Environmental Engineering Reference
In-Depth Information
Two experiments are accomplished for observ-
ing the relationship between dross formation and
time interval adopted for the collection of rejected
parts and grids. Two different time intervals were
tested as 1.5 and 3.0 hour. The current grid cast-
ing procedure was also observed for comparison.
Before the experiment, the dross layer from the
surface of the molten lead alloy was completely
removed from the melting pot by the use of a ladle
with holes in it. The amount of dross formed in
the melting pot during 1.5 and 3.0 hour periods
was observed under the conditions in which the
excess parts and rejected grids were collected. At
the end of the collection period, the production
of grids was stopped and the dross formed on the
surface was removed and weighed. Following
this, the collected excess parts and rejected grids
were weighed and added to the melting pot. Since
the excess parts and rejected grids weighed about
700 kg, the manual addition of the rejects to the
pot took approximately 25 minutes. Then, the
formation of dross for an hour was allowed under
the conditions of no production. Finally, the dross
formed in an hour was collected and weighed. The
summation of the dross collected before and after
the addition of rejects is considered as the total
weight of dross obtained from the experiment with
the considered time interval. The results from this
experiment show that the ratio of rejects to the
ratio of product produced is about 58-60%, which
is considerably high.
Following up these steps, the current grid
casting procedure was observed for the same time
intervals; specifically 1.5 and 3 hours time periods.
Accordingly, the production of grids was started
with continuous feeding of the melting pot with
the excess parts and rejected grids by conveyor
belt. The amount of dross formed at the end of the
time interval was measured by collecting and then
weighing the dross. During this step, the amount
of excess parts and rejected grids could not be
measured since they are automatically added to
the melting pot.
According to the results obtained from the
two experiments with different time intervals, it
is proved that the suggested work practice reduces
the amount of dross significantly. It has also been
shown that decreasing contact time decreased the
amount of dross formed. Therefore, the longest
possible time period for collection is expected to
decrease the dross amount most. The cash flow
analysis of the batch mode and the current system
are compared in Table 1. The calculations in Table
1 take into basis that dross is reduced by 48%
with the use of the batch mode alternative. The
cost of raw material loss is calculated based on
the unit cost of dross, which is 1.18 $/kg. Since
it took 3 hours to obtain 41 kg of dross in the
experiment, the production of dross is assumed to
be 13.6 kg/hour. Additionally, the unit labor cost
for the company is calculated to be 1.63 $/hour,
based on the base wage rate. Given the unit labor
cost and unit production rate of dross, it can be
concluded that the unit direct labor cost of dross
is 0.11 $/kg. Based on the monthly quantity of
Table 1. Comparison of cash flow analysis of the current system and the batch mode for t=3.0 hours
Current System
Batch Mode
Quantity of dross (kg/month)
12,299
6,396
Cost of raw material loss ($/month)
14,605
7,593
Direct labor cost associated to dross ($/month)
1,352
759
Overhead cost associated to dross ($/month)
2,712
1,410
Labor cost associated to the collection of parts ($/month)
-
285
