To Polarize or Not to Polarize: Practical Advice on How …
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refinery. Awareness of these changes enables an operator to prepare to counteract
them and maintain high CE.
Some operations use small test cells to determine whether the plant is at the
optimum polarization. Usually three cells are run—(1) electrolyte, (2) electrolyte
+ gelatin, and (3) electrolyte + Sb. The cell conditions are designed to guide an
operation in regard how structure and/or current efficiency change with polarization.
Test cells, however, are limited because they do not represent the physical condition
of the cellhouse (cathode surface, cell maintenance, etc.). While test cells can be
helpful in providing guidance about the electrolyte, the information generated can
be misleading. It is better to make observations in the cellhouse. “Read the metal” and
use the correlations developed over time to determine the direction to go regarding
polarization.
It is good to have a polarizing/depolarizing lever to make quick changes if needed.
Most zinc cellhouses use gelatin as their lever. The gelatin addition rate can be
altered by changing the speed of a pump. Some cellhouses add Sb tartrate (called
“strip eaze” in some places) directly to cells. Gelatin and Sb are actually polarizers
and depolarizers. They are used to modify the polarization and control the deposit
structure. For example, if the zinc deposit is very rough and dendritic, this indicates
that the depolarization is needed. This can be done by adding Sb tartrate, decreasing
the addition rate of gelatin, increasing the temperature, decreasing the current density,
etc. (see Fig. 4d). Changing gelatin and/or Sb concentrations are typically the easiest
parameters to modify without disrupting the operation. With that said, there are times
when changes in temperature or current density might be warranted.
How to Read a Deposit
This practical guide relies on one’s ability to “read the metal” and understand what
it is saying. The metal changes structure and morphology as the system shifts in
polarizing or depolarizing directions. Observing zinc morphology and structure will
provide information to make process changes to counteract what has occurred.
It is best to check multiple samples from different areas in the cellhouse to obtain
an overall picture. Observe the morphology (surface appearance), the back of the
deposits, and growth structure of the deposits. This will likely require manual stripping of individually pulled cathodes. The structure of the deposit can be observed
by breaking the deposit and looking at the cross section. If a cellhouse, due to crane
constraints or safety concerns, does not permit manual pulling of cathodes, then
observations at the stripping machine and breaking of stripping zinc plates can be
used as a substitute. The consequence of using end-of-plating cycle observations is
that they reflect what has occurred further in the past then pulling plates with hours
of growth.
Different zinc deposit morphologies and structures have been given different
names by different authors. Table 1 summarizes some the descriptors used in the
119
refinery. Awareness of these changes enables an operator to prepare to counteract
them and maintain high CE.
Some operations use small test cells to determine whether the plant is at the
optimum polarization. Usually three cells are run—(1) electrolyte, (2) electrolyte
+ gelatin, and (3) electrolyte + Sb. The cell conditions are designed to guide an
operation in regard how structure and/or current efficiency change with polarization.
Test cells, however, are limited because they do not represent the physical condition
of the cellhouse (cathode surface, cell maintenance, etc.). While test cells can be
helpful in providing guidance about the electrolyte, the information generated can
be misleading. It is better to make observations in the cellhouse. “Read the metal” and
use the correlations developed over time to determine the direction to go regarding
polarization.
It is good to have a polarizing/depolarizing lever to make quick changes if needed.
Most zinc cellhouses use gelatin as their lever. The gelatin addition rate can be
altered by changing the speed of a pump. Some cellhouses add Sb tartrate (called
“strip eaze” in some places) directly to cells. Gelatin and Sb are actually polarizers
and depolarizers. They are used to modify the polarization and control the deposit
structure. For example, if the zinc deposit is very rough and dendritic, this indicates
that the depolarization is needed. This can be done by adding Sb tartrate, decreasing
the addition rate of gelatin, increasing the temperature, decreasing the current density,
etc. (see Fig. 4d). Changing gelatin and/or Sb concentrations are typically the easiest
parameters to modify without disrupting the operation. With that said, there are times
when changes in temperature or current density might be warranted.
How to Read a Deposit
This practical guide relies on one’s ability to “read the metal” and understand what
it is saying. The metal changes structure and morphology as the system shifts in
polarizing or depolarizing directions. Observing zinc morphology and structure will
provide information to make process changes to counteract what has occurred.
It is best to check multiple samples from different areas in the cellhouse to obtain
an overall picture. Observe the morphology (surface appearance), the back of the
deposits, and growth structure of the deposits. This will likely require manual stripping of individually pulled cathodes. The structure of the deposit can be observed
by breaking the deposit and looking at the cross section. If a cellhouse, due to crane
constraints or safety concerns, does not permit manual pulling of cathodes, then
observations at the stripping machine and breaking of stripping zinc plates can be
used as a substitute. The consequence of using end-of-plating cycle observations is
that they reflect what has occurred further in the past then pulling plates with hours
of growth.
Different zinc deposit morphologies and structures have been given different
names by different authors. Table 1 summarizes some the descriptors used in the