Plasma recovery of tin from smelter dust

Disclosed are a method and apparatus for recovering tin from tin smelter dust without the need for pre-agglomerating the dust particles. Dust containing tin oxides is directed into the tail flame of a plasma reactor, and the tin oxides are reduced to liquid tin using hydrogen and/or hydrocarbon gases supplied to the reactor as dust carrier and plasma gases. Tin is removed from a collection vessel at the bottom of the plasma reactor, preferably while in liquid form so that the recovery process may be run continuously. The plasma recovery process has demonstrated yields of about 94.5% and metal containing 99.7% tin, and analysis has indicated low operating costs relative to the current market price of tin.

BACKGROUND OF THE INVENTION 
This invention relates to recovery of tin from dust produced during tin 
smelting, and particularly to an efficient method of recovery utilizing a 
plasma reactor. 
The smelting of tin-bearing ores in metallurgical furnaces generates large 
quantities of dust consisting largely of tin oxides (SnO and SnO.sub.2). 
This flue dust or smelter dust is typically collected by filtration of hot 
gases leaving the furnace, then is stored or further processed for 
recovery of tin. Since the smelter dust may amount to up to about ten 
percent by weight of the ore feed material, long-term storage or disposal 
of the large quantities generated presents problems. Known techniques for 
reprocessing the flue dust, however, also are unsatisfactory. Because 
direct return of the dust to the smelting furnace would result in a large 
fraction of the dust being again carried off with furnace gases, current 
practice is to form the dust into pellets or briquettes and then charge 
these agglomerates into the furnace. This intermediate processing of the 
dust typically includes multiple steps such as mixing of the dust with 
water and chemical binders, then roasting and/or drying the mixture. Also, 
the resulting pellets often have low mechanical strength and partially 
crumble during storage or transportation, again yielding flue dust. 
Overall efficiency of recovery, therefore, may be less than fifty percent. 
A more direct technique for processing smelter dusts consisting chiefly of 
metal oxides of low melting point is the so-called "flash agglomeration" 
method described in U.S. Pat. No. 4,013,456. In this process, flue dust 
from lead smelting furnaces is fused at relatively low temperatures, 
heated with additives, solidified by cooking, then fed to a furnace. This 
method, however, is not readily applicable to dust containing tin oxides 
since their melting points are considerably higher than those of lead 
oxides. 
Accordingly, it is an object of the invention to provide an improved method 
of recovery of tin from dust produced during tin smelting. 
It is a particular object of the invention to provide a method of 
recovering tin from smelter dust without prior agglomeration of the dust. 
It is also an object of the invention to provide a method of recovering tin 
as a liquid from flue dust utilizing a single reactor operating at high 
yield and low cost. 
SUMMARY OF THE INVENTION 
The invention is an improved method of recovering tin from tin fines 
(smelter dust) produced during smelting of tin. In general terms, the 
improved method involves the generation of a plasma arc and high 
temperature plasma tail flame, feeding a mixture of flue dust and a 
carrier gas into the tail flame in a reactor tube so that tin oxide in the 
dust is heated and reduced to liquid tin in passing through the reactor 
tube, and then removing liquid tin from the reactor tube. 
In a preferred process according to the invention, hydrogen is used as the 
plasma gas. The carrier gas is a reducing gas such as natural gas, 
hydrogen, or a mixture thereof so as to assist in reduction of tin oxide 
to liquid tin. A reactor tube having an inner wall of graphite is used to 
form a reaction zone, and a portion of this wall near the point of entry 
of the flue dust is shaped to converge in the downstream direction. 
Alternatively, the inner wall of the reactor tube first diverges then 
converges in approximate conformance with the shape of the plasma tail 
flame. 
To facilitate removal of liquid tin produced in the recovery process, a 
collection vessel is positioned adjacent to the downstrem end of the 
reactor tube. The tin may be withdrawn as liquid through a port in the 
side wall of the collection vessel, or it may be allowed to solidify and 
then be remelted to separate tin from slag in the tin product. 
It is an important aspect of the recovery process that no agglomeration or 
other pre-treatment of the flue dust is necessary prior to its reduction 
in the plasma reactor. Also, the process can be run continuously, 
providing high yields of about 95%, and metal with a tin content of about 
99.7%.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In FIG. 1 there is shown a plasma reactor 20 suitable for processing tin 
dust according to the invention. The reactor 20 includes a plasma head 22 
for generating a plasma arc and hot tail flame and a reactor tube 24 
downstream of the plasma head 22 for receiving the tail flame and a flow 
of reactants. 
The plasma head 20 of FIG. 1 may be of conventional construction and 
similar, for example, to the plasma reactor head of U.S. Pat. No. 
4,335,080, of common assignee with the present invention and whose 
disclosure is incorporated herein by reference. The head 20 includes an 
anode 26 and a cathode 28 which are connected by electrical lines 30 and 
32 to a D.C. power source 34 and which are cooled by suitable means (not 
shown) such as a coil through which water is circulated. A suitable 
cathode 28 is a copper rod having a tungsten tip 36 in its downstream end. 
The cathode 28 is positioned at one end of a central plasma arc passage 40 
and is separated from the anode 26 by a spacer 42. A suitable anode is a 
copper ring 48 surrounding the downstream end of the passageway 40, the 
ring 48 in turn being contained within a block 50 of stainless steel. 
To permit the production of plasma within the head 20, a plasma gas inlet 
duct 52 is provided to feed a plasma gas into the upstream end of the 
plasma arc passage 40. Hydrogen is preferred as a plasma gas in the 
recovery of tin from flue dust according to the process of the invention 
since it participates directly in the reduction of tin oxides (SnO and 
SnO.sub.2) to tin (Sn) and is inexpensive and non-toxic. Many other gases 
which might be useful in other plasma processes have one or more 
characteristics which make them less preferred and in some cases 
undesirable for use in the present invention. For example, nitrogen, 
though inexpensive, would not contribute to the reduction of tin oxides 
and thus would lower efficiency of the process by diluting other 
reductants. Moreover, nitrogen as a plasma gas would produce NO.sub.x and 
possibly toxic cyanide gases in the reactor effluent. Hydrocarbon gases 
such as natural gas or methane, though available at low cost, are less 
preferable than hydrogen as plasma gases since they would tend to condense 
in the plasma head 20 and eventually interfere with arcing and flow 
through the head. 
The lower portion of the plasma head 20 is intersected by one or more feed 
lines 60 through which flue dust is introduced into the reactor tube 24 
and into the tail flame 62 discharged from the plasma arc passage 40 into 
the reactor tube 24. A carrier gas under pressure is used to direct the 
tin oxide-bearing flue dust through the lines 60 into the tail flame. The 
carrier gas preferably is a reductant such as natural gas, hydrogen, or a 
mixture thereof which participates in the reduction of tin oxides to tin. 
The use of a hydrocarbon for at least a portion of the carrier gas may be 
preferable to hydrogen alone since carbon's greater relative strength as a 
reductant should produce a higher yield of tin than would hydrogen alone. 
As indicated in FIG. 1, the reactor tube 24 may comprise a substantially 
straight tube whose inner wall 64 includes a convergent section 66 near 
the upstream end of the tube 26 and a constant diameter section 70 over 
essentially the remaining portion of the tube. The converging section of 
the reaction chamber 72 formed by the wall 64 increases the flow velocity 
of the reactants, thus helping to avoid sticking of reactants and eventual 
plugging of the reaction chamber 72. It also concentrates the heat input 
provided by the tail flame 62, facilitating complete reduction of the tin 
oxides to tin with a moderate tube length and power input. 
The reactor tube 24 may be constructed by any of various high temperature 
materials, such as graphite, alumina, zirconia, or tungsten carbide. 
Graphite is a preferred material which, in addition to high temperature 
resistance, has good thermal shock resistance, is relatively inexpensive, 
readily machineable, and non-reactive with tin. To minimize the loss of 
heat from the tube 24 and thus the power required to operate the plasma 
reactor 20, insulation 76 is provided around the outer wall of the tube 
24. The insulation 76 may be formed of any suitable high temperature 
insulating material. 
As indicated above, the flue dust or smelter dust from which tin is 
recovered according to the process of the invention consists largely of 
tin monoxide (SnO) and (in smaller amounts) tin dioxide (SnO.sub.2). The 
dust typically has a tin content of about 60-72 percent by weight and may 
also contain metals such as tungsten, iron, zinc, aluminum, and calcium in 
the form of oxides, sulfides, and/or carbides, and may also contain 
non-metals such as silicon in the form of SiO.sub.2. 
During operation of the plasma reactor 20, this flue dust and a suitable 
gas are introduced into the tail flame 62 within the reaction chamber 72 
to be heated to temperatures at which tin oxides are reduced to tin. It is 
considered important to the invention and the yields obtained thereby that 
the tin oxide reactants are in the form of small particles (dust) since 
this facilitates rapid heating in the very short residence time provided 
by the plasma reactor 20. 
Reduction of the tin oxides occurs according to one or more of the 
following reactions, depending on the form(s) of tin present and on 
whether the carrier gas includes natural gas (considered to be essentially 
CH.sub.4), hydrogen, or both: 
EQU SnO.sub.(s) +H.sub.2(g) .fwdarw.Sn.sub.(l) +H.sub.2 O.sub.(g) (1) 
EQU 4SnO.sub.(s) +CH.sub.4(g) .fwdarw.CO.sub.2(g) +2H.sub.2 O.sub.(g) 
+4Sn.sub.(l) (2) 
EQU SnO.sub.2(s) +2H.sub.2(g) .fwdarw.Sn.sub.(l) +2H.sub.2 O.sub.(g) (3) 
EQU 2SnO.sub.2(s) +CH.sub.4(g) .fwdarw.2Sn.sub.(l) +CO.sub.2(g) +2H.sub.2 
O.sub.(g) (4) 
where s, l, and g denote, respectively, solid, liquid, and gaseous states. 
Equations (1) and (2) show the reduction reactions for tin monoxide (SnO), 
the major constituent of the flue dust, and equations (3) and (4) show 
similar reactions for tin dioxide (SnO.sub.2), which comprises a minor 
proportion of the flue dust. 
The reduction reations proceed to completion as the reactants pass through 
the converging and then the constant diameter sections of the reaction 
chamber 72 so that the tin oxide in the smelter dust is converted to 
liquid tin in the reactor tube 24. Conversion to liquid tin may be 
essentially complete in approximately the upper half of the reactor tube 
24, and for such operation the lower half of the tube 24 is merely 
maintained at a temperature sufficiently high that the tin remains a 
liquid as it falls through the lower portion of the chamber 72 and runs 
down the inner wall 64 of the tube 24. A collection vessel 80, which may 
also be formed of graphite, is positioned at the downstream end of the 
reactor tube 24 to receive liquid tin emerging from the tube outlet 82. 
Exhaust gases such as water vapor, carbon dioxide, and unreacted hydrogen 
and natural gas are directed out of the reactor system in any convenient 
manner such as by passing between the outer portion of the reactor tube 24 
and a sidewall 86 of the vessel 80 as indicated by the arrows 90 shown in 
FIG. 1. The exhaust gases, which typically also contain a small amount of 
solid particles, are then preferably passed through a dust collector and 
scrubber (not shown). 
Liquid tin from reduction of the tin monoxide is collected in the vessel 
80, which if necessary may be heated by external means to maintain the tin 
in liquid form. Maintaining the tin as a liquid rather than allowing it to 
cool and solidify in the vessel 80 facilitates continuous operation of the 
method, which is preferred in order to efficiently process large 
quantities of dust. Continuous operation also avoids the need to remelt 
the solidified tin product to remove slag from it. 
Liquid tin may be removed from the vessel by means of an outlet line 94 
which extends through an opening 96 in the vessel sidewall 86. Withdrawal 
of tin from a position intermediate between the top and bottom of the pool 
of liquid tin product separates tin from slag produced in the plasma 
reactor 20. The slag, which is formed in quantities up to about 5 percent 
of the tin product and includes materials such as oxides of aluminum, 
calcium, and silicon, may, if the vessel 80 is maintained at a temperature 
sufficiently high that the slag remains molten, be periodically removed 
through a slag removal port 98. 
After being withdrawn from the collection vessel 80, the liquid metal, 
which tests have shown contains up to about 99.7 percent tin, may be cast 
into ingots, and/or delivered to a location for further refining, or used 
directly such as in the preparation of alloys. The process is considered 
particularly useful when employed in a tin smelting plant. In such 
applications the liquid tin produced by the plasma reduction process of 
the invention is simply combined with the liquid tin output of a smelting 
furnace. 
To begin operation, the plasma reactor 20 is started utilizing argon as the 
plasma gas. A few seconds after an arc is established, a flow of hydrogen 
gas is started and gradually increased while the flow of argon is steadily 
decreased so that hydrogen gradually replaces argon as the plasma gas. To 
maintain a stable plasma arc and tail flame, the argon flow preferably is 
not shut off completely, however, but instead a small flow of argon is 
maintained equal to about 1/2 to 1 percent by volume of the total plasma 
gas flow. Then, after the plasma arc and tail flame have stabilized and 
run for a period of time sufficient to heat the reactor tube 24 to 
operating temperatures--preferably a minimum temperature of about 
1000.degree. C. at the downstream or output end of the reactor tube--a 
feed of carrier gas and flue dust to the reactor tube 24 is started and 
tin oxides are reduced to liquid tin in the tube. During steady-state 
operation, the flow rate of plasma gas may, by way of example, be 
approximately three times the flow rate of carrier gas. 
FIG. 2 illustrates a lower portion of a plasma reactor 100 according to an 
alternate embodiment of the invention. The reactor tube 102 shown therein 
includes an inner wall 104 whose upper portion first diverges in the 
downstream direction and then converges to a constant diameter lower 
section. The divergent/convergent portion is shaped similar to an 
unconfined tail flame emerging from the plasma head 106 so that "dead 
zones" or areas of low gas velocity near the plasma head are avoided. This 
in turn minimizes any tendency for crustlike deposits to build up on the 
reactor tube inner wall 104 or the plasma head 106, which could embrittle 
the head and/or plug up outlet ports of the feed lines 108. 
Practice of one embodiment of the invention is illustrated by the following 
tests conducted to demonstrate feasibility of the process for recovering 
tin from smelter dust. A plasma reactor was provided similar to the 
reactor 20 shown in FIG. 1, though lacking the outlet line 94 and the slag 
removal port 98. The reactor included a water-cooled plasma head having a 
tungsten-tipped copper cathode and an anode comprising a disk-shaped 
copper ring surrounded by a stainless steel block. The anode and cathode 
were separated from each other by a teflon spacer and were connected to a 
D.C. power source having an available power rating of 85-90 KW. The plasma 
head had a single feed tube extending therethrough to direct flue dust and 
carrier gas into a reaction chamber at approximately a 45-degree angle 
with respect to the vertically-oriented reaction chamber 72. The reactor 
tube below the plasma head comprised a cylindrical tube of graphite having 
an outer diameter of about 5 inches and a length of about 30 inches and 
wrapped with layers of graphite felt insulation. The tube also had, in 
approximate dimensions, an inner diameter of 31/2 inches at its upstream 
end adjacent to the plasma head, tapering to an inner diameter of 2 inches 
in the first 5 inches of length and remaining at this diameter over its 
remaining length. A graphite receptacle was used as a collection vessel, 
and in the tests liquid tin product was allowed to solidify in the vessel. 
After each test run the vessel was removed and the product remelted to 
separate tin from slag. 
Approximately one hundred pounds of tin smelter dust was obtained for 
testing of the above-described embodiment of the invention. Analysis of a 
random sample of the powder revealed a tin content of 72.0 percent by 
weight. The sample also contained the following percentages of other 
metals--tungsten (3.0), iron (0.7), zinc (0.4), aluminum (0.2), and 
calcium (0.1). 
Various parameters and results of three test runs are set forth in Table 1. 
The data on tin yield of Table 1 (Run No. 3) shows that a recovery of 94.5 
percent of the tin present in smelter dust was achieved using the process 
of the invention. The yield computation included an assumption that tin 
content of the dust was 72.0 percent, i.e., the value measured in the 
random sample analyzed, and the amount of dust processed was based on the 
weighed amount consumed by the plasma reactor rather than being determined 
from the approximate dust feed rate and run length set forth in Table 1. 
TABLE 1 
______________________________________ 
Run No. 1 2 3 
______________________________________ 
Plasma gas hydrogen* 
hydrogen* hydrogen* 
Plasma gas 300 300 300 
flow rate (SCFH) 
Plasma gas 40 40 40 
pressure (psi) 
Carrier gas hydrogen hydrogen & hydrogen 
natural gas** 
Carrier gas 5 5 5 
pressure (psi) 
Carrier gas 100 100 100 
feed rate (SCFH) 
Dust feed rate 
10 12 20 
(lb/hr) 
Reactor power 
85 90 87 
rating (KW) 
Length of run 
30 30 30 
(min) 
Weight of product 
2.5 3.5 6 
ingot (lb) 
Tin content of 
99.4 99.7 99.5 
product ingot 
(% by weight) 
Yield of tin 
NM*** NM*** 94.8 
(% by weight) 
______________________________________ 
*A small amount of argon (about 1/2-1%) was included as stabilizer gas 
**In approximately equal volumes 
***NM = not measured precisely (but yields were high) 
In the test runs no fluxing agents were added to the feed (dust). However, 
it is considered within the scope of the invention and desirable in 
commercial practice that small amounts of fluxing agents such as lime, 
borosilicate glass, or calcium fluoride be added to the flue dust to 
facilitate separation of slag from tin in the products of the plasma 
reaction process and to increase tin yield. Also, in an actual large scale 
recovery process it is desirable that slag be recycled into a smelting 
furnace and scrubber dust recycled into the plasma reactor or the furnace, 
which should also increase overall recovery of tin. 
Table 1 also shows that after separation of slag from the tin product of 
the plasma reactor, tin content of the resulting ingot was as high as 99.7 
percent (Run No. 2). Thus the tin recovered utilizing the method of the 
invention is of a purity sufficient for certain uses without further 
refining, and is equal or better in purity than tin produced in some 
typical smelting furnaces. Additional details of the results of the 
chemical analysis of the ingot of Run No. 1 are given in Table 2, which 
shows the major constituents of the remaining 0.6 percent of the ingot to 
be lead and iron. 
TABLE 2 
______________________________________ 
Element Percent (by weight) 
______________________________________ 
Sn 99.4 
Fe 0.055 
Si &lt;0.001 
Sb &lt;0.01 
Mg &lt;0.001 
Pb 0.45 
In 0.005 
Ni 0.010 
Bi 0.002 
Al &lt;0.001 
Cu 0.015 
Ag &lt;0.001 
______________________________________ 
Analysis of the slag separated from the tin product in Run No. 1 revealed a 
tungsten content of 14.2 percent as compared with a tungsten content of 
about 3 percent in the smelter dust. The process of the invention thus can 
be used not only in recovering tin but also in recovering tungsten as a 
by-product. 
In addition to providing high yields of good quality tin from smelter dust 
as demonstrated by the above-described test results, the process of the 
invention can effect this recovery at a cost quite attractive compared to 
the cost of tin. For example, for a plasma reactor system employing the 
disclosed process and of a size sufficient to recover 150 lbs. of tin per 
hour, energy and material (hydrogen and argon) costs of the process are 
conservatively estimated as only five to seven percent of present market 
price of tin (over five U.S. dollars per pound). 
Accordingly, there has been described an improved method of recovering tin 
from tin smelter dust wherein tin monoxide is reduced to liquid tin in a 
plasma reactor. The method avoids any need for pelletizing or other 
preparation of the smelter dust prior to introduction of the dust into the 
reactor, provides high yields of metal containing up to about 99.7% tin, 
is readily usable as a continuous process, and permits recovery at 
operational costs far below current market prices for tin.