Apparatus for refining lead

An apparatus for continuously refining molten crude lead including a furnace for removal of antimony from the crude lead regardless of whether the crude lead does or does not contain arsenic and/or tin. The furnace is of the reverberatory type having a plurality of transverse walls dividing the interior thereof and thus the lead into a number of distinct but connected compartments with the passageways through the transverse walls being staggered so that molten lead passes along a zigzag or tortuous path through the furnace which is lined with a refractory material. Air is injected into the molten crude lead as it passes from the inlet end of the furnace to the discharge end for progressive softening of the crude lead. The low antimony content slags produced near the lead discharge end of the furnace becomes progressively enriched in antimony as they react with the higher antimony content metal as they pass toward the lead inlet end where they are discharged at a slag overflow device. A solid reducing agent, such as coal, may be added to the surface of the slag or an organic gas capable of reducing lead oxide to lead may be injected into the crude lead or the slag layer to increase its antimony content and reduce the weight of the slag.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to an apparatus for continuously 
refining molten crude lead and more particularly to a furnace for the 
removal of antimony from the molten crude lead which may or may not 
contain arsenic and/or tin. 
2. Description of the Prior Art 
Softening, as the process is termed in this field of endeavor, is usually 
carried out in a reverberatory furnace or in a kettle as a batch process. 
In its simplest form, air is blown into the molten metal to oxidize the 
impurities which, dependent upon the temperature, rise to the surface of 
the bath in the form of a powdery scum or as a molten slag. If substantial 
quantities of tin are present, a dry powdery scum is always produced at 
the normal working temperature below 1000.degree. C. until the tin 
content, which oxidizes preferentially to arsenic and antimony, is reduced 
to the order to 0.2%. Below this amount, tin is oxidized together with 
arsenic and antimony and provided that the temperature is maintained at a 
high enough level, the oxidation products form a molten slag which floats 
on the surfaces of the metal. 
The addition of fluxes such as caustic soda enable the oxidation to be 
carried out at lower temperatures thus making the process suitable for 
operation in a steel pot or kettle rather than in a more expensive 
refractory-lined furnace. 
However, at nearly all lead smelters in the world, softening, whether at a 
higher temperature in a furnace or at a lower temperature in a kettle, is 
carried out as a batch operation which makes this process expensive in 
terms of the equipment used and the time required in carrying out the 
process. In one notable exception, air is blown into a bath of lead held 
in a small reverberatory furnace to oxidize the arsenic and antimony. Lead 
containing approximately 0.2% arsenic and 0.8% antimony is added to the 
bath at one end and softened lead with 0.03% antimony overflows through a 
specially shaped siphon device at the other. Lead is softened at the rate 
of nearly 30 tons per hour with a very small labor requirement and little 
other expense except for supplying the air and repairing the furnace. More 
importantly, good hygiene can be maintained around a small continuous unit 
compared with the difficulties of large batch units. One disadvantage of 
this type of operation is the necessity of maintaining the antimony 
content of the bath at the same level as desired in the softened product. 
In the case of the above mentioned process, the antimony content is 
maintained at 0.03%. It is well known in the art that a form of 
equilibrium exists between the antimony content of the lead oxide-antimony 
oxide slag and the antimony content of the lead that it is in contact 
with. Maintaining the bath of lead at a low antimony content inevitably 
means that the slag produced has a low antimony content or put in other 
terms for a given amount of antimony oxidized, a larger weight of 
by-product, which will incur an expense in its re-treatment will be 
produced. The relationship between the antimony content of a bath of lead 
and the slag in contact with it is well known and was presented in a 
publication of Dr. T.R.A. Davey-Proc. A.I.M.E. Symposium Lead-Tin Zinc '80 
Las Vegas, February 1980, p. 489. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to soften lead continuously while 
producing the smallest possible quantity of slag with a high antimony 
content so that re-treatment costs are kept at a minimum which can be 
accomplished by utilizing a novel type of reverberatory furnace. 
Another object of the invention is to provide an apparatus for continuously 
refining molten crude lead employing a reverberatory furnace having a 
plurality of transverse walls defining interconnected compartments to 
define a zigzag flow path through the furnace together with means for 
introducing air into the molten lead as it flows from an inlet end to the 
discharge end for progressive softening thereof with the low antimony 
content slags produced near the discharge end of the furnace becoming 
progressively enriched in antimony content as they react with the higher 
antimony content metal as the slags pass toward the lead inlet end from 
which they are discharged at an overflow device at the lead inlet end of 
the furnace. 
Still another object of the present invention is to provide a lead refining 
apparatus which is efficient in operation, reduces labor and cost and 
maintains clean environmental conditions in the area associated with the 
apparatus. 
These together with other objects and advantages which will become 
subsequently apparent reside in the details of construction and operation 
as more fully hereinafter described and claimed, reference being had to 
the accompanying drawings forming a part hereof, wherein like numerals 
refer to like parts throughout.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now specifically to the drawings, the lead refining apparatus of 
the present invention includes a reverberatory furnace 10 in which FIG. 1 
and 2 illustrate one embodiment of this invention. 
The furnace 10 comprises a steel shell 12 lined with a refractory material 
14 of a type resistant to the corrosive action of lead oxide-antimony 
oxide slags. Three cross walls or transverse walls 16, 18 and 20 are 
illustrated, but the invention is not limited to this number. These cross 
walls create four separate compartments 22, 24, 26 and 28 containing the 
molten lead 30 in the lower portion thereof. The chambers are connected by 
openings 32 in the base of each cross wall with the openings being 
staggered so that the molten lead is forced to take the longest path as 
illustrated in FIG. 2. 
Molten crude lead, preferably at a temperature above 650.degree. C., enters 
the furnace 10 through an inlet opening 34 at one end as illustrated and 
overflows by means of a siphon device 36 at the other end. The lead level 
38 within the furnace 10 can be controlled by altering the level of the 
overflow weir 40 by either adding or subtracting thin layers of brick or 
clay. Air is blown into the molten crude lead 30 by a number of lances 42 
constructed of mild steel, cast iron or slag resistant refractory material 
with at least one lance being positioned to discharge in each chamber or 
compartment. A lead oxide-antimony oxide slag layer 44 is generated and 
flows back along the furnace 10 toward the chamber 28 in which the 
unsoftened lead was received. The slag 44 overflows by means of a weir or 
taphole 46 into an awaiting receptacle 48. 
In flowing back along the furnace, the low antimony content slag produced 
in chamber 22 where it has been in contact with low antimony content 
metal, reacts with the progressively higher antimony content metal to give 
a higher antimony content slag. By reacting in chamber 28 with the 
incoming lead, the highest possible antimony content in the slag is 
obtained. The exact content of antimony in the slag will be dependent on 
the antimony content of the incoming lead with which it is in contact. 
In a variation of the process, the antimony content of the slag in chamber 
28 can be enhanced further by the addition of a small quantity of a solid 
reducing agent, such as coal, charcoal or other carbon-containing reducing 
agent of low ash content, sprinkled over the surface of the slag 44 
through the opening 34 or the like. In this case, the amount of air blown 
into the molten unsoftened lead in chamber 28 is limited to providing a 
stirring action or turbulence to ensure mixing of the slag 44 and reducing 
agent. The action of the limited quantity of reducing agent is to cause a 
preferential reduction of lead oxide from the lead oxide-antimony oxide 
slag before the slag leaves the furnace, thus reducing still further the 
quantity of slag required to be retreated and at the same time enhancing 
its antimony content. 
Alternatively, this same effect can be obtained by omitting the addition of 
the reducing agent and omitting the injection of air into chamber 28 and 
in their place, injecting a hydrocarbon gas or other organic gas of 
similar reducing ability, which will reduce lead oxide preferentially from 
the lead oxide-antimony oxide slag. 
Most crude lead arising from primary sources, such as obtained by smelting 
lead minerals, contains arsenic and antimony in varying proportions but 
little or no tin. However, lead obtained from secondary sources, such as 
from scrap or recycled materials can contain tin and/or arsenic in 
addition to the antimony which must be removed if soft lead is required. 
The presence of tin above 0.2% reduces the fluidity of lead oxide-antimony 
oxide slags and above 0.5% tin, powdery scums are produced instead of a 
liquid slag. 
The presence of tin above 0.2% in the unsoftened lead can be used to 
advantage by the adoption of a further variation in which one or more 
additional chambers are added to a furnace for dealing with the arsenic 
and antimony as previously described. FIGS. 3 and 4 illustrate such a 
modified furnace 50 in which additional chambers 52 and 54 have been 
added. These two chambers are interconnected by a submerged opening 56 and 
in a similar manner are connected to chamber 28. Crude lead at a 
temperature in excess of 650.degree. C. enters chamber 54 and after 
reacting with air injected by one or more supply pipes 42 passes to 
chamber 52 where it reacts with further air injected into the bath. The 
metal then passes through the chambers 28 to 22 inclusive before 
overflowing as nearly soft lead. However, the wall 58 between chamber 28 
and chamber 52 is constructed with additional bricks or refractory 
material to provide a height sufficient that the slags arising from the 
softening in the chambers 22 to 28 inclusive cannot come into contact with 
the products of the oxidation carried out in chambers 52 and 54. As 
illustrated in FIG. 4, the slag arising from the oxidation in chambers 22 
to 28 overflows from a taphole or weir 60 located in one external wall of 
chamber 28 and the products of oxidation in chambers 52 and 54 can be 
removed over a weir 62 in one of the external walls of chamber 54. The 
products of oxidation in chambers 52 and 54 in the absence of any fluxing 
agent will be a powdery scum, but by the addition of sufficient fluxing 
agent to either chamber 52 or chamber 54 or to both, a fluid reaction 
product can be obtained. This is advantageous in that it can be tapped 
either continuously or intermittently without the inherent difficulties of 
handling dry powdery lead-containing materials, especially from the point 
of view of controlling the amount of lead in the working atmosphere or 
environment. 
By dividing the furnace 50 into two distinct parts, one part dealing with 
the removal of tin contained in the incoming crude lead, comprising 
chambers 52 and 54, and the other part dealing with the removal of 
antimony contained in the incoming crude lead, comprising chambers 22 to 
28 inclusive, a tin-containing by-product reasonably low in antimony 
content can be obtained suitable for the recovery of tin, and an 
antimony-containing by-product low in tin content can be obtained suitable 
for the recovery of antimony as antimonial lead or with further treatment 
as lead and antimony. In order to achieve the optimum separation between 
tin and antimony, the oxidation of tin should take place in the presence 
of sufficient fluxing agent, such as caustic soda, to give a liquid 
product and the degree of oxidation controlled by the amount of air 
injected in a sufficient number of chambers, that the partially softened 
lead passing from the tin removal section to the antimony removal section, 
such as from chamber 52 to chamber 28, does not exceed 0.2% tin. 
Throughout the description, the term "air" has been used since it is the 
most common and cheapest form of oxygen-containing gas. In certain 
circumstances, when tonnage oxygen is available for other purposes, or 
inexpensive oxygen is available, oxygen could be used alone or in addition 
to the air injected into the bath. In certain circumstances, it may be 
advantageous to vary the volume of molten lead contained in any one or 
more chambers relative to any other to facilitate an increase in the 
number of lances to inject the air or oxygen-enriched air or the time 
available for partial reduction at that point in the furnace. 
The foregoing is considered as illustrative only of the principles of the 
invention. Further, since numerous modifications and changes will readily 
occur to those skilled in the art, it is not desired to limit the 
invention to the exact construction and operation shown and described, and 
accordingly, all suitable modifications and equivalents may be resorted 
to, falling within the scope of the invention.