530 



UNITED STATES MINERAL RESOURCES 



terial in a mortar and mix it with 10-20 times its 

 weight of powdered ZnO and then to compare with 

 commercial pigments treated similarly to disclose 

 the tinting strength. Dilution with white ZnO is 

 called "letting down" the pigment and is done to 

 obtain desired shades in finished pigments. Another 

 test is to determine the effect of particle size on the 

 color. Each pigment has an optimum particle size, 

 close to 1 micron, below which light scattering 

 causes some undertones to be lost. On the other hand 

 particle size above 2 microns causes a rough paint 

 film. Tests for other properties, such as oil absorp- 

 tion, hiding power, and mass color are also impor- 

 tant and are described by Siegel (1960). 



RESOURCES OF IRON OXIDES FOR PIGMENTS 



Because synthetic and byproduct iron oxides are 

 rapidly replacing the use of natural oxides, specu- 

 lation here about resources of the natural oxides 

 that are discussed in the iron chapter of this volume 

 is probably not significant. However, it is well to 

 remember that red and yellow ocherous materials, 

 such as in gossans, have a market, if they occur in 

 sufficient quantity to be recovered easily. Ladoo and 

 Myers (1951, p. 348) describe an operation in 

 which pigment is extracted from acid coal-mine 

 drainage : 



... In Pennsylvania, where pigment recovery is attempted, 

 logs and rocks are placed in the discharge of mine waters 

 pumped from the mine. These obstacles agitate the water 

 and aerate the solution, introducing oxygen and thereby 

 accelerating the precipitation of the iron oxide. Economical 

 recovery can be obtained only where there is a substantial 

 tonnage within a few hundred feet of sufficient thickness 

 to permit movement by ordinary shoveling . . . The yellow 

 mud is shoveled from the stream, which may be diverted, or 

 in some cases from the floor of the mine tunnel, with care 

 to avoid contamination by soil or coal . . . 



Acid mine drainage charged with iron salts is 

 one of the many pollution problems facing us today. 

 Perhaps a more efficient process than that described 

 above of aerating and recovering the precipitate 

 from such water might provide a salable resource 

 while aiding in cleaning the streams of iron in 

 these problem areas. 



MISCELLANEOUS PIGMENTS 



Graphite, magnetite, and some carbonaceous slates 

 have been used for black pigments, but the market 

 for these materials as pigments is very small, with 

 graphite the leading commodity. In 1969, graphite 

 in the amount of 182 tons valued at $28,000 was 

 consumed for paints and polishes. Magnetite is one 

 of the few minerals that grinds to a black powder, 

 but statistics on its current consumption as a pig- 

 ment are not available. 



MANUFACTURED PIGMENTS 



Zinc ores, ilmenite and rutile, and fossil fuels are 

 used as raw materials in the production of ZnO, 

 TiOa, and carbon black, respectively — all major 

 pigments. 



ZINC PIGMENTS 



Production of zinc compounds used for pigments 

 in 1969 consumed 120,700 short tons of zinc ore of 

 which 89,900 tons was domestic and represented 16 

 percent of total zinc mine production in the United 

 States. The percentage used for pigments has been 

 steadily increasing, and this use should continue as 

 an important market for zinc ores. 



The principal zinc compounds used as pigments 

 are zinc oxide, zinc chloride, zinc sulfate, leaded 

 zinc oxide, and lithopone, a coprecipitate of zinc 

 sulfide and barium sulfate. Shipments of zinc oxide 

 and zinc sulfate, produced mainly from zinc ore, 

 have increased sharply as zinc oxide production has 

 increased since 1960. The other zinc compounds are 

 produced mainly from slab zinc and scrap and as 

 byproducts. Consumption of zinc pigments other 

 than zinc oxide and zinc sulfate is decreasing 

 (Moulds, 1969). 



Zinc oxide is used mainly in the manufacture of 

 white rubber products such as white sidewall tires. 

 The rubber industry consumed 116,000 tons of zinc 

 oxide in 1969, representing 53 percent of lead-free 

 zinc oxide production (Moulds, 1969). The manu- 

 facture of photosensitive copying paper consumed 

 27,600 tons of zinc oxide, and the manufacture of 

 paint, particularly alkyd-latex paint used 25,200 

 tons. Other zinc compounds used as pigments were 

 consumed mainly by the paint industry. 



The total value of zinc pigments produced in the 

 United States from zinc ores in 1969 was about 

 $77.5 million out of about $80 million for zinc pig- 

 ments from all zinc materials. During the year the 

 United States imported an additional 23,500 tons 

 of zinc pigments, an increase over previous years, 

 and these imports were valued at $4.5 million 

 (Moulds, 1969). Resources and other aspects of the 

 zinc industry are discussed in the chapter of this 

 volume on zinc. 



TITANIUM DIOXIDE 



Titanium dioxide, a very important pigment, is 

 made from ilmenite, rutile, and titanium slag that 

 is the residue after smelting ilmenite. All titanium 

 dioxide for use as pigment is recrystallized after 

 chemical treatment of the mineral concentrates. 

 The resulting Ti02 is synthetic rutile or anatase or 

 a mixture of the two. Both have very high reflect- 



