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CHAPTER
II.
ON THE BURNING OF LIMESTONE IN THE LARGE WAY.
25. LIMESTONE becomes lime on being deprived
of its carbonic acid, and of the water it contained,
whether
hygrometrically or in combination.
agent employed to effect this is heat.
The
(App. V I I I . )
26. With the same heat, the calcination is effected
with more ease and rapidity in proportion as the stone
is of a less compact texture, to the smallness in bulk
of the fragments into which it is reduced, and to its
being impregnated with a certain degree of humidity.
(App. I X . )
27. The contact of the air is not indispensable,
but it exercises a useful influence, especially in regard
to argillaceous limestone.
Moreover, no limestone can
be converted into lime in a vessel so close as to render
the escape of the carbonic acid impossible.
(App. X . )
28. Limestone which is pure, or nearly so, supports
a
a white heat without inconvenience ; the compound
limestone, on the other hand, alloyed in the propor­
tions necessary to form hydraulic or eminently hy­
draulic lime, fuses easily.
certain precautions :
a
U n d e r the
Its calcination demands
the heat ought never to be
intense heat o f the h y d r o - o x y g e n b l o w p i p e this
substance affords the brilliant light, the
beautiful application o f
which to the m i c r o s c o p e is n o w so well k n o w n . — T R .
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14
BURNING
[CHAP.
OF L I M E S T O N E
pushed beyond the common red heat, the intensity
being made up for by its duration.
29. The compound limestone, when too much burnt,
is heavy, compact, dark-coloured, covered with a kind
of enamel, especially about the
angular parts;
it
slakes with great difficulty, and gives a lime carbon­
ized and without energy; sometimes it will not slake
at all, but becomes reduced, after some days exposure
to the air, to a harsh powder altogether inert.
b
30. The pure and compound limestones when in­
sufficiently burnt, either refuse to slake, or slake only
partially, leaving a solid kernel, a kind of sub-carbo­
0
nate with excess of base, which possesses properties
of which I shall have to speak elsewhere.
b
(App. X I . )
This is called dead lime, and should b e carefully distinguished
from the sub-carbonates, o r unburnt limes, mentioned in the next
paragraph.
M r . A n d r e w s ( M e c h a n i c s ' Magazine, N o . 1 2 6 ) notices
four cases in which dead lime m a y b e f o r m e d : 1st, when the lime­
stone contains m u c h alumina, and it has been heated so as to harden
the mass; 2nd, when the limestone contains silica, and it has been
strongly heated after the expulsion o f the carbonic acid ; 3rd, when
the limestone is so strongly heated as to b e in some measure melted,
in w h i c h case the carbonic acid is frequently not entirely separated
from the central parts o f the large lumps, and they o f course effer­
vesce with a c i d s ; 4-th, when the limestone is strongly heated
after
the total separation o f the c a r b o n i c acid, so that the lime neither splits
when water is added to it, nor does it effervesce with a c i d s . — T R .
c
A sub-carbonate is a combination o f one equivalent o f any
alkali with less than a full equivalent o f carbonic acid.
Thus a
combination o f one equivalent o f lime, with half an equivalent o f
c a r b o n i c acid ( i f such a c o m p o u n d could b e f o r m e d ) , w o u l d b e a
sub-carbonate o f lin*e.
T h e meaning o f the term equivalent, is the
definite proportion o f acid required to neutralise exactly a given
quantity o f alkali, o r o f alkali t o neutralise a given quantity o f a c i d .
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«•]
IN
THE
LARGE W A Y .
15
31. The calcining of calcareous minerals consti­
tutes the art of the lime-burner. According to situa­
tion, either firewood, faggots, brushwood, turf, or coal
is used.
32. It would be tedious, as well as useless, to de­
scribe in this place all the limekilns which have
been suggested or tried within the last few years.
I shall content myself with saying, that the forms of
interior most generally adopted are, 1st, the upright
rectangular prism (pi. I. figs. 1 and 2 ) ; 2nd, the cy­
linder (pi. I. figs. 3 and 4 ) ; 3rd, the cylinder sur­
mounted by an erect cone slightly truncated (pi. I. fig.
5 ) ; 4th, a truncated inverted cone (pi. I. figs. 7 and
8 ) ; 5th, an ellipsoid of revolution variously curvated,
or egg-shaped kiln. (PI. I. figs. 7> 8, 11, and 12.)
33. The rectangular kilns are in use in Nivernais,
and in the south of France: they burn in them, at
the same time, limestone and bricks. The limestone
occupies very nearly the lower half of its capacity.
The upper is filled with bricks, or tiles, laid and
packed edgeways.
34. The cylindric kilns are principally employed
upon works which consume a large quantity of lime
in a short time. They are termed " field-kilns"
(*' fours de campagne"); their construction is expe­
ditious and economical, but precarious. Above a
pointed ( " ogive") or oven-shaped vault, they raise, in
the form of a tower, a high stack of limestone, which
they enclose by a curtain of rammed earth, and supT h u s 2 2 grains o f c a r b o n i c acid c o m b i n e with, and exactly neutralise
2 8 grains o f lime ; the equivalents o f carbonic acid and lime, therefore,
are always in the proportion to one another o f 2 2 : 2 8 . — T R .
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16
BURNING
OF LIMESTONE
[CHAP.
port outwardly by a coarse wattling, in which care is
taken to leave an opening to introduce the fire be­
neath the vault.
35. The kilns of the third kind are constructed
in a solid and durable manner, like the four-sided
kilns: no bricks are burnt in these;
the largest
stones occupy the lower part of the cylinder, the
smaller pieces and fragments are thrown into the
cone which surmounts it.
36. The kilns of the fourth and fifth kind are spe­
cially intended for burning with coal.
37. The interior wall of the kiln is generally built
with bricks, or other material unalterable by heat,
cemented throughout a thickness of from thirty-two
to forty centimetres ( 1 2 to 15 inches nearly), with a
mixture of sand and refractory clay beaten together.
38. In the flare-kilns fed by logs or brushwood,
the charge always rests upon one or two vaults built
up dry with the materials of the charge itself.
Un­
derneath these vaults they light a small fire, which
they gradually increase as they retire, in proportion
as the draught establishes itself, and gains force.
On
reaching the exterior they adjust the aperture at the
eye of the kiln suitably, and then keep it constantly
filled with the combustible.
The air which rushes
in, carries the flame to a distance over every point of
the vaults : it insinuates itself by the joints, and is not
long in extending the incandescence by degrees to the
highest parts.
39. There are some kinds of stone which the fire,
however well regulated, seizes suddenly, and causes
to fly with detonation: we cannot, without running
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17
IN T H E LARGE W A Y .
II.]
the risk of spoiling the charge, use these for the
construction of the vaults and piers in loading the
kiln.
In such a case, we employ for that purpose
materials which are free from such an inconvenience.
40. Practice can alone indicate the time proper for
the calcination.
It varies with a multitude of cir­
cumstances, such as the more or less green, more or
less dry quality of the wood ;
the direction of the
wind, if it favour the draft, or otherwise, &c.
d
The
master burners usually judge by the general settling
of the charge, which varies from ^ to ^.
In a kiln
6
of the capacity of from 6 0 to 7 5 cubic metres, the
fire lasts from 100 to 150 hours : every cubic metre
of lime consumes (on an average) 1.66 steres in fire­
wood, 22.00 steres in faggots, and 3 0 steres in fascines,
furze, or other brushwood/ ( A p p . X I I . )
4 1 . In the coal kilns by slow heat, the stone and
coal are mixed.
Of all the methods of burning lime,
this is certainly the most precarious and difficult;
more especially when applied to the argillaceous lime­
stone.
A mere change in the duration or intensity
of the wind, any dilapidation of the interior wall of
d
General Treussart remarks concerning the burning o f the
Obernai lime, that b e i n g calcined with w o o d , it is very difficult t o
obtain a homogeneous result; there are always lumps o f lime w h i c h
are over-burnt, and others which are n o t sufficiently s o .
T h e lime-
burners, he adds, ought to keep up the calcination o f their lime for
a longer period, but with a less intense h e a t ; this w o u l d n o t c o n ­
sume more w o o d , and a m u c h better result w o u l d b e obtained.—
TR.
e
F r o m 211.8 to 2 6 4 . 7 5 c u b i c feet
f
A stere is equal t o o n e c u b i c metre, o r 35.3 English c u b i c
TR.
feet.—TR.
c
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18
BURNING
OF LIMESTONE
[CHAP.
the kiln, a too great inequality in the size of the frag­
ments, are so many causes which may retard or acce­
lerate the draft, and occasion irregular movements in
the descent of the materials, which become locked to­
gether, form a vault, and precipitate at one time the
coal, and another the stone, upon the same point;
hence an excess or deficiency in the calcination.
4 2 . Sometimes a kiln works perfectly well for many
weeks, and then all at once gets out of order without
any visible cause.
A mere change in the quality of
the coal is sufficient to lead the most experienced limeburner into error.
In a word, the calcination by
means of coal, and the slow heat, is an affair of cau­
tious investigation and habit.
(App. X I I I . )
4 3 . The capacity of a furnace contributes, no less
than does its form, to an equable and proper calcina­
tion.
There are limits beyond which we cannot en­
large it without serious evils.
W e have drawn and
marked in plate I., figs. 9 , 1 0 , 1 1 , and 1 2 , the sections
of four kilns executed and tried in the Department of
the Maine and Loire, by Messrs. Ollivier Brothers,
manufacturers of hydraulic limes.
Number 9 has been
abandoned in consequence of producing lime always
too much or too little burnt; number 1 0 answered
tolerably;
numbers 1 1 and 1 2 answered perfectly.
(App. X I V . )
4 4 . The bulk of coal burnt to produce a cubic
metre of lime, necessarily varies with the hardness of
the limestone used; but within narrow limits.
When
not referring to the chalks, or friable marles, we cal­
culate on an average upon three cubic metres of lime
from one of coal.
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45.
19
IN T H E LARGE W A Y .
II.]
The calcination of limestones presents other
important problems for solution; but the reasonings
of the closet, unaided by experiment, will always be
insufficient in a matter of this kind. It is for this rea­
son, that we here abstain from the discussion of a
number of projects more or less ingenious, but wholly
theoretical.
W e must beg the reader nevertheless to
refer to the notes; he will there find some details use­
ful to be known, when he is called upon to apply him­
self specially to the burning of lime.
(App. X V . )
c 2
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