FIELD OF THE INVENTION
The present invention relates to the recovery of zinc from top dross formed in galvanizing
processes and recycling of the same in the molten bath. In particular, the invention
relates to a process of recovery of Zinc from the Dross produced during galvanization by
selective addition of exothermic compounds/powder in the bath during operation itself
using heat of liquid zinc in-situ. The process further advantageously favours the possible
reuse the recovered zinc from the dross in the molten bath to an extent of over 75% out
of said dross in bath. The present invention is thus economic, energy saving and
preventing loss of expensive metal and an on-line process improving productivity and
product quality in galvanizing process and thus having potential for wide industrial
application.
BACKGROPUND ART
It is well known in the related art that dross generation is a continuous process in
galvanizing line, which is rich in zinc content. The top dross is removed frequently from
galvanizing bath. This results in continuous depletion of zinc form of dross as well as zinc
alloy in the bath. Normally, dross is a skim formed on the top of the molten bath of metal
and in the case of galvanizing the molten bath of zinc, consisting of a mush-like mixture
of iron, aluminum and zinc intermetallic particles, oxide skims and liquid zinc alloy. Dross
carries huge amount of zinc in form of oxide/intermetallics or mechanical entrapment.
The dross generated in galvanizing process consists of about 98% zinc and 2% each of
aluminum and iron. The dross is frequently removed for processing to prevent fouling the
surface of the galvanizing strip product and to recover the zinc retained in the dross. The
removal process generally consists of having an operator with a long-handled paddle or
rake to skim the surface of the molten bath. The dross is then scooped into a nearby
mold.
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In conventional method of recovery, the dross is retreated separately to recover zinc
from zinc dross. Zinc is primarily recovered from dross either by pyro-metallurgical
method or by electrochemical process. It has been observed that if molten dross is
stirred mechanically, about 15% zinc is recovered. It is also observed that if dross is
heated with sawdust, zinc can be recovered from dross up to an extent of 55%. For pyro-
metallurgical method a separate shaft type furnace is required in which dross is reheated
for recovery of zinc. For very low zinc bearing dust, electrolytic method is useful for
recovery of zinc.
Persons skilled and engaged in the art of galvanizing have put in a lot of effort in
treatment of the formation of dross on the top of the molten zinc bath gainfully. A
number of prior art dealt in the related field out of which some relevant documents are
discussed below as reference.
WO03023077 is directed to a process for the recovery of zinc metal from a zinc mineral
includes the steps of leaching the zinc mineral in a solution including a halide species
formed from two or more different halides, to leach the zinc Into the solution. The zinc-
bearing solution is then electrolysed to yield zinc metal and to generate the halide
species and further treatment to precipitate manganese dioxide etc.
US2007095169 publishes a method for leaching zinc-containing materials in connection
with the electrolytic recovery of zinc. According to this method, the feed materials i.e.
zinc calcine and zinc sulphide, are leached in three stages, in which the sulphuric acid
content of the stages rises in accordance with the direction in which the solids are
moving. The solids and solution formed in the leaching stages are routed throughout the
process countercurrently in relation to each.
Document corresponding to ZA200505981( also WO2005/031014 dated 07.04.2005
based on PCT/EP200409785) reveals that relates to a process for the separation and
recovery of non-ferrous metals from zinc-bearing residues, in particular from residues
produced by the zinc manufacturing industry, comprising subjecting the residue to a flash
or agitated bath fuming step, thereby producing an Fe bearing slag and Zn and Pb-
bearing fumes; and extracting the Zn- and Pb-bearing fumes and valorising Zn and Pb;
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characterised in that CaO, Si02 and IMgO are added as a flux before or during the fuming
step etc.
CN1878879 and CA2539166 belonging to the same family of patents document directed
to same process.
GB269321 directs to the dross obtained in melting zinc with a flux containing ammonium
chloride during refining is roasted or calcined to volatilize the chlorides therein, the zinc
and ammonium chlorides being condensed separately or together and reused as flux.
This prior art although deals with treating dross produced while melting zinc with a flux
containing ammonium chloride during refining, roasted and calcined to volatilize the
chlorides, and thus it is not same in the inventive merit of the present invention which is
directed to treat dross with exothermic compound in the bath itself without any external
heat addition, as of roasting or calcinations, and achieve recovery and recycling of the
zinc from the dross to the molten bath during the galvanization process itself in
operation, utilizing heat of the bath.
The present invention does not use any electrolytic means or leaching while recovering
zinc from dross during galvanizing. It is also observed that none of the above prior art
documents deals with processes, while recovering/separating zinc from zinc bearing
minerals or residues, involves recovery and recycling of zinc from any zinc bearing
material such as dross produced during galvanizing by advantageous addition of
exothermic compound/powder in-situ and in operation in molten bath itself.
There has been ,therefore, a continuous need to develop an efficient method for recovery
of zinc in higher proportions, as compared to conventional means, from the dross formed
during galvani2ing process in the bath itself and recycling the same in the molten bath
and thereby save depletion or loss of zinc in dross or alloy which is wasted. It would
further help conserving the waste full expenditure of the scarce metal and prevent
deterioration o' quality of galvanized products and rejection therefore, due to dross
deposition in galvanized surface.
4

OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide a process to recover
maximunn zinc from dross in molten bath during galvanizing operation itself involving
simple and cost-effective procedures.
A further object of the present invention is directed to a process of recovery and
recycling of zinc from dross in galvanizing bath by selective process which can be readily
adapted and used in relation to conventional galvanizing processes/baths.
A further object of the present invention is directed to a process of recovery and
recycling of zinc from dross in galvanizing bath wherein the skimmed dross and the
additives are mixed properly and thereby the heat of the molten bath is utilized and also
involve in-situ generation of excessive heat during such recovery of zinc from dross.
A further object of the present invention is directed to a process of recovery and
recycling of zinc from dross in galvanizing bath wherein the heat developed results the
reversal of entrapped zinc from dross to zinc bath and thereby facilitating recycling zinc
in bath in-situ.
A still further object of the present invention is directed to a process of recovery and
recycling of zinc from dross in galvanizing bath wherein oxides in dross are readily
converted to halides/oxides known as zinc ash which is suitable for agriculture purposes.
A still further object of the present invention is directed to a process of recovery and
recycling of zinc from dross in galvanizing bath wherein approximately 75% zinc from top
dross could be reversed back to galvanizing bath.
5

SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a process
for zinc recovery from Zinc dross generated in galvanizing lines and the like comprising:
skimming the top zinc dross from the top of galvanizing bath and gathering in a
heap;
spreading exothermic powder over the heap of zinc dross and mixing thoroughly;
heat generated during the exothermic reaction favouring the reversal of the
entrapped zinc from the dross to zinc bath and this results in the formation of zinc
ash which floats on the top of the bath for easy removal.
Another aspect of the present invention directed to said process for zinc recovery from
Zinc dross wherein during reversal of entrapped zinc from dross to bath, the oxides in
dross are converted to halides or oxides comprising said zinc ash.
A further aspect of the present invention directed to a process for zinc recovery from Zinc
dross wherein said mixing of the exothermic powder with the zinc dross is carried out
using a mixing system for environment friendly mixing of the exothermic powder with the
zinc dross.
6

A further aspect of the present invention directed to said process for zinc recovery from
Zinc dross wherein about 75% zinc from the top dross is reversed back to galvanizing
bath.
A still further aspect of the present invention directed to said process for zinc recovery
from Zinc dross wherein the quantity of dross reduces to about one-fourth as zinc ash.
A still further aspect of the present invention directed to a process for zinc recovery from
Zinc dross wherein the composition of zinc dross comprise Zn in amounts of 98.4 to
99.4% by wt, Al in amounts of 0.75 to 0.05 % by wt., Pb in amounts of 0.05 to 0.22 %
by wt. and Fe in amounts of 0.70 to 0.04% by wt.
According to yet another aspect of the present invention directed to a process for zinc
recovery from Zinc dross wherein the composition of zinc ash comprises Zn in amounts of
55 to 70 % by wt.
A still further aspect of the present invention directed to a process for zinc recovery from
Zinc dross generated in galvanizing lines and the like substantially as hereindescribed
and illustrated with reference to the accompanying examples.
The present invention and its objects and advantages are described in greater details
with reference to following non limiting exemplary illustration.
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DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process of recovery of Zinc from tine Dross produced
during galvanization by selective addition of exothermic compounds/powder in the bath
during operation itself using heat of liquid zinc in-situ. The process is further
advantageously adapted to reuse the recovered zinc from the dross in the molten bath to
an extent of over 75% out of said dross in bath. The present invention is thus economic,
energy saving and preventing loss of expensive metal and an on-line process improving
productivity and product quality in galvanizing process.
As already discussed, the generation of zinc dross is a continuous process in any
galvanizing plant. The top dross is frequently removed manually from the galvanizing
bath to avoid the deterioration of zinc-coated sheet due to entrapment of dross. The
dross formed during galvanizing, carries huge amount of zinc in form of
oxide/intermetallics/ mechanical entrapment. The present process of recovery and
recycling of zinc from dross in galvanizing bath is achieved by selective addition of
exothermic compounds/powder after top dross is skimmed at one corner of zinc bath to
form a heap and said exothermic powder is spread over the heap of zinc dross. The
exothermic powder mixed with the skimmed dross, utilizes the heat of molten zinc and
also releases huge quantity of heat on reacting with dross and separating zinc for
recycling to molten bath. The mixing system used is useful for proper mixing of dross
and the exothermic compound and to maintain the environment clean. The heat
generated helps recycling of entrapped zinc from dross to molten zinc bath. The zinc
recovery process of the invention adapted to recover and recycle about 75% zinc from
top dross that is reversed back to galvanizing bath. By the chemical reaction of dross
with the exothermic powder flux, the oxides in dross are converted to halides /oxides
known as zinc ash. The quantity of dross produced in the bath reduces to approximately
one-fourth as zinc ash. Zinc ash contains maximum zinc as and is a suitable product for
agricultural purposes. The following non-limiting example illustrates the process of
recovery and recycling of zinc from the dross produced during galvanizing.
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EXAMPLE
The present process of zinc recovery from zinc dross using exothermic compound
according to an embodiment of the present invention, said recovery of zinc entrapped in
dross is recovered and recycled back to the molten zinc bath is achieved by the following
process steps:
a. Top dross comprising the compositicn in Table 1, is skimmed at one corner of zinc
bath to form a heap;
b. Exothermic powder/compound having selective composition comprising —
(provide necessary details in Table 3), is spread over the heap of zinc dross and
mixed properly, using a system which keeps the environment clean;
c. During mixing, the exothermic powder use the heat of molten bath and also
develops excessive heat;
d. The heat released on reaction results the reversal of entrapped zinc from dross
to zinc bath to an extent of about 75 wt.% out of about 98wt.% present in said
dross;
e. The oxides in dross are converted to halides /oxides known as zinc ash
f. The zinc ash contains maximum zinc and being lighter in Density, therefore it
floats on molten zinc bath which can be removed easily;
g. The dross generated during galvanizing is reduced to approximately one-fourth as
zinc-ash substantially rich in zinc, used advantageously for agricultural purpose;
The composition(wt%) of zinc dross produced during galvanizing, to which the selective
exothermic compound is mixed in desired proportion, has been shown in following
Table-1.
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Table-1 :
Composition of
Zinc dross Zn A! Pb Fe
Wt % 98.41 0.70 0.117 0.64
The composition of the zinc ash is given in the following Table 2 , produced by the
reaction of exothermic compound with zinc dross directed to recovery of zinc, such that
said zinc ash containing substantial quantity of zinc halides is used for agricultural
purpose.

Table-2 :
Composition
of
Zinc Ash Zn Al ^ IMa Fe Pb ^ Mg CI F Ca Si K
Wt.% 59.19 6.32 2.54 2.16 0.18 1.10 2.74 0.81 0.50 0.47 0.12

Table-3 :
Chemical composition of Flux, consisting of following elements and their halides.
Composition of
Flux Si Na Zn Al Mg
Wt.% 9.0 3.0 11.0 9.5 12,5
Oxides and halides in gaseous form are as follows:
-Oxygen: 12%
-Chlorine:32%
-Fluorine: 9%
-Organic material:2%
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It is thus possible by way of tliis invention to aclnieve a method of recovery and recycling
of zinc from zinc dross produced during galvanization process by selective addition of
exothermic compound to the molten zinc bath in safe and environment friendly manner,
and thereby recycling zinc up to about 75 wt% out of over 98wt% present in said dross.
The heat of reaction further provide advantageous conversion of oxides of zinc in to
halides of lighter density that floats on top of bath and is thus easily separated. The zinc
halides so produced, called the zinc-ash, being substantially rich in zinc content, is
capable of being gainfully utilized for agricultural purposes. The process of the invention
is providing means for economic use of zinc as a expensive metal for galvanizing,
reducing time of processing, input costs and also effecting savings by way of providing
agricultural bi-product in an environment friendly manner and thus having the potential
of wide industrial application.
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WE CLAIM:
1. A process for zinc recovery from Zinc dross generated in galvanizing lines and the
like comprising:
skimming the top zinc dross from the top of galvanizing bath and gathering in a
heap;
spreading exothermic powder over the heap of zinc dross and mixing thoroughly;
heat generated during the exothermic reaction favouring the reversal of the
entrapped zinc from the dross to zinc bath in the form of zinc ash which floats on
the top of the bath for easy removal.
2. A process for zinc recovery from Zinc dross as claimed in claim 1 wherein during
reversal of entrapped zinc from dross to bath, the oxides In dross are converted
to halides or oxides comprising said zinc ash.
3. A process for zinc recovery from Zinc dross as claimed in anyone of claims 1 or 2
wherein said mixing of the exothermic power with the zinc dross is carried out
using a mixing system for environment friendly mixing of the exothermic powder
with the zinc dross.
4. A process for zinc recovery from Zinc dross as claimed in anyone of claims 1 to 3
wherein about 75% zinc from the top dross is reversed back to galvanizing bath.
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13
5. A process for zinc recovery from Zinc dross as claimed in anyone of claims 1 to 4
wherein the quantity of dross reduces to about one-fourth as zinc ash.
6. A process for zinc recovery from Zinc dross as claimed in anyone of claims 1 to 5
wherein the composition of zinc dross comprise Zn in amounts of 98.4 to 99.4%
by wt, Al in amounts of 0.75 to 0.05 % by wt, Pb in amounts of 0.05 to 0.22 % by
wt. and Fe in amounts of 0.70 to 0.04 % by wt.
7. A process for zinc recovery from Zinc dross as claimed in anyone of claims 1 to 6
wherein the composition of zinc ash comprises Zn in amounts of 55 to 70% by wt.
8. A process for zinc recovery from Zinc dross generated in galvanizing lines and the
like substantially as hereindescribed and illustrated with reference to the
accompanying examples.

The invention is directed to a process for recovery of zinc from top dross formed in
galvanizing processes and recycling of the same in the molten bath. The invention
recovery of Zinc from the Dross produced during galvanization is achieved by selective
addition of exothermic compounds/powder in the bath during operation itself using heat
of liquid zinc in-situ. The process is further advantageously adapted to reuse the
recovered zinc from the dross in the molten bath to an extent of over 75 wt% out of said
top dross containing zinc over 98 wt%. The present invention is thus economic, energy
saving and preventing loss of expensive metal and an on-line process improving
productivity and product quality in galvanizing process and thus having potential for wide
industrial application.