The Role of Gels in Self Organisation of Slagheaps
from the Arsenic Production Site, Muldenhütten,
Freiberg, Saxony, FRG

D. Rammlmair BGR, Stilleweg 2, 30655 Hannover, Germany

Surface samples of the 30 to 40 years old slagheap from the arsenic production site at Muldenhütten are significantly enriched in As (>5 wt.%), Pb (>10 wt.%), Cu (>2 wt.%) and other elements. The grain size distribution, obtained by dry sieving, shifts to finer fractions applying a wet sieving method and shows that some 30 wt. % of the fragments are aggregated fines, agglutinated by gels. Sample material of the fraction >6.3 mm is very heterogeneous. It is composed of some 67 wt.% slag (50% magnetic, 17% unmagnetic), 8 wt.% coke, 5 wt.% rock fragments and 20 wt.% concrete, fireclay and brick fragments.

The slag fragments (magnetic and unmagnetic) consist of silicate slag (25 wt.%) and aggregates (75 wt.%). The latter are aggregates of silicate slag (a), Pb-Fe-spheres (b) and sulphur free roasted arsenopyrite (c).

Each of the components underwent different styles of alteration:

a) The first contact with rain water outlines the shape of a silicate slag as a thin skin (<1 µm in size) iron hydroxide which remains stable throughout the process of alteration of the fragment. As alteration proceeds the glass component quantitatively goes into solution along a 0.1 µm water-slag interface. Locally simultaneous formation of gel on a water-gel interface might occur. The phenocrysts (chromite, Zn-rich spinel, magnetite, fayalite, plagioclase) can be in situ embedded in colloform gel that incorporates Zn from the glass (and from spinels), As, Cu, etc. from the sulphide spheres in the glass, and Pb and some of the As from sources b) and c).

b) Alteration of Pb-Fe-(hollow)-spheres shows a different pattern. Leaching of Pb occurs from either sides (core and rim) and migrates towards the centre of the shell letting an iron hydroxide sponge behind. Pb locally forms pure anglesite or cerrusite gel or reacts with solutions from sources a) and/or c) to polymetallic gels in the inter- and intra-fragmental space.

c) The first contact with water as in a) outlines the shape of the ex-arsenopyrite. Further reaction leads to a mesh-like type of alteration with As joining the solution and Fe forming a 3D-network. Pb-gel might fill the available space and the mesh traces. The micropores locally are filled by poly- or monometallic gels. The formation of iron hydroxide 3D-network results in an extreme increase of surface area and microporosity. The water retaining capacity increases dramatically and the overall flow direction changes into an upwards dominated migration of the solutions due to the increased capillary forces.

The seasonal climatic changes are reflected in an oscillating evaporation front resulting in a precipitation of gels that
coagulate fragments on edge and surface contacts, or form agate-type textures in inter-fragmental areas and fill micropores. Within the gel and at the gel-air interface crystallisation of Fe-oxides and -hydroxides, Fe-, Pb-, Ca-, Zn-, Cu- and polymetallic arsenates, chlorides, sulphates and carbonates takes place.