A well designed batch plant
can improve proﬁtability
High quality mixed batch is essential for producing quality glass and
maximising production yields. Here D D Burgoon* of Toledo Engineering
Company points out that the glass making process starts at the batch plant
and describes the critical features of good batch plant design.
n Mr Burgoon’s article in Glass
Fig 1. Basic process steps of a glass
International, September 19991, the manufacturing operation.
point was made that the glass man-
ufacturing process is only as good 0.0002 gm/cc range. Increasingly, direct
as the weakest element in the glass analysis, such as x-ray defraction,
process, that is, batching, melting/condi- allows direct tracking of the key oxide
tioning, forming, annealing, cold end percentages. With the desired glass char-
treatment, or packaging (ﬁg 1). Similarly, acteristics established, the design of the
batch plant performance is only as good batch weighing sys-
as its weakest element, whether it be, silo inlet to minimise tem can proceed.
materials handling, weighing, mixing or segregation during Feeding devices
the electronic controls (ﬁg 2). This article ﬁlling. In addition, (not conveying
is written to highlight the essential ele- particle size for all devices) are used to
Fig 2. Basic process steps of a glass batching
ments of good batch plant design. raw materials needs operation.
feed raw materials
to be in the same to the scale(s). The
Materials handling range and this is controlled during the feeder type, for example, vibratory,
During unloading of the various raw mate- procurement phase. screw or gate is dependent on the raw
rials, care must be taken to avoid both The ﬁnal control stage in segregation material characteristics. Free ﬂowing/low
contamination and segregation. takes place at the silo bottom, where mass permeability materials are usually han-
Contamination shows up in the glass as ﬂow devices such as bin activators, special dled by vibratory feeders (ﬁg 3), whereas
off-chemical composition, or as stone type ﬂow control inserts and special bin bottom erratic ﬂowing/high permeability (easily
defects. Segregation will show up as stria- shapes are used to minimise or eliminate aerated) materials are handled by special
tion (cord or ream). funnel ﬂow discharge. Ideal mass ﬂow in a screw feeders, with a ﬂush control device
Contamination can usually be avoided silo would be for all material in the cylin- (ﬁg 4).
with good operational practices, such as drical section to draw down at the same All scale feeding devices must be sized
making sure the unloading hopper is rate, thereby keeping the surface proﬁle for both feed rate and necessary weigh-
clean before use, using a non-residual unchanged. Achieving mass ﬂow at dis- ment resolution (ﬁg 3). The design para-
or non-contaminating bucket elevator charge helps to compensate for segrega- meter for necessary resolution (variation
boot, and using a distribution system tion created during the ﬁlling operation. in angle of repose at cut-off, plus feeder
that cannot leak between adjacent silo Furthermore, mass ﬂow assures a uniform coast) are usually more than sufﬁcient to
positions. For critical applications, dedi- source of material at the weigh feeder satisfy the feeder capacity requirements.
cated unloading facilities are used for inlet, which is essential for accurate, Many years ago, TECO developed the
each material. repeatable ingredient weighments. Superﬁne feeder for handling ﬁne
Each raw material is subject to segrega- mesh/high permeability materials (ﬁg 4).
tion during the silo ﬁlling and withdrawal Batch weighing A rotary vane feeder serves as an air-lock
operations. Silos should be kept as full as Before designing the batch weighing sys- to guard against material ﬂushing. The
possible to minimise segregation created tem, the glass technologist establishes the rotary vane feeds a small surge hopper
by the impact of the material striking the batch composition required to obtain the and when full, the material overﬂows and
pile during the ﬁll operation. In some desired glass characteristics. The technol- provides a fast-feed rate for ﬁlling the
applications, diffusers are installed at the ogist also establishes the statistical toler- scale. Once near the desired weight, the
ance limits to which each raw rotary vane feeder stops and a special
material must be weighed in order screw feeder continues to feed material
to stay within the suitable range of from the surge hopper, until the batch
properties. One common property weight is satisﬁed. A safety butterﬂy valve
used for statistical process control is closes to assure that no additional material
glass density. Typical glass density will ﬂow to the scale. The Superﬁne feeder
limits for container glass are in the can deliver almost any feed rate capacity
0.0020 gm/cc range, whereas the desired. This feeder type has been used in
limits for ﬂoat glass are in the conjunction with minor ingredient scales,
Fig 3. Vibratory feeder with cut-off
as small in capacity as 20lb. with scale res-
resolution shown. olution of 0.02lb.
GLASS INTERNATIONAL NOVEMBER/DECEMBER 2000 21
Modern batch plant for the production of ‘E’ glass
Batch PLANT (continuous) glassﬁbre.
Fig 4. Superﬁne feeder with mixer. In recent years, however, batch
rotary vane feeder, surge w/ plant structural height (and cost) are
overﬂow, special screw being reduced, by eliminating the
feeder and butterﬂy shut-
batch surge hopper and mounting
load cells under the mixer or
The glass compo- pneumatic blender/trans-
sition and statistical porter. This requires spe- attention at the time of design is the elec-
limits set by the cial know-how to negate tronic control system. Strain gauge load
glass technologist erroneous forces created cells and digital scales are universally used
provide the basis by mixer motion, con- for weight detection. PLCs, with update
for determining scale resolution and duits and piping, which time in the 25 to 50 millisec range, are
accuracy, to which each ingredient is can affect the scale accu- commonly used to obtain adequate con-
weighed. Scale resolutions of 1:5000 are racy and repeatability. trol resolution for the weighing process.
common and in some cases, 1:10,000 or Each ingredient is weighed under the
more are used. The weigh hopper must Mixing supervision of real-time statistical control,
also be designed to assure cleanout at dis- The design of the mixing which monitors each weighment and auto-
charge. The weighing system must be system is critical since it is the ﬁnal stage matically adjusts the ingredient cut-off, in
enclosed to contain dust, thus the scale in developing necessary mixed batch line with the trend of previous batches,
system must be designed to be immune homogeneity. Today, pneumatic blending thereby minimising process upsets.
from internal pressure effects, which can is typically used for ﬁne mesh ingredient The furnace operator is commonly
be created by the operation of a dust col- applications, like E-glassﬁbre batch, and responsible for the batch plant opera-
lector, bucket elevator, and/or mixer. the turbine-type mixer for the coarse tion. Information and data from the
Pressure effects on an improperly mixes, such as soda-lime batch. Both batch plant is transmitted electronically
designed weighing system can create methods ﬂuidise the batch and introduce to the furnace control room, as well as to
errors that are not immediately apparent, a turbulent force to create the mixing the plant and company-wide informa-
since the scale would be a combination action. The mixing time to achieve the tion systems.
gravimetric plus pressure force detector. desired degree of mixed batch homogene-
One method of obtaining pressure immu- ity is established at start-up. Results
nity is to have the weigh feeders enter the Cullet is generally introduced into the To test how well a new batch plant (or
hopper side (not through the top) (ﬁg 5), process after the mixing operation, to modernisation) is performing, one can
or by using a feeder ring, which is a pres- reduce wear and maintenance of the compare glass density variation with a
sure counter-balancing scheme. mixer, and to avoid cullet/mixed batch counterpart, that is, the obsolete batch
A check scale is a vital element of any segregation. TECO prefers to batch- plant or a similar facility within the
batch processing plant and it is a simple weigh cullet and blend it with the mixed organisation. TECO recently provided,
task if a separate batch surge hopper is batch as it is conveyed to the storage bin on a design/build basis, a new batch
used between the weigh gallery and the at the furnace. plant for an existing three-furnace glass
Mixed batch, regardless of how container plant. 30 days before
it is mixed, is subject to segrega- switchover to the new batch plant, the
tion during conveying to storage glass density data was noted for all three
at the furnace, and subsequent furnaces. After switchover, the density
delivery to the furnace charger. was noted for an additional 30 days. The
Proper design of the conveying results of the ‘before and after’ glass den-
system and transfer points will sity conditions are shown in ﬁg 6.
minimise segregation. Most soda- Obviously, the -30 day is glass density
lime batch is wetted with water in before switchover and +30 day are the
the mixer, as a ﬁnal step to pro- results with the new batch plant. An
Fig 5. Pressure balanced vs pressure sensitive weigh vide cohesion, minimise particle approximate ten-fold improvement in
hopper/feeder arrangement. segregation, and dusting. glass density variation was obtained with
Typically, mass ﬂow the new batch plant.
bins are used for Who said well performing batch plants
batch/cullet storage aren’t beneﬁcial? In the above example,
at the furnace. This the ten-fold improvement in glass density
is very important for stability increased the pack rate a few per-
batches that cannot centage points, which had a positive and
be wetted and must direct impact on proﬁtability.
be conveyed dry.
Electronic 1 Better plant performance begins at the
controls batch house, D D Burgoon, Glass
The last process ele- International September 1999, pg 9.
* D D Burgoon
Fig 6. Glass density his-
Toledo Engineering Co Inc, Toledo, Ohio,
tory 30 days before/after
new batch plant. USA. Fax+1 419 537 1369.
22 GLASS INTERNATIONAL NOVEMBER/DECEMBER 2000