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EC number: 294-409-3
CAS number: 91722-09-7
Substance formed during processing of liquid steel or during production of iron castings. Consists primarily of fused silicates and trace elements as oxides as well as trace of alloying elements.
Slags are inorganic solid UVCB. Consequently, single data cannot represent the complex and varying processes leading to the release of traces of analytes from the solid material.The key values for release of ions from ferrous slags are estimated to be heavy metals: insoluble 0.01 mg/lcalcium: moderately soluble (0.1-600 mg/L)other alkaline and alkaline earth elements, aluminium: slightly soluble (0.1-100 mg/L)Sulfate and thiosulfate: slightly to moderately soluble (0.1-1000 mg/L) The solubility of slag analytes decreases during aging of slag.
Concentrations of important analytes in leachates (L/S 10/1, DIN
38414-4) of ferrous slags used for testing of ecological or
toxicological properties of ferrous slags (Table compiled from 2009 and
2010 toxicological and ecotoxicological testing)
Tested grain size (mm)
8 - 11
0 - 10
Cr total (mg/L)
Only a small part of the inorganic material contained
in slags is released (Larm 1998).
pH of leachates of ferrous slags
The pH of slag leachates is basic (pH
11.2 -12.2). The pH value may be due to
The conductivity of slag leachates is mostly caused
by the high mobility of the OH ions which are predominantly a result of
the presence of free alkaline earth oxides/hydroxides
Heavy metals occur, if at all, only in traces in
ferrous slag leachates because the heavy metals are bound into crystal
matrices (e.g. spinel) (Tossavainen and Lind 2005, Tossavainen et al.
2005) or absorbed in glass phase(s).
Leachates of blast furnace slags may contain sulfate,
thiosulfate, sulfide and polysulfides in the form of their calcium,
sodium, and potassium salts. The concentration of reduced sulfur ions in
the leachates decreases in the time course of leaching and in the
presence of oxygen (LECES 1991).
Thiosulfate and sulfate were detected in the
leachates of ABS. Thiosulfate (reported concentrations were calculated
as sulfate) decreased from > 9 g/L to approximately 0.1 g/L after a
dozen leaching cycles. Sulfate decreased from starting levels at
approximately 1 g/L to approximately 0.1 g/L within 5 leaching cycles.
Other sulfur compounds (e.g. sulfides) were not detectable (no limit of
detection available). In outdoor lysimeter experiments sulfate and
thiosulfate concentrations were one order of magnitude less, e.g. the
leaching rates of thiosulfate from ABS decrease sharply from 320 mg/L to
approximately 10 mg/L within some 5 months whereas sulfate leaching
rates were only slightly decreased during 20 months of outdoor exposure
The sulphate concentration was 0.168
g/L in ABS leachates, 0.030 g/L in GBS leachates, 0.070 g/L in SMS, and
less than 0.02 g/L in the other ferrous slags (Geiseler 1998).
Only a small part of the trace
substances present in solid slag is leached. F concentrations in
leachates attain some mg/L in BOS and SMS (Geiseler 1998, 2000)
Several measurements were performed but
cyanide could not be detected (Larm 1998)
Due to the formation of ferrous slags at
temperatures > 1000 °C, formation of organics is limited and expected to
be very low. Consecutively, the concentrations of organics in slag
leachates are expected to be negligible. However, to increase the
possibility of detection, TOC, hydrocarbons, EOX, sum of BTEX (benzene,
toluene, ethylbenzenes, xylenes), sum of long chain hydrocarbons, sum of
16 PAH compiled by the US EPA (naphthalene, dihydroacenaphthalene,
acenaphthalene, fluorene, phenanthrene, anthracene, fluoranthene,
pyrene, benz(a)anthracene, chrysene, benz(b)fluoranthene,
benz(k)fluoranthene, benz(a)pyrene, dibenz(a,h)anthracene,
benz(g,h,i)perylene and indeno(1,2,3,c,d)pyrene), and sum of 6 PCB
(polychlorinated biphenyls) were measured in solid slags as a worst case
for slag leachates. In general, no organics were detected (Oekoinstitut
COD (chemical oxygen demand)
The COD is due to sulfide and thiosulfate in the
slags and is relevant only for cooled blast furnace slag. These sulfur
compounds are oxidised in the COD test to yield sulfate (Larm 1998).
For the assessment
of inorganic (waste) materials the
DIN 38414-4 DEV S4 leaching method was compared with pH4 stat method.
The later simulates worst case conditions of acidic rain and yields the
acid neutralization capacity (ANC).
Under the acidic
conditions of the pH4 stat method, the mobility of the heavy metals is
significantly higher than measured by the DIN 38414-4 DEV S4 method.
However, it is possible to use natural rock and glasses containing heavy
metals to make a comparative assessment of the heavy metal leachability
of the inorganic matrix.
For most materials a
very tight binding of heavy metals is observed which is similar to the
properties of inert glasses. This
is especially valid for Cr in ferrous slags. In most materials, lead is
more mobile (leachable) than As,
Ba, Cd, Co, Cr, Cu, Mn, Ni, Sb and Zn which
are classified as less mobile.
The mobility of
these trace elements can be controlled by the production method of the
material e.g. slow or rapid cooling for the production of crystalline or
vitreous phases, respectively. There are also differences between the
leaching methods: In general, the DIN 38414-4 DEV S4 method yields lower
heavy metal concentrations in the leachate of crystalline slags, the pH4
stat method of vitreous materials.
The mobility of
heavy metals as measured by the pH4 stat method depends on the acid used
for pH adjustment. The use of sulfuric acid instead of nitric acid
increases the mobility of heavy metals as sulfate leads to an increased
destruction of silicates. In contrast, the leaching of lead and barium
is decreased by sulfuric acid as their insoluble sulfates are formed.
The grain size used
in the pH4 stat method is of minor importance for the results of the
leaching experiments. However, a comparable grain size distribution
should be used, e.g. less than 10 mm.
A multivariate data
analysis including data clusters was performed which suggests a
structure for both leaching methods. For hydraulic engineering stones it
was shown that the data of the 15 elements included in this study, can
be reduced to 3 characteristic factors. For other construction
materials, there are 2 factors each sufficient for the almost complete
description of their properties. An assessment of these factors enables
the recognition of distinct types as well as the classification of these
materials into the LAGA classes.
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