rate sufficient to provide 150-170 ppm. 
NH.sub.3 to the boric acid. The physical properties of the boric acid in 
the force feeder changed dramatically and the boric acid began to move 
through the compactor, yielding a product of good physical integrity. 
EXAMPLE 3 
Boric acid fines were fed continuously to the force feeder of a 
Komareck-Greaves compactor and ammonia gas was injected at the force 
feeder. The boric acid passed through the compactor giving a cake with 
good physical integrity. This compacted cake was analyzed by ion 
chromotography and found to contain 61 ppm. ammonia. 
EXAMPLE 4 
Dust collector fines from boric acid manufacture were compacted 
continuously in a Komareck-Greaves compactor with ammonia added at the 
force feeder. The recycle of fines passing the compactor and passing 
through a 12 mesh screen was approximately 30 percent of the compactor 
discharge. The compacted cake contained 79 ppm. ammonia, analyzed by ion 
chromotography. 
EXAMPLE 5 
In a large scale study, using a smooth roll compactor without a force 
feeder, boric acid dust was fed through a pugmill, dropping the dust 
through a conical hopper, which in turn fed the compactor rolls. The rolls 
were 18 inches in diameter by 18 inches wide. Initially the boric acid 
dust would not move through the pugmill satisfactorily, and it would not 
pass the compactor rolls at all. Anhydrous ammonia gas was then fed into 
the pugmill, resulting in immediate response and good discharge from the 
pugmill. At first the product passed the compactor rolls only with 
difficulty. The ammonia addition was increased and the boric acid dust 
began to move through the compactor at a very satisfactory rate. Ammonia 
concentrations were measured in the product cake, as the ammonia 
concentration was increased. These NH.sub.3 concentrations were found to 
be 0.11, 0.11, 0.16, 0.17, 0.22, 0.23, 0.28, and 0.54 percent, 
respectively. In each case the product was well compacted with good 
physical integrity. 
Various changes and modifications of the invention can be made, and to the 
extent that such variations incorporate the spirit of this invention, they 
are intended to be included within the scope of the appended claims.