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.

متقدم

Weeks (weathering) plus days

التعليمات

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.

المواد لهذه الخطوة:

طَفل الشبّ1 as required

Extract

Wash the weathered mass with water, step by step, until the solution reaches a specific gravity of 1.16.

المواد لهذه الخطوة:

ماء1 as required

الأدوات المطلوبة:

مقياس كثافة

Settle

Let the solution stand so calcium sulphate and basic iron sulphate settle out, then draw it off.

الأدوات المطلوبة:

خزّان الترسيب

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.

الأدوات المطلوبة:

حوض التبخير

مقياس كثافة

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.)

المواد لهذه الخطوة: