The keu is a production residue deriving from the treatment of the sludge produced by the scraps of the leather tanning; it is characterized as a sintered granulate.
Based on the Consolidated Law on environmental and the related ministerial regulation, followed by a specific explanatory circular, it can be classified as a by-product, that is as a production residue not constituting waste, provided that the necessary treatment operations of the tanning sludge have been adequately carried out.
In this case, from the production point of view it qualifies as a secondary raw material.
The ministerial decree of 5 February 1998 includes, among the waste for which material recovery is envisaged, the “sludge from the purification of water resulting from the processing of dried leather”.
The sludge produced by the tanning activity, when it comes out of the clarification plants (in which it is separated from the rest of the tanning slurry), is characterized as thickened sludge (humidity around 96%). They are then subjected to various treatments:
a possible chemical conditioning with polyelectrolytes, which coagulates the solid substances suspended in the supernatant (the upper part of the suspension);
centrifugation (or subjecting to a plate filter press), consisting in a mechanical dehydration of the sludge (remaining humidity around 70%, in the case of centrifugation);
drying, which involves thermal dehydration of the mud (remaining humidity around 5%) through the evaporation of liquid substances, carried out inside turbo-dryers with the use of diathermic oil (while the gaseous mixture is introduced into a scrubber and then into a capacitor);
pyrolysis-gasification, heat treatment which leads to the splitting of chemical bonds and which renders the resulting material inert;
sintering, which allows the welding of the remaining powders.
This is how keu is obtained: on average, from 12-14 tons of dehydrated sludge, 1.8-1.9 tons of inert material can be obtained.
The keu is mixed with calcium carbonate giving rise to a conglomerate known as plastofill: this can be of the bituminous type (HCB, keu 30%), used for the production of asphalts, or of the cement type (HSC, keu 10%), used as an additive to concrete. Keu can also be used as a filling material, for example for the construction of road embankments.
Risks to health
Untreated or inadequately treated sludge is classified as waste and contains elements that are harmful to health. In cases of “chrome tanning”, in which, unlike what happens in “vegetable tanning”, the tanning agent is usually constituted by chromium salts in trivalent form, such as dichromium trioxide, Cr2O3, or from basic chromium sulphates, such as chromium hydroxy sulphate, Cr (OH) (SO) 4, the materials deriving from sludge also contain a high concentration of chromium, in addition to the various pollutants used in the other processing phases of the leather (such as antimony). The resulting material, even when mixed with bricks or crushed stone, if buried can contaminate groundwater and the groundwater, since heavy metals do not dissolve and are absorbed by the soil.
In this regard, the ministerial decree of 5 February 1998 provides for “transfer tests”, to be carried out “at least every start of business and, subsequently, every two years, and in any case whenever substantial changes occur in the recovery process some waste”. According to this legislation, the sludge must have a chromium content of less than 0.3% on dry matter and a humidity of less than 20%, so that the content of trivalent chromium (Cr +++, chromium in the oxidation state +3), in the mixture of mud with clay does not exceed 250 mg / kg on dry matter.
Over the years, various judicial inquiries have been opened regarding the illegal disposal of waste