Page 48 from: April 2005
Glass recycling in Europe is a highly successfulactivity although collection and recycling rates
vary greatly from one country to another. Across the
continent, some 2.5 million tonnes of packaging glass
is recycled in glass furnaces every year and, given
that this amount is on the increase, quality control
has to be ever more stringent to avoid production
losses. Typical concentrations of cullet contaminants
include 5 grams per tonne of iron, 25-50 grams per
tonne of ceramics, stone and porcelain (CSP), and 4-
10 grams per tonne of non-ferrous metals.
outlines the glass cullet recycling
process. Firstly, all magnetic material is removed
using a magnet while hand-sorting removes non-
glass items such as bricks, heat-resistant glass and
waste. The maximum cullet size most glassworks
can accept in their feed hoppers is 55 mm. This over-
size material is crushed and light, liberated materi-
als such as paper and foil are removed using suction.
The remaining material is sized into three classes:
25-55 mm, 10-25 mm and -10 mm. In some plants,
the -10 mm fraction is screened at 4 mm and materi-
al measuring less than 4 mm is only processed by a
metal sensor.
Each size fraction is first cleaned using an eddy
current separator to remove non-ferrous metals. The
next processing step removes CSP contamination,
which is typically 1-10 kg per tonne and which is
detected using an array of laser diodes or a camera
. An initial separator takes out some
90-95% of the CSP while, in a second processing step,
a metal sensor is added to the CSP sensor (CSP-M)
for removal of the non-ferrous metals remaining in
the product from the eddy current separator. If all
the equipment works properly, CSP concentration
will subsequently be around 25 grams per tonne
while non-ferrous concentration will be less than 4
grams per tonne. If CSP contamination in the feed
increases to 20 kg per tonne, CSP concentration in
the cullet will increase to 50 grams per tonne.
Figure 2
Figure 1
Colour sorting
In the case of separate collection of green, white
and brown glass, feedstock will still contain bottles of
the wrong colour. White glass collected in bins
throughout Europe typically contains 3-10% of other
colours and also CSP. Processing plant for white glass
is the same as for mixed cullet except that a colour
separator with a camera sensor is added to the line.
Depending on colour sorting machine settings, a
white cullet product can be made with 0.1-0.3% cont-
amination by other colours. If the concentration of
other colours in the feedstock exceeds 10%, then pro-
cessing costs generally increase to uneconomic levels.
Heat-resistant glass
Heat-resistant glass is difficult for the average
consumer to spot and so, unfortunately, this type of
material is regularly deposited in normal glass col-
lection bins. In existing recycling processes, heat-
resistant glass is not recognised as a contaminant
and ends up in the glass furnace. In the furnace,
heat-resistant glass does not melt and the heat-resis-
tant particles reach the bottle production point and
lead to stoppages due to clogging. Most heat-resis-
tant glass contains elements with a high atomic
number to give it its special properties and these can
be detected using an X-ray transmission (XRT) sen-
sor. The final stage in a glass recycling plant is the
quality control system (see also article in Recycling
International, October 2003).
Quality, recovery and optimisation
The increasing demand for higher-quality cullet
has resulted in lower glass cullet recovery rates in
processing plants.
Suction removes all light materials such as paper
and plastics; depending on the air flow, the light
waste concentrate contains 50% glass. By proper set-
ting of the eddy current separator, very little glass
will be left in the non-ferrous concentrate. However,
T E C H N O L O G Y
Recycling International • April 2005 48
Clear opportunities to i
Improvements in sensors and in the speed of data processing, combined with
decreasing costs, will make it possible to improve the quality and recovery rates
of glass. For example, it has been found that new XRT sensors can be used in plant
control and for the removal of unwanted heat-resistant glass. The following article
expands on the opportunities open to glass recycling specialists.
By Prof. Ir. Wijnand Dalmijn &
Dr. Ir. Tako P.R. de Jong
Delft University of Technology,
The Netherlands
