6. Purifying absorbent

General

The industrial absorbents in Europe are native ores or synthetic materials containing iron hydroxide or hydrated iron oxides. They are employed under such names as "H-Masse", "Lux-Masse", "Wasserwerks-Masse" and "Raseneisenerz".

Iron content

The iron content calculated as the oxide is decisive and should be at least 40-50% by weight. If possible the proportion of "active iron" should be more than 70% of the total iron. "Active iron" is hydrated iron oxide having a particular crystal modification, for instance, alpha-FeOOH and beta-FeOOH. Certain industrial, iron-containing substances such as iron pyrites and slags etc., do not react with H₂S even though they have a similar chemical composition. They are "inactive". Under certain conditions they can be activated by grinding or other mechanical processing. This partially destroys the crystal structure.

Natural materials

Some naturally occurring, ferruginous soils are suitable for use as absorbents. Those ores or soils containing large concentrations of hydrated ferric oxides (Fe₂O₃H₂O or FeO(OH)) are suitable. Such ores are known as brown ironstone. There are various subtypes:

goethite (needle iron ore, pyrrhosiderite): a-FeO(OH),
lepidocrocite (ruby glance): b-FeO(OH),
limonite: (by appearance) amorphous.

They occur mainly as conglomerates and are generally classified as limonites.
The ores in question have various local names:
brown haematite (xanthosiderite),
limonite,
pea ore,
conglomerate ore,
minette (iron ooliths),
sea ore,
bog ore,
stilpnosiderite,
yellow clay ironstone,
yellow ochre.
The ores have various appearances and abundances.

Deposits

Laterite soils occur widely in tropical and subtropical areas. The iron content varies according to location: some of these laterite deposits are large enough to be mined for the production of iron.
Since laterite consists mainly of goethite, it is probably suitable for use as an absorbent. It is, of course, necessary to break or grind the material mechanically to achieve the right particle size. Since areas with laterite soils are usually cultivated for intensive crop production, i.e. the basis exists for a biogas plant, this soil appears likely for the purification agent.
The "terra rosa" found in the Mediterranean region is probably also suitable. It contains 10% of iron, again in the form of goethite.

Activation by regeneration

Some absorbents are less efficient when used for the first time. After they have been regenerated once the purification effect can be improved. The chemical processes involved have activated the material.