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Potash Alum from Alum Shale
Alum shale is a mix of iron pyrites, clay (aluminium silicate) and bituminous matter. Left to weather in the air, the pyrites oxidise and form sulphuric acid, which attacks the clay and turns it into aluminium sulphate. Water draws this out as a solution, which is settled, boiled down until iron sulphate crystallises out on cooling, then boiled down further to a specific gravity of 1.40. Stirred with the right amount of potassium sulphate, it throws down potash alum as a fine powder, alum meal. Alum crystallises in regular octahedra and is very soluble in water. Shale was also roasted to speed this up, which gives off sulphur dioxide; that route is not used here.
Advanced
Weeks (weathering) plus days
Instructions
1
1
Weather the shale
Weather the shale
Spread the broken shale in the open and let it weather, so the pyrites oxidise and the acid formed turns the clay into aluminium sulphate.
Materials for this step:
Alum Shale1 as required2
2
Extract
Extract
Wash the weathered mass with water, step by step, until the solution reaches a specific gravity of 1.16.
Materials for this step:
Water1 as requiredTools needed:
Hydrometer3
3
Settle
Settle
Let the solution stand so calcium sulphate and basic iron sulphate settle out, then draw it off.
Tools needed:
Settling Tank4
4
Remove the iron
Remove the iron
Boil it down until iron (ferrous) sulphate crystallises on cooling; draw off the liquor and boil it down to a specific gravity of 1.40.
Tools needed:
Evaporating PanHydrometer5
5
Precipitate the alum
Precipitate the alum
Let it stand, decant it from any sediment, add the calculated amount of potassium sulphate and stir well: the alum falls as a fine powder, alum meal. (With much iron present, potassium chloride is used instead.)
Materials for this step:
Potassium Sulphate1 as requiredTools needed:
Settling TankMaterials
3- 1 as requiredPlaceholder
- 1 as requiredPlaceholder
- 1 as requiredPlaceholder
Tools Required
3- Placeholder
- Placeholder
- Placeholder
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