Ind. Health, 1964, 2, 181.

DETERMINATION OF FREE SILICA BY

PHOSPHORIC ACID METHOD (I)

Akira HAMADA

National Institute of Industrial Health, Kizuki-Sumiyoshi, Kawasaki

(Received May 11, 1964)

The phosphoric acid method for the determination of free silica mixed in silicates

was improved in its reproducibility by applying the copper mantle as a heat radiator.

The superiority of this method to the electric heater method was proved by the observa-

tion of the dissolution velocities of several kinds of quartz samples using the two

methods.

The dissolution velocities were studied on quartz powders which were produced by

three different methods by observing their dissolution in hot phosphoric acid using

copper mantle and it was cleared that the surface of quartz powder was denaturalized

by the grinding of quartz and this part was dissolved in phosphoric acid with far higher

velocity than the original quartz. The appearances of these quartz were observed

before and after the treatment with phosphoric acid under the optical microscope, and

the denaturalized surface was demonstrated clearly by the phase contrast microscope

with polarizer.

The obtained value of non-soluble part in phosphoric acid had been taken as free

silica but the observations in this experiment showed that the part of quartz denatura-

lized during the grinding was soluble and this part was accounted as non-free silica.

The phosphoric acid method 1,2,3) is a simple and useful method for the deter-

mination of free silica mixed in silicates. The advantage of this method is that

it can be practised without any special apparatus and techniques. The principle

of this method is to obtain the difference of the weight before and after the treat-

ment with hot phosphoric acid, because the solubility of free silica differs from

that of silicates in hot phosphoric acid.

This method, however, contains some factors which are liable to affect the

reproducibility ; that is, the particle size of the samples, the method of pretreat-

ment of the samples, the method of removing the gel precipitate in the acid which

disturbs the following processes, the problem of the silicates which dissolve in

phosphoric acid at different velocities and especially the problem of the steady heating technique.

Talvitie 1) developed the precision heater to improve reproducibility. Schmidt 2)

used the heater using copper mantle which is easier to be constructed than the

heater which Talvitie used.

181

A. HAMADA

The loss of quartz by dissolution in hot phosphoric acid is generally a few

per cent of the whole quartz in the case of large particles but it reaches as far

as more than ten per cent in the case of fine powder.1,4, 5) The estimation of the

amount of loss is difficult in the latter case.

In the present paper, the effect of the heating method on the dissolution

velocity was studied using the electric heater or the copper mantle with electric

heater. The effect of the preparing methods of quartz powder on the reproduci-

bility of the determination of free silica was also observed. Besides, the solubilities

of some kinds of quartz powders in phosphoric acid were measured. The appear-

ances of quartz powders before and after the treatment by phosphoric acid were

studied under the obtical microscopy 6, 7).

EXPERIMENTALS AND RESULTS

Materials and Apparatus

Electric heater : 600W electric heater for the home use was covered with plates

and fibers of asbestos. The temperature was regulated with the slide regulator

and was indicated by the thermometer.

Copper mantle : Copper column of 100 mm in diameter and 120mm in height hav-

ing a heating hole of 40 mm in both of diameter and depth, was placed on the

electric heater. The temperature was regulated in the same manner as the

electric heater.

Pulverizing tools : For the mechanical grinding, agate pestle was driven mechani-

cally at the rotation speed of 120 rpm in the agate mortar of 150 mm in diameter.

For handgrinding, agate mortar of 90 mm in diameter and agate pestle were used.

For crushing, lumps of quartz were treated at the speed of 75 blows per minute

in a stamp mill with iron mortar of 110 mm in diameter and 5.8 kg pestle.

Sieve : Brass sieves of 40 and 200 mesh were used.

Microscopy : Negative-medium-contrast phase microscope was used with the light

source of 6 volts and 5 amperes.

Light polarizer : Polarizing plate of 30 mm in diameter was used in front of the

mirror.

Refractometer : Abbe's refractometer was used to measure the refractive index of

liquids in the immersion method. The standard liquid was the mixture of the

ƒ¿-mono-chloronaphthalene and terpene oil, this immersion liquid had the refractive

index of 1.548, which was the same with the mean value of ƒÃ and ƒÖ of quartz.

Platinum crucible : 10 cc in the capacity was used for the hydrofluoric acid

treatment.

Quartz : Massive blocks from Ishikawayama were used.

Reagents : Phosphoric acid, hydrofluoric acid, hydrocloric acid, boric aicd and

sulphuric acid of guaranteed grade were used. Filter paper with no ashes was used

182

DETERMINATION OF FREE SILICA

without any further treatment.

Method and Results

The following three experiments were carried out ; 1) the comparison of the

two heating methods, 2) the measurment of the dissolution velocities of the quartz

particles produced by the three different methods and 3) recovering efficiency of quartz in the known mixed samples of quartz and feldspar by phosphoric acidmethod.

1. Comparison of the electric heater method and the copper mantle heat radiator

method as a heating apparatus.

To compare the reproducibility of these two methods, quartz in phosphoric

acid was heated by the electric heater and by the copper mantle heat radiator

(shown in Fig. 1) placed on the electric heater, and the dissolution velocity of

quartz by each method was determined. Experimental procedure was as follows.Crushed quartz powder (100mg ; smaller than 200 mesh)

•© Add phosphoric acid (6cc)

Heating by electric heater (200•Ž, 290•Ž, 300•Ž and 410•Ž ; f 10•Ž)

by copper mantle on electric heater (280•Ž ; •}2•Ž)

Washing of the residue

Weighing of the dried residue (P mg)

Fig. 1. Copper mantle.

183

A. HAMADA

Then, the dissolved amount of the sample was expressed as percentage by the

following equation,

W(%)= (100-P)

The results are shown in Fig. 2, which indicates that the dissolution of quartz

progressed linearly with heating time in the copper mantle heat radiator but it

did not linearly in the method of electric heater only.

Beside the inhomogeneity of heating of quartz, the different states of the quartz

surface may be another cause of disturbing the precise analysis of free silica, as

quartz particles show the various surface states according to the producing methods

of the particles. Dissolution velocities of the quartz particles which were from

the same source but pulverized by three different methods were determined by the

phosphoric acid method using the copper mantle.

2. Dissolution velocities of quartz particles prepared by three different methods.

The methods of particle preparation were as follows.

(1) Crushing : quartz blocks of 2 cm size were crushed by a stamp mill, and

the particles of 10-50/ƒÊ size produced in this treatment were used as

experimental specimens.

(2 Braying by hand : quartz particles smaller than 40 mesh were brayed in

a small agate mortar by hand, and two sorts of particles smaller than

200 mesh and 2ƒÊ respectively were used as experimental samples.

(3) Grinding quartz particles smaller than 40 mesh were ground in an agatemortar by the grinding machine and the 2 samples of powder under

200 mesh and 2ƒÊ were prepared.

The whole method was the same as described in the former experiment. The

Fig. 2. Amount of dissolved quartz by heating with two different methods.

184

DETERMINATION OF FREE SILICA

results are shown in Fig. 3. The dissolved amounts of quartz powders smaller

than 200 mesh were as follows : 7.6-8.0% in the case of crushed powder, 17.85-31.65

% in the ground powder. Generally, quartz dissolved rapidly until about ten minutes, and then slowly in all methods. The amounts of dissolved quartz were

Fig. 3. Weight loss of various quartz samples by heating.

Fig. 4. Micrograms of quartz particles in the 1.548 liquid as observed with

a 20 •~ (0.40 N.A.) negative-medium-contrast phase objective and a 15 •~

eyepiece.

185

Left : Before treatment with phosphoric acid.

Right : After treatment with phosphoric acid.

A. HAMADA

differerent according the preparing method of particles and the mechanicaly ground

quartz showed a highest value of the dissolved quartz, and the crushed the lowest.

As this difference was assumed to be produced by the surf ace characters of quartz

particles prepared by the three methods, the particles were examined optically.

The surface of the ground particles were optically denaturalized and the refractive

index of that part dropped to about 1.544 or smaller. Their appearances studied

by the negative-medium-contrast phase microscope with polarizer and immersion

liquid of suitable refractive index are shown in Fig. 4.

3. Test of the method using the known samples of quartz.

The denaturalized surface of quartz by grinding is easily dissolved in phos-

phoric acid contrary to the original quartz, and this part of quartz is determined as

non-free silica by the phosphoric acid method. Therefore, if we neglect this

denaturalized surface, amounts of quartz determined by the phosphoric acid method

would not coinside with the previously weighed amount of quartz in the artificial

samples of mixed minerals. To investigate this problem the artificial samples of

the following compositions were analysed by the phosphoric acid method.

The mixtures of crushed quartz and crushed feldspar (both were

smaller than 200 mesh), containing quartz of 65, 25, 15, 8, 6, 4 and 2%

of the whole weight.

The quartz in this sample contained non-soluble part of 91.73 % was determined

in the previous experiment. Feldspar was already proved to be dissolved comple-

tely in phosphoric acid.

6m1 of orthophosnhoric acid in a 20-ml beaker

Heating for 6 Minutes by copper mantle (280•Ž; •}2•Ž)

•© Add sample powder (100mg; smaller than 200 mesh)

Heating for 6 minutes by copper mantle (280•Ž ; •}2•Ž),

swirling the beaker for 3 seconds at 1 minute intervals

•© Dissolve the sirapy phosphoric acid, and add 2ml

of fluoboric acid

Standing the beaker for 1 hour

Washing of the residue

Weighing of the dried residue (P1 mg)

•© Add 1 drop of sulfulic acid (1 : 1)

and 2m1 or more of hydrofluoric acid

Volatilizing the acids

Weighing of the residue (P2 mg)

Free silica =P1 - P2 (%)

Fig. 5. Analytical processes.

186

DETERMINATION OF FREE SILICA

These samples were analysed by the processes shown in Fig. 5. The results

obtained coincided with the estimated values as shown in Table 1. The differences

between the results and the estimations were in the range of 0.36-0.66 %.

DISCUSSIONS

Change of the Dissolution Velocity with Time

Fig. 3 shows that the dissolution velocity of the quartz powder is large at the

initial period of dissolution and after a certain period it dropps to a very small

value. The amount dissolved during initial period is about 7-40% of the samples.

And these values are affected by the preparing conditions of the powder. Generally,

the dissolved amount of quartz increases with the grinding time and also increases

inversely with the particle size.

Denaturalization of the Surface of Quartz

Denaturalization of quartz surface was observed under the negative-medium-

contrast phase microscope in the immersion method. If refractive index of the

immersion liquid is 1.548, the original quartz should show pink or pale yellow.

The quartz powder really showed these characteristic colours after the treatment

with phosphoric acid, but blue was observed at the outer layer of the ground

particles. The refractive index of this blue part was about 1.544 or smaller These

denatura-lized parts increased with the grinding time of the particles. When

the outer layer was completely changed by long time grinding, the original colours

of quartz could not be observed. These parts were dissolved by phosphoric

acid with high velocity and counted as the non-free silica contrary to its original

part in the determination of the free silica by phosphoric acid method.

Table 1. Results of the test analyses.

187

A. HAMADA

ACKNOWLEDGEMENT

The author is gratefully obliged to Dr. Hiroyuki Sakabe for his encouragement

to start this work.

REFERENCES

1) Talvitie, N. A. (1951). Anal. Chem., 23, 623.

2) Schmidt, K. G. (1954). Ber. Deut. Keram.Ges., 31, 402.

3) Hamada, A. (1952). Ann. Rept. Silicosis Res. Lab., 34. (in Japanese)

4) Jephcott, C. M. & Wall, H. F. V., (1955). A. M. A. Arch. Ind. Health, 11, 425.

5) Schmidt, K. G. (1960). Staub, 20, 404.

6) Schmidt K. G. (1955). Staub, Heft 41, 436.

7) Hamada, A. (1955). Ann. Rept. Silicosis Res. Lab., 24. (in Japanese)

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