Method for the production of cadmium pigments
cadmium sticks
Cadmium to be dissolved: 100 kilos
1) Middle Yellow (PY-37)
a) Preparation of a cadmium chloride solution
150 kilos of cadmium sticks are put into a FRP reactor, where are added 200 liters of demineralized water, then 200 liters of hydrochloric acid (32-33% concentration). To start the reaction 30 liters of HNO3 -technical grade nitric acid, 51% concentration- this acid will be added 1/3 of the volume each half an hour.
Cadmium is left 3 hours to dissolved-about 100 kilos of cadmium has been dissolved-, then the previously polished solution is pumped to the precipitation tank.
The final pH will be above 1.1, and the solution has a density of about 1.35 g/cm3, this is a solution containing 241 grams per liter of dissolved cadmium.
Note.-Reaction is exothermic, and in these proportions it reaches about 60°C.
b) Precipitation
Previously to this step, in a 2m3 tank with mixer, approximately 450 liters of demineralized water are added and 116 kilos (1.16 times the dissolved cadmium metal) of sodium sulfide flakes (60-62% concentration) and dissolved. Solution is heated up to 50°C.
This solution is pumped to the precipitation tank (about 15 minutes), where it will mix with the cadmium chloride solution. Discharge pipe will be below the level reached by the cadmium chloride solution to allow a better reaction. Mix is stirred 15 minutes after precipitating solution has been pumped.
The precipitation step is done (really the cadmium chloride solution is at about 40°C). Solution is heated up to 75°C.
c) Filtration
Precipitated pigments are pumped to filter.
d) Calcination
Medium yellows (PY-37) are not properly pigments, as these are not calcined. The pigments are washed until wash water indicates 0 chlorides. After this operation the filter cake is dried in a spin flash dryer.
2) Primrose and lemon yellows (PY-35)
a) Preparation of a cadmium chloride solution (id. As 1)
b) Precipitation
Once the cadmium chloride solution has been pumped to the precipitation tank, n kilos of zinc chloride or zinc sulfate are dissolved in a separate tank. This solution is added to the cadmium chloride solution. The amount of zinc added will be relative to the dissolved cadmium and to the hue to be obtained (up to 15% of zinc content-relative to cadmium-for a greenish yellow).
The sodium sulfide solution is added to precipitate the pigment.
In the cases where different amounts of barium sulfate need to be cooprecipitated, it should be added a solution of barium chloride to the cadmium chloride + zinc sulfate or zinc chloride solution In turn, where zinc chloride has been used, instead of sulfate, the sulfate ion will be needed- adding, after the pigment has been formed, an ammonium sulfate or sodium sulfate solution. Final pH: about 7.
c) Calcination
The filtered pigments (unwashed cake) are calcined, with no forced extraction of gases (only convection), to avoid oxidation, at a temperature of 360-400°C. The material in contact with these pigments should be AISI 310 stainless steel (other types of stainless steels will react with the pigments and flake out). Calcination time is relative to power of kiln (experience shows a need of at least 0.06 kw. per liter of chamber volume). When ready these pigments show a reddish hue within the kiln. After calcination step pigments are quenched in water, where sodium chloride is readily dissolved freeing pigments particles in such a way that there is no need of milling. (This is the case of the cadmium chloride, where sodium chloride is formed along with the pigment).
d) Washing
Quenched pigments are washed until free from soluble salts.
e) Filtration
Pigments are filtered and filter cake is derived to drying step.
f) Drying
Filter cake is dried in a spin flash dryer. Pigments are ready to be packed.
Oranges and Reds (PO 20 and PR-108)
a) Preparation of a cadmium chloride solution (id. As 1)
b) Precipitation
After preparing the sodium sulfide solution, as indicated above, several amounts of selenium metal are dissolved in the solution of sodium sulfide.
For oranges: From 5% to 14% of the weight of cadmium in solution.
For reds : From 15% to 25% of the weight of cadmium in solution.
After all the selenium has been dissolved, the solution is pumped to the precipitation tank.
When different amounts of barium sulphate need to be cooprecipitated, a barium chloride solution will be added after precipitation, and, afterwards the sulfate ion will be added (adding a solution of ammonium sulfate or sodium sulfate).
c) Filtration
Precipitate is filtered-not washed-.
d) Calcination
Pigments are calcined at a temperature of 360 to 375°C with a moderate extraction of gases, so as to extract the not used selenium metal (about 9% of selenium added, which will be washed and condensed in a washing tower). Calcination is finished when pigments, in the kiln, do not emit selenium gases anymore.
Afterwards, pigments are quenched in water ground to desired particle size and washed free from soluble salts.( max.Cd soluble salts allowance=65 ppm. )
e) Washing
Quenched pigments are washed until free from soluble salts.
f) Filtration
Pigments are filtered and filter cake is derived to drying step.
g) Drying
Filter cake is dried in a spin flash dryer. Pigments are ready to be packed.
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