BLOWERS AND BLOWING ENGINES
ROTARY BLOWERS VS. BLOWING ENGINES FOR LEAD SMELTING
(April 27, 1901)
A note in the communication from S. E. Bretherton on “Pyritic Smelting
and Hot Blast,” published in the _Engineering and Mining Journal_
of April 13, 1901, refers to a subject of great interest to lead
smelters. Mr. Bretherton remarked that he had been recently informed
by August Raht that by actual experiment the loss with the ordinary
rotary blowers was 100 per cent. under 10 lb. pressure; that is, it
was possible to shut all the gates so that there was no outlet for the
blast to escape from the blower and the pressure was only 10 lb., or in
other words the blower would deliver no air against 10 lb. pressure.
For that reason Mr. Raht expressed himself as being in favor of blowing
engines for lead blast furnaces. This is of special interest, inasmuch
as it comes from one who is recognized as standing in the first rank of
lead-smelting engineers. Mr. Raht is not alone in holding the opinion
he does.
The rotary blower did good service in the old days when the air was
blown into the lead blast furnace at comparatively moderate pressure.
At the present time, when the blast pressure employed is commonly
40 oz. at least, and sometimes as high as 48 oz., the deficiencies
of the rotary blower have become more apparent. Notwithstanding the
excellent workmanship which is put into them by their manufacturers,
the extensive surfaces of contact are inherent to the type, and
leakage of air backward is inevitable and important at the pressures
now prevailing. The impellers of a rotary blower should not touch
each other nor the cylinders in which they revolve, but they are made
with as little clearance as possible, the surfaces being coated with
grease, which fills the clearance space and forms a packing. This
will not, however, entirely prevent leakage, which will naturally
increase with the pressure. Even the manufacturers of rotary blowers
admit the defects of the type, and concede that for pressures of 5
lb. and upward the cylinder blowing engine is the more economical.
Metallurgists are coming generally to the opinion, however, that
blowing engines are probably more economical for pressures of 4 lb. or
thereabouts, and some go even further. With the blowing engines the
air-joints of piston and cylinder are those of actual contact, and
the metallurgist may count on his cubic feet of air, whatever be the
pressure. Blowing engines were actually introduced several years ago
by M. W. Iles at what is now the Globe plant of the American Smelting
and Refining Company, and we believe their performance has been found
satisfactory.
The fancied drawback to the use of blowing engines is their greater
first cost, but H. A. Vezin, a mechanical engineer whose opinions carry
great weight, pointed out five years ago in the _Transactions_ of the
American Institute of Mining Engineers (Vol. XXVI) that per cubic
foot of air delivered the blowing engine was probably no more costly
than the rotary blower, but on the contrary cheaper, stating that the
first cost of a cylinder blower is only 20 to 25 per cent. more than
that of a rotary blower of the same nominal capacity and the engine
to drive it. The capacity of a rotary blower is commonly given as the
displacement of the impellers per revolution, without allowance for
slip or leakage backward. Mr. Vezin expressed the opinion that for the
same actual capacity at 2 lb. pressure, that is, the delivery in cubic
feet against 2 lb. pressure, the cylinder blower would cost no more
than, if as much as, the rotary blower.
In this connection it is worth while making a note of the increasing
tendency of lead smelters to provide much more powerful blowers than
were formerly considered necessary, due, no doubt, in large measure to
the recognition of the greater loss of air by leakage backward at the
pressure now worked against. It is considered, for example, that a 42 ×
140 in. furnace to be driven under 40 oz. pressure should be provided
with a No. 10 blower, which size displaces 300 cu. ft. of air per
revolution and is designed to be run at about 100 r.p.m.; its nominal
capacity is, therefore, 30,000 cu. ft. of air per minute; although
its actual delivery against 40 oz. pressure is much less, as pointed
out by Mr. Raht and Mr. Bretherton. The Connersville Blower Company,
of Connersville, Ind., lately supplied the Aguas Calientes plant (now
of the American Smelting and Refining Company) with a rotary blower
of the above capacity, and duplicates of it have been installed at
other smelting works. The force required to drive such a huge blower
is enormous, being something like 400 h.p., which makes it advisable
to provide each blower with a directly connected compound condensing
engine.
In view of the favor with which cylindrical blowing engines for driving
lead blast furnaces are held by many of the leading lead-smelting
engineers, and the likelihood that they will come more and more into
use, it will be interesting to observe whether the lead smelters will
take another step in the tracks of the iron smelters and adopt the
circular form of blast furnace that is employed for the reduction
of iron ore. The limit of size for rectangular furnaces appears to
have been reached in those of 42 × 145 in., or approximately those
dimensions. A furnace of 66 × 160 in., which was built several years
ago at the Globe plant at Denver, proved a failure. H. V. Croll at
that time advocated the building of a circular furnace instead of the
rectangular furnace of those excessive dimensions and considered that
the experience with the latter demonstrated their impracticability. In
the _Engineering and Mining Journal_ of May 28, 1898, he stated that
there was no good reason, however, why a furnace of 300 to 500 tons
daily capacity could not be run successfully, but considered that the
round furnace was the only form permissible. We are unaware whether
Mr. Croll was the first to advocate the use of large circular furnaces
for lead smelting, but at all events there are other experienced
metallurgists who now agree with him, and the time is, perhaps, not far
distant when they may be adopted.
ROTARY BLOWERS VS. BLOWING ENGINES
BY J. PARKE CHANNING
(June 8, 1901)
In the issues of the _Engineering and Mining Journal_ for April
13th and 27th reference was made to the relative efficiency of
piston-blowing engines and rotary blowers of the impeller type, and in
these articles August Raht was quoted as saying that, with an ordinary
rotary blower working against 10 lb. pressure, the loss was 100 per
cent. I have waited some time with the idea that some of the blower
people would call attention to the concealed fallacy in the statement
quoted, but so far have failed to notice any reference to the matter. I
feel quite sure that Mr. Bretherton failed to quote Mr. Raht in full.
The one factor missing in this statement is the speed at which the
blower was run when the loss was 100 per cent.
The accepted method of testing the volumetric efficiency of rotary
blowers is that of “closed discharge.” The discharge opening of the
blower is closed, a pressure gage is connected with the closed delivery
pipe, and the blower is gradually speeded up until the gage registers
the required pressure. The number of revolutions which the blower
makes while holding that pressure, multiplied by the cubic feet per
revolution, will give the total slip of that particular blower at that
particular pressure. Experience has shown that, within the practical
limits of speed at which a blower is run, the slip is a function of
the pressure and has nothing to do with the speed. If, therefore, it
were found that the particular blower referred to by Mr. Raht were
obliged to be revolved at the rate of 30 r.p.m. in order to maintain a
constant pressure of 10 lb. with a closed discharge, and if the blower
were afterward put in practical service, delivering air, and were run
at a speed of 150 r.p.m., it would then follow that its delivery of air
would amount to: 150-30 = 120. Its volumetric efficiency would be 120
÷ 150 = 80 per cent. The above figures must not be relied upon, as I
give them simply by way of illustration.
About a year ago I had the pleasure of examining the tabulated results
of some extensive experiments in this direction, made by one of the
blower companies. I believe they carried their experiments up to 10 lb.
pressure, and I regret that I have not the figures before me, so that
I could give something definite. I do, however, remember that in the
experimental blower, when running at about 150 r.p.m., the volumetric
efficiency at 2 lb. pressure was about 85 per cent., and that at 3 lb.
pressure the volumetric efficiency was about 81 per cent.
It is unnecessary in this connection to call attention to the
horse-power efficiency of rotary blowers. This is a matter entirely
by itself, and there is considerable difference of opinion among
engineers as to the relative horse-power efficiency of rotary blowers
and piston blowers. All agree that there is a certain pressure at which
the efficiency of the blower becomes less than the efficiency of the
blowing engine. This I have heard placed all the way from 2 lb. up to 6
lb.
At the smelting plant of the Tennessee Copper Company we have lately
installed blast-furnace piston-blowing engines; the steam cylinders
are of the Corliss type and are 13 and 24 in. by 42 in.; the blowing
cylinders are two in number, each 57 × 42 in.; the air valves are all
Corliss in type. These blowing engines are designed to operate at a
maximum air pressure of 2½ lb. per square inch.
At the Santa Fe Gold and Copper Mining Company’s smelter we have
recently installed a No. 8 blower directly coupled to a 14 × 32 in.
Corliss engine. This blower has been in use about five months and is
giving very good results against the comparatively low pressure of 12
oz., or ¾ lb.
During the coming summer it is my intention to make careful volumetric
and horse-power tests on these two types of machines under similar
conditions of air pressure, and to publish the results; but in the
meantime I wish to correct the error that a rotary blower of the
impeller type is not a practicable machine at pressure over 5 lb.
BLOWERS AND BLOWING ENGINES FOR LEAD AND COPPER SMELTING
BY HIRAM W. HIXON
(July 20, 1901)
In the _Engineering and Mining Journal_ for July 6th I note the
discussion over the relative merits of blowers and blowing engines for
lead and copper smelting, and wish to state that, irrespective of the
work to be done, the blast pressure will depend entirely on the charge
burden in any kind of blast-furnace work, and that the charge burden
governs the reducing action of the furnace altogether. Along these
lines the iron industry has raised the charge burden up to 100 ft. to
secure the full benefit of the reducing action of the carbon monoxide
on the ore.
In direct opposition to this we have what is known as pyritic smelting,
wherein the charge burden governs the grade of the matte produced to
such an extent that if a charge run with 4 to 6 ft. of burden above the
tuyeres, producing 40 per cent. matte, is changed to a charge burden of
10 or 12 ft., the grade of the matte will decrease from 40 per cent. to
probably less than 20 per cent. I can state this as a fact from recent
experience in operating a blast furnace on heap-roasted ores under the
conditions named, with the result as above stated.
I was exceedingly skeptical about pyritic smelting as advocated by
some of your correspondents, and still continue to be so; but on
making inquiries from some of my co-workers in this line, Mr. Sticht
of Tasmania, and Mr. Nutting of Bingham, Utah, I have arrived at the
following conclusion, to which some may take exception: That pyritic
smelting without fuel, or with less than 5 per cent., with hot
blast, is practically impossible; that smelting raw ore with a low
charge burden to avoid the reducing action of the carbon monoxide,
thereby securing oxidation of the iron and sulphur, is possible and
practicable, under favorable conditions; and that a large portion of
the sulphur is burned off, and the iron, without reducing action,
goes into the slag in combination with silica. These results can be
obtained with cold blast.
A blowing engine would certainly be much out of place for operating
copper-matting furnaces run with the evident intention of oxidizing
sulphur and iron and securing as high a grade of matte as possible,
for the reason that to do this it is necessary to run a low charge
burden, and with a low charge burden a high pressure of blast cannot
be maintained. With a 4 to 6 ft. charge burden the blast pressure at
Victoria Mines at present is 3 oz., produced by a No. 6 Green blower
run at 120 r.p.m.; and a blowing engine, delivering the same amount
of air, would certainly not give more pressure. Under the conditions
which we have, a fan would be more effective than a pressure blower,
and a blowing engine entirely out of the question as far as economy is
concerned.
I installed blowing engines at the East Helena for lead smelting where
the charge burden was 21 ft. and the blast pressure at times went up
as high as 48 oz. Under these conditions the blowing engines gave
satisfaction, but I am of the opinion that the same amount of blast
could have been obtained under that pressure with less horse-power by
the best type of rotary blower. I do not believe that the field of
the blowing engine properly exists below 5 lb., and if this pressure
cannot be obtained by charge-burden conditions, their installation is a
mistake.
I wish to add the very evident fact that varying the grade of the matte
by feeding the furnace at different hights varies the slag composition
as to its silica and iron contents and makes the feeder the real
metallurgist. The reducing action in the furnace is effected almost
entirely by the gases, and when these are allowed to go to waste,
reduction ceases.
BLOWING ENGINES AND ROTARY BLOWERS—HOT BLAST FOR PYRITIC SMELTING
BY S. E. BRETHERTON
(August 24, 1901)
I have just read in the _Engineering and Mining Journal_ of July 20th
an interesting letter written by Hiram W. Hixon in regard to blowing
engines versus the rotary blowers, and also the use of cold blast for
pyritic smelting.
The controversy, which I unintentionally started in my letter in
the _Engineering and Mining Journal_ of April 13th last, about the
advantages of using either blowers or blowing engines for blast
furnaces, does not particularly interest me, with the exception that I
have about decided, in my own mind, to use blowing engines where there
is much back pressure, and the ordinary up-to-date blower for pyritic
or matte smelting where much back pressure should not exist. I fully
appreciate the fact that so-called pyritic smelting can be done to a
limited extent, even with cold blast. Theoretically, enough oxygen
can be sent into the blast furnace, contained in the cold blast, to
oxidize both the fuel and the sulphur in an ordinary sulphide charge,
but I have not yet learned where a high concentration is being made
with unroasted ore and cold blast. I experimented on these lines at
different times for three years, during 1896, 1897, and 1898, making
a fair concentration with refractory ores, most of which had been
roasted. I was myself interested in the profits and as anxious as any
one for economy. We tried, for fuel in the blast furnace, coke alone,
coke and lignite coal, lignite coal alone, lignite coal and dry wood,
coal and green wood, and then coke and green wood, all under different
hights of ore burden in the furnace.
A description of these experiments would, no doubt, be tiresome to your
readers, but I wish to state that the furnace was frozen up several
times on account of using too little fuel, when the cold blast would
gradually drive nearly all the heat to the top of the furnace, the
crucible and between the tuyeres becoming so badly crusted that the
furnace had to be cleaned out and blown in again, unless I was called
in time to save it by changing the charge and increasing the fuel. We
were making high-grade matte under contract, high concentration and
small matte fall, which would, no doubt, aggravate matters.
After the introduction of hot blast, heated up to between 200 and 300
deg. F., we made the same grade of matte from the same character of
ore, with the exception that we then smelted without roasting, and
reduced the percentage of fuel consumption, increased the capacity of
the furnace, and almost entirely obviated the trouble of cold crucibles
and hot tops. I write the above facts, as they speak for themselves.
I nearly agree with Mr. Hixon, and do not think it practical to smelt
with much less than 5 per cent. coke continuously; but there is a
great saving between the amount of coke used with a moderately heated
blast and cold blast. Regardless of either hot or cold blast, however,
the fuel consumption depends very much on the character of the ore
to be smelted, the amount of matte-fall and grade of matte made. It
is not always advisable or necessary to use hot blast for a matting
furnace; that is, where the supply of sulphur is limited. It may then
be necessary to use as much fuel in the blast furnace to prevent the
sulphur from oxidizing as will be sufficient to furnish the heat for
smelting. Such conditions existed at Silver City, N. M. , at times,
after our surplus supply of iron and zinc sulphide concentrates was
used. I understand that they are now short of sulphur there, on account
of getting a surplus amount of oxidized copper ore, and are only
utilizing what little heat the slag gives them, without the addition of
any fuel on top of the forehearth.
Before closing this, which I intended to have been brief, I wish to
call your attention to a little experience we had with alumina in the
matting furnace at Silverton, Col., where I was acting as consulting
metallurgist. The ore we had to smelt contained, on an average, about
20 per cent. Al₂O₃, 30 per cent. SiO₂, with 18 per cent. Fe in
the form of an iron pyrite, and no other iron was available except some
iron sulphide concentrates containing a small percentage of zinc and
lead.
The question naturally arose, could we oxidize and force sufficient
of the iron into the slag, and where should we class the alumina, as
a base or an acid? My experience in lead smelting led me to believe
that Al₂O₃ could only be classed as an acid in the ordinary
lead furnace, and that it would be useless to class it otherwise in
a shallow matting furnace; and E. W. Walter, the superintendent and
metallurgist in charge, agreed with me.
We then decided to make a bisilicate slag, classing the alumina as
silica, and we obtained fairly satisfactory results. The slag made
was very clean, but treacherous, which was attributed to two reasons:
First, that it required more heat to keep the alumina slag liquid
enough to flow than it does a nearly straight silica slag; and, second,
that we were running so close to the formula of a bisilicate and
aluminate slag (about 31½ per cent. SiO₂, 27 per cent. Fe, 20 per
cent. CaO, and 18 per cent. Al₂O₃, or 49½ per cent. acid) that a
few charges thrown into the furnace containing more silica or alumina
than usual would thicken the slag so that it would then require some
extra coke and flux to save the furnace. At times the combined SiO₂
and Al₂O₃ did reach 55 and 56 per cent. in the slag, which did
not freeze up the furnace, but caused us trouble.