Process for sintering lead concentrates

An improved process for increasing (1) the removal of sulfur from lead concentrates and (2) the ratio of fresh, lead concentrate in the feed to a conventional lead concentrate sintering machine. Oxygen is added to the air of the updraft ignition section in predetermined amounts coupled with predetermined reductions and finally, termination of oxygen enrichment at a predetermined distance from the point of downdraft ignition. Additional cooling is supplied by use of finely dispersed, liquid water introduced into preselected wind boxes at predetermined positions downstream of the termination of oxygen enrichment.

BACKGROUND 
1. Field of the Invention 
This invention relates to the sintering of lead concentrates and, more 
particularly, to an improved process for sintering lead concentrates using 
programmed oxygen enrichment coupled with the use of fog nozzles in 
selected wind boxes to obtain the highest permissible sintering 
temperatures for increased production while achieving a relatively high 
degree of desulfurization. 
2. The Prior Art 
For the past several decades, the pyrometallurgical production of lead from 
lead sulfide concentrates has been accomplished by (1) the autogenous 
updraft sintering of the lead sulfide concentrates for sulfur removal 
followed by (2) blast furnace smelting of the sinter using coke as the 
fuel. With particular reference to the schematic illustration of a 
conventional sintering process shown in FIG. 1, the sintering operation 
takes place in a conventional sintering machine wherein an initial, thin 
layer (about 30 millimeters) of balled mix (having a ratio of about 137 kg 
concentrates, 76 kg recycle slag, and 263 kg return sinter) is laid on the 
moving grate of the sinter machine. The grate is continuous and moves to 
the right at a rate of about 1 meter per minute. This initial layer is 
heated to ignition temperature by downwardly fired oil or gas burners. The 
combustion gases from the burners are drawn downwardly through the layer 
and removed. 
Immediately following the ignition position, the remainder of the mix bed 
is spread on top of the ignition layer to a total height of about 280 
millimeters. Air is blown upwardly through both the hot, ignition layer 
and the overlying fresh mix. Heat from the hot, ignition layer is 
sufficient to commence ignition of the adjacent material with the 
resulting hot combustion gases passing upwardly through the balance of the 
fresh mix. The upwardly moving, hot gases, depleted in oxygen, rise from 
the ignition zone to enter the balance of the fresh mix, preheating and 
drying the same. As the bed moves to the right, the region of ignition 
forms a reaction band that moves upwardly through the bed resulting in a 
band of reacting material located generally diagonally through the moving 
bed. The thickness of the reacting band depends upon the reaction rate. At 
a point a few minutes after ignition, air rising upwardly through the 
grate is heated as it passes through the reacted sinter and cools the same 
prior to entering partially reacted mix and fresh mix to react therewith, 
passing thence into additional unreacted material which is dried and 
preheated by the reaction gases. 
The gas issuing from the top of the bed contains oxides of sulfur and is 
collected and sent to the acid plant where the sulfur oxides are converted 
into sulfuric acid. Approximately 84 percent of the sulfur is removed by 
this conventional sintering process. 
Great care must be taken to prevent excessive temperature rise in the 
sintering process since temperatures above about 1300 K. result in the 
production of excessive amounts of liquid oxides which plug the 
interstices and impede the passage of oxidizing gases. Importantly, the 
initial composition of the charge to the sintering machine is carefully 
predetermined in order to control the temperature that is reached in the 
sintering process. For example, the relatively high ratio of cooled, 
return sinter (263 kg) assures that the richness of the charge will be 
sufficiently diluted to maintain temperatures below the 1300 K. limit. The 
cooled sinter also (1) provides the necessary porosity for the mix and (2) 
acts as a heat sink. The recycled slag (76 kg) serves as a flux for the 
process. 
It would, therefore, be an advancement in the art to provide improvements 
in the process for sintering lead concentrates. It would also be an 
advancement in the art to increase the percentage of sulfur removed from 
the sinter. Another advancement in the art would be to provide 
improvements in the process for sintering lead concentrates at an 
increased rate of production. Such a novel, improved process is disclosed 
and claimed herein. 
BRIEF SUMMARY AND OBJECTS OF THE INVENTION 
The present invention relates to novel improvements in the sintering 
process for increasing sulfur elimination to about 95 percent. The 
conventional sintering process is improved by the carefully controlled use 
of oxygen enrichment at a rate to avoid excessive temperature rise while 
preventing overheating by adding atomized water to certain of the wind 
boxes beneath the grate of the sintering machine when temperatures will 
otherwise be too high. The improved process thereby accommodates an 
increased rate of production, the increased rate of production tending to 
offset the additional cost of the oxygen needed to maximize sulfur removal 
from the sinter. 
It is, therefore, a primary object of this invention to provide 
improvements in the process for removing sulfur from lead concentrates. 
Another object of this invention is to provide an improved process for 
sintering lead concentrates and removing sulfur therefrom by the carefully 
programmed addition of oxygen. 
Another object of this invention is to provide an improved process for 
preventing overheating of the sinter in a sintering machine by means of 
cooling with atomized water added to preselected wind boxes in the 
sintering machine. 
These and other objects and features of the present invention will become 
more fully apparent from the following description and appended claims 
taken in conjunction with the accompanying drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is best understood by reference to the drawing in combination 
with the following description. 
General Discussion 
Sulfur is present in lead sinter in two forms: as (a) sulfate, either 
PbSO.sub.4 or basic sulfates PbSO.sub.4.nPbO where n is 1, 2, or 4, and as 
(b) unreacted sulfide. The oxidation of the lead sulfide at low 
temperatures leads to sulfate formation; at intermediate temperatures to 
basic sulfates; and at high temperatures to lead oxide or metallic lead. 
This temperature/composition relationship is shown more fully in FIG. 2 
wherein it is assumed that the sulfur dioxide pressure approximates 0.2 
atmospheres and air is used as the oxidant. Referring particularly to FIG. 
2, if a particle of lead sulfide, PbS, is exposed to an oxidizing 
atmosphere, the products formed will be found by following the upper 
boundary of the field that is labeled PbS. Thus, for temperatures below 
about 670.degree. C., the product formed is lead sulfate, PbSO.sub.4, and 
no sulfur is removed. Between 670.degree. and 730.degree. C., half of the 
sulfur in PbS is eliminated, forming PbSO.sub.4.PbO. From 730.degree. to 
800.degree. C., two-thirds of the sulfur is removed forming 
PbSO.sub.4.2PbO. Between 800.degree. and 830.degree. C., four-fifths, of 
the sulfur is eliminated with the most basic sulfate, PbSO.sub.4.4PbO 
being the product. Above 830.degree. C., metallic lead is the equilibrium 
product, but the oxygen content in equilibrium with it is practically 
zero, meaning that any excess oxygen will result in PbO being formed as a 
secondary reaction product above 920.degree. C. where Pb and PbO are in 
equilibrium. Thus, metallic lead is only found occasionally in lead sinter 
where there is a slight deficiency in the stoichiometric oxygen supplied 
for reaction with PbS. 
From the foregoing, it is clear that the proportion of the sulfur in the 
concentrate that is removed depends upon the temperature reached where the 
sulfide and the oxidant come in contact. One technique for modifying the 
temperature locally in a section of the sinter strand would be the use of 
carefully controlled, predetermined oxygen enrichment at different levels 
in the wind boxes. Advantageously, it was discovered that even the 
addition of 100 percent oxygen following ignition with air (21 percent 
oxygen) did not result in excessive temperatures in the first wind boxes. 
With reference to Table I, a comparison was made of the best use of a 
limited amount of commercially pure oxygen to enrich the air in the wind 
boxes of each succeeding section of the sintering machine downstream of 
the ignition section. The conclusion obtained from the foregoing is that 
it is definitely better to enrich the section immediately following the 
ignition section. However, splitting the oxygen amongst the two sections 
following ignition seems to be an even greater improvement while the 
latter two options illustrated in Table I are not as good, although 
showing better desulfurization than use in the ignition batch alone. 
TABLE I 
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BEST USE OF A LIMITED AMOUNT OF COMMERCIALLY 
PURE OXYGEN 
Total amount of enrichment oxygen 0.025 kg per 
100 kg of new Pb, distributed in several ways 
to sinter strand sections of 1.2 m 
Overall Sulfur 
Percent Oxygen Removed 
by Volume Percent of 
in Wind to Each Section Sulfide 
Ignition 
1 2 3 4 Balance 
Charged 
______________________________________ 
100 21 21 21 21 21 87.6 
21 100 21 21 21 21 89.7 
21 34.7 34.7 21 21 21 91.5 
21 28.5 28.5 28.5 21 21 89.1 
21 26.2 26.2 26.2 26.2 21 89.1 
______________________________________ 
The principal effects of oxygen enrichment in the ignition section is that 
the enrichment increases the temperature of both the gas and the solid. 
However, the heat loss in the gas is decreased because the volume of gas 
is lower because of the decrease in its overall nitrogen content. As a 
consequence of increasing the ideal temperature (a) the fraction reacted 
is calculated to be higher, (b) the reaction rate is also assumed to be 
proportional to oxygen content, causing a further increase in fraction 
reacted. The overall effect is to decrease the remaining unreacted 
sulfide. However, this more complete reaction in the ignition zone is not 
necessarily an unmitigated blessing since it decreases the fuel supply to 
the next or updraft sections of the sinter strand. 
Experimental runs were conducted using 70 percent oxygen initially 
following ignition at 21 percent oxygen and then judiciously decreasing 
the oxygen percentage so that temperatures of 1300 K. were not exceeded. 
If the temperature limitation of 1300 K. is accepted, maximum permissible 
enrichment would be approximately 60 percent beyond which so much melting 
would be likely to occur that porosity of the sinter strand would be lost. 
Referring now more particularly to FIG. 3, the results obtained using 
oxygen-enriched air are compared with a standard run using air only. The 
bottom curve represents the oxygen content of the wind in both cases 
plotted against meters after the end of ignition. The run using air only 
is represented as a straight line at 21 percent and the run using 
oxygen-enriched air is shown as a stepwise reduction in oxygen content 
representative of 70, 60, 50, 40, 24, and 21 percent oxygen by volume. The 
upper two curves are for temperature as a function of height above the 
grate while the middle two curves represent the removal of sulfur also as 
a function of height above the grate. 
Temperatures for the oxygen-enriched air are uniformly higher than for the 
standard run and remain just below 1300 K. It was found that oxygen could 
not be further enriched to increase temperatures at the beginning since 
the temperatures tended to exceed 1300 K further along the sinter strand. 
As shown, the sulfur removal was also much better using oxygen-enriched 
air than for the standard run, averaging 96.6 percent sulfur removal as 
compared to 84.4 percent when air only was used. 
In both sets of upper curves, a sharp dip is noticed in one line of each 
set of curves. This sharp dip represents the sharp change experienced when 
the upper, thick layer (about 250 millimeters) of cold, moist feed is 
spread on top of the ignition layer at a position about one meter 
downstream of the commencement of the downstream ignition segment (see 
also FIG. 1). 
Referring also to FIG. 4, the relationship between sulfur removal and 
oxygen consumption is shown. The highest desulfurization achieved without 
exceeding 1300 K was 96.6 percent using the programmed oxygen addition 
system as shown in FIG. 3. However, it will be noted from FIG. 4 that 
additional sulfur can be removed although at the expense of a temperature 
above 1300 K. Advantageously, the combination of programmed oxygen 
addition, as shown, in combination with selective cooling allowed for a 10 
percent increase in concentrate content of the mix. In particular, efforts 
were directed toward selectively removing heat from those portions of the 
sinter strand where temperatures tended to be excessive so that the 
production rate might be increased by using a charge richer in 
concentrates. However, if grate speed were increased without changing the 
wind supply to each wind box, the total length available for subsequent 
cooling (see FIG. 1) would correspondingly decrease. Advantageously, if 
extra cooling was supplied, it was found possible to still discharge at 
normal temperature thus increasing productivity without adversely 
affecting sulfur removal. 
It was determined that the additional cooling could be obtained by the use 
of finely dispersed liquid water added to the current of wind blown 
through the sinter strand on the grate. A conventional fog nozzle was 
found to be the appropriate equipment for such an application. 
Accordingly, it was determined that fog nozzles should be installed in the 
wing boxes lying below regions where excessive temperatures were known to 
occur. In passisng through the previously reacted materials, the water 
particles are evaporated so that the heat content of the wind reaching the 
hot areas will be lowered by the energy needed to evaporate the water. The 
overall reaction temperatures will be lowered thereby. 
While the precise amount of water addition necessary can be readily 
calculated, the following considerations may assist in the calculation. 
For example, with 100 percent of excess air (over and above that 
theoretically required for oxidation of sulfides), as used in normal 
operating procedures, saturation at 25.degree. C. would yield three 
percent steam by volume, or 0.03 mole fraction. Since about 3 moles of air 
are blown per 100 kg of mix, saturated air would contain about 0.1 moles 
of steam per 100 kg mix. Accordingly, in the absence of a fog nozzle or 
other suitable equipment, it would not be possible to add twice this 
amount, or 0.2 moles of water, and keep it suspended long enough to reach 
layers of the bed where it can be vaporized. 
Referring particularly to Table II, sinter bed cooling using fog nozzles in 
selected wind boxes is shown. Advantageously, the average sulfur removed 
was about 96.7 percent. These results indicate that the usage of fog in 
the amounts suggested will allow a 10 percent increase in production of 
the sinter machine together with removal of about 96.7 percent of the 
sulfur initially charged. If a smaller sulfur elimination figure is 
acceptable, 95 percent of the sulfur can be eliminated by using 0.06 kg 
oxygen per kg of sulfide sulfur. This is accomplished by replacing 
oxygen-enriched air, Table II, with ordinary air after 3.6 meters, and 
using fog cooling in the amount of 0.1 moles per 100 kg mix in the region 
8.4 to 12.0 meters after ignition. 
TABLE II 
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SINTER BED COOLING USING FOG NOZZLES IN 
SELECTED WIND BOXES 
Concentrate Level 110% of Normal. Oxygen Usage 0.085 kg/kg 
Pb in Concentrates 
Distances Fog Added 
from Percent (Mols H.sub.2 O 
Maximum % of 
Ignition Oxygen Per 100 Temperature 
Sulfur 
Position (m) 
in Wind kg Mix) (Degrees K) 
Removed 
______________________________________ 
0 to 1.2 
70 0 1246 99.3 
1.2 to 2.4 
65 0 1245 99.2 
2.4 to 3.6 
55 0 1291 99.5 
3.6 to 4.8 
34 0 1302 98.5 
4.8 to 6.0 
28 0 1293 96.4 
6.0 to 7.2 
23 0 1293 94.0 
7.2 to 8.4 
21 0.1 1292 93.8 
8.4 to 9.6 
21 0.2 1289 94.5 
9.6 to 10.8 
21 0.1 1307 95.3 
10.8 to 24.0 
21 0 falling -- 
______________________________________ 
Average sulfur removed 96.7% 
While the amounts of oxygen needed for significant improvements in 
desulfurization is likely to be minimal with respect to cost per kg of 
lead in the finished sinter, the overall cost calculated on the basis of 
tonnages becomes significant. This significant increase in the overall 
production cost may be at least partially offset by the expedient of 
increasing the percentage of concentrates in the sinter machine feed as an 
attractive measure to offset the foregoing increased oxygen costs. 
Accordingly, increased grate speed coupled with fog cooling and oxygen 
enrichment not only results in improved desulfurization but also an 
increased production rate of the particular sinter machine. 
In summary, the variables explored indicate that it would be difficult to 
obtain in excess of 97.5 percent elimination of sulfur with the best 
results obtained indicating about 96.6 percent elimination by the use of 
programmed oxygen-enrichment using a normal concentrate level while 
maintaining the maximum temperature in any part of the sinter bed below 
1300 K. The same degree of sulfur elimination was obtained by increasing 
concentrate level to 110 percent of normal while using programmed 
oxygen-enrichment in the first 7.2 meters after ignition and fog additions 
of 0.1 to 0.2 moles per 100 kg mix in the region 7.2 to 10.8 meters after 
ignition. Correspondingly, if sulfur elimination of 95 percent is 
adequate, then this also requires oxygen additions of about 0.08 kg oxygen 
per kg of new lead rather than the 0.11 required for 96.6 percent removal. 
With concentrate level increased to 110 percent of normal, 95 percent 
sulfur removal was obtained with oxygen usage of 0.06 kg and a lesser 
amount of fog cooling. 
The invention may be embodied in other specific forms without departing 
from its spirit or essential characteristics. The described embodiments 
are to be considered in all respects only as illustrative and not 
restrictive and the scope of the invention is, therefore, indicated by the 
appended claims rather than by the foregoing description. All changes 
which come within the meaning and range of equivalency of the claims are 
to be embraced within their scope.