THOMAS-PHOSPHATE OR BASIC SLAG.
In this substance we have a most important addition to our phosphatic
manures. It has been in the market since 1886, and the consumption alone
in Germany in 1887 amounted to nearly 300,000 tons. In this country it
is only now beginning to be used to any extent.
_Its Manufacture._
_Thomas-slag_ is a bye-product obtained in the manufacture of steel by
what is known as the "basic" process. In the year 1879 an improvement in
the well-known "Bessemer" process was patented by Messrs Gilchrist &
Thomas. It must be explained that in the manufacture of steel from
pig-iron certain impurities in the raw material have to be got rid of in
order to produce a good steel. Among these impurities one of the most
important is _phosphorus_. This is owing to the fact that even a very
small percentage of phosphoric acid in steel has the effect of
rendering it brittle. The extraction of the phosphorus from the raw
material was formerly, however, attended with very serious difficulties,
and had the effect naturally of rendering steel a costly article,
inasmuch as only the purer kinds of pig-iron could be used for the
purpose.
By the introduction in 1879, however, of the "Thomas-Gilchrist" or
"basic" process, these difficulties were very largely overcome, and the
employment of even such impure irons as the Cleveland (containing
comparatively a large percentage of phosphorus) was rendered possible,
and the price of steel consequently generally very much reduced. The
process consists of submitting the molten pig-iron to a very great heat
in a pear-shaped vessel (known technically as the "converter"). This is
open at the top, and is supported on hinges, which permit of its being
moved so as to pour off the scum which rises to the surface at the end
of the operation, and which, we may explain, consists of "basic slag."
In the original process the sides of the "converter" were lined with
fire-bricks, consisting largely of silica. This process was known as the
"acid" process. In the "Thomas-Gilchrist" process, however, the sides of
the "converter" are lined with _lime_ (dolomitic limestone being largely
used), lime being also added to the pig-iron. An air-blast is injected
through the molten mass, and the impurities are burnt, or oxidised as it
is chemically termed. The phosphorus in the iron is thus converted into
phosphoric acid, and, uniting with the lime, forms phosphate of lime,
which rises, as we have already said, to the surface in the form of a
scum, and is separated from the steel by being poured off.
_Not at first used._
This, then, is how the _Thomas-slag_ is obtained. It did not seem,
however, for some years after the introduction of this ingenious
process, to have struck any one that this rich phosphatic bye-product
might prove a valuable addition to our artificial fertilisers. The
result was, that the Thomas-slag was treated as another of the only too
numerous valueless bye-products which seem to be necessarily incidental
to most of our chemical and other manufactures, and was allowed to
accumulate in large quantities without being used for any purpose.
_Discovery of its Value._
In 1883 some short articles published in Germany on the subject were the
means of first drawing the attention of the public to its importance as
a manure. During the years 1884 and 1885 numerous experiments were
carried out on the subject in the same country; and from then up till
the present hour it has become more and more extensively used in
Germany, till in 1887, as already stated, its consumption amounted to
nearly 300,000 tons.
_Composition._
It consists mainly of phosphate of lime, silicate of lime, free lime,
free magnesia, and oxides of iron and manganese. Its composition, of
course, naturally varies; but the following may be taken as an average
analysis:[233]--
Per cent.
*Phosphoric acid 17
Lime in combination with phosphoric, silicic,
sulphuric, and carbonic acids 40
Free lime 15
Oxides of iron 12
*Equal to tricalcic phosphate 37
As a rule, the phosphoric acid varies considerably, ranging from 10 to
20 per cent--that is, from 22 to 44 per cent tricalcic phosphate. This
is owing to the difference in the percentage of phosphorus in the raw
material and the quantity of lime added. Attempts have been made in
Germany during the last two or three years to obtain a slag richer in
phosphoric acid than that obtained heretofore, and a process for this
purpose has been patented by Professor Scheibler. This consists of a
slight modification in the ordinary process. Instead of treating the
pig-iron with an excessive quantity of lime, the amount added is not
sufficient to effect the complete dephosphorisation of the iron. The
resulting slag is very rich in phosphoric acid, and is correspondingly
poor in iron. The iron is then again treated with fresh lime, and the
phosphorus completely removed, while the same lime may be used over
again. Such slag forms a very much more concentrated phosphatic manure
than the ordinary slag, and is known as _patent phosphate meal_.
A point which not only renders the slag a product of peculiar interest
from a chemical point of view, but has a most important bearing on its
value as a manure, is the nature of the compound formed by the union of
the lime with the phosphoric acid.
In the ordinary so-called raw phosphates, such as bone-meal, bone-ash,
coprolites, &c., the lime and phosphoric acid are combined in the form
of what is known, in chemical phraseology, as _tribasic phosphate of
lime_. That is to say, that for every equivalent of phosphoric acid
there are three equivalents of lime. Now it was naturally concluded at
first that the tribasic phosphate was the form in which these two
substances existed in the slag. This, however, was found out not to be
the case, in the following way. On allowing the slag to cool, it was
found that small but perfectly defined crystals were formed. These
crystals, by careful analysis, were shown, first by Hilgenstock, to
consist of a form of phosphate of lime hitherto unknown, in which four
equivalents of lime were combined with one equivalent of phosphoric
acid, and which was therefore called "tetrabasic phosphate."
_Processes for preparing Slag._
As soon as the idea of utilising the slag as a manure was suggested,
various plans for extracting its phosphoric acid, and rendering it
available as plant-food, were devised. These were deemed necessary, it
was thought, by the very insoluble nature of the phosphates in the slag,
as well as by the supposed injurious action which would be exerted on
plant-life by the protoxide of iron it contained. Accordingly, a large
number of patents were taken out, "covering almost every conceivable
method for treating the slag, whether practicable or not. They all in
the main are combinations or variations of the following processes:--
"1. _Preliminary preparation of the Slag._
(_a_) By treating molten, or otherwise, with superheated steam, or
cooling when hot with water, to reduce it to small pieces or
to a fragile state.
(_b_) Grinding.
(_c_) Treating with water to wash out free lime, or with sugar
solution.
(_d_) Roasting in the air, or with some oxidising agent.
"2. _Solution of the Slag._
(_a_) _Completely_ in weak or strong acids (hydrochloric, sulphuric,
&c.)
(_b_) _Partially_, so as to dissolve the phosphates and silicates of
lime, and leave most of the iron and manganese oxides.
"3. _Precipitation_ of the phosphoric acid, with lime
or iron salts: or,
"Processes in which the slag is smelted with charcoal,
to reduce phosphates to phosphides, treated with
acid, and the phosphuretted hydrogen burnt to phosphoric
acid; and,
"Processes in which the slag is fused with soda or
potash salts,--caustic, chlorides, sulphates, carbonates,--with
or without steam being forced through, to form soluble alkaline
phosphates."[234]
Many of these processes were tried; but it was found by experiment that
the best and most economical way was by applying the slag direct to the
ground in a state of very fine powder. Experiments further showed that
it had _not_ the injurious effect on vegetation which it was feared it
would have from the protoxide of iron it contained. The discovery that
its phosphoric acid existed, as has been already explained, as a
tetrabasic phosphate of lime, has strengthened the opinion that this is
the best method of application.
A good deal has been found to depend upon the fineness of the ground
slag, with the result that it is now commonly sold on a mechanical as
well as a chemical analysis--_i.e._, the slag is guaranteed to pass
through a sieve of a certain fineness.
_Solubility of Slag._
Professor Wagner of Darmstadt has carried out some extremely interesting
experiments on the solubility of slag. He found that very finely
powdered slag was dissolved in carbonic acid water to the extent of 36
per cent, while, similarly treated, phosphorite only dissolved to the
extent of 8 per cent.[235] Another very important solvent is _citrate of
ammonia_. Reverted (or precipitated) phosphate is entirely soluble in
it, and phosphate soluble in it ought to be valued as worth more than
that which is not. Now, the solubility of Thomas-slag in citrate of
ammonia was found by Professor Wagner to be no less than 74 per cent,
while that of phosphorite only amounted to 4 per cent. These results
were corroborated by Professor S. W. Johnson, who found that of the
19.87 per cent of phosphoric acid contained in a sample of basic slag,
no less than 19.57 per cent was soluble in ammonium citrate, while a
finely ground sample of phosphatic rock yielded, on analysis, only 1.81
per cent soluble in citrate of ammonia, of a total of 29.49 per cent
phosphoric acid which it contained. Professor Fleischer has also tested
the comparative solubility of basic slag and phosphorite, by boiling
them in a solution of acetic acid. The former was found to have been
dissolved to the extent of 19 per cent, while the latter to only 5 per
cent. A highly interesting and most important experiment was performed
by Mr Heinrich Albert, of Biebrich. One gramme of basic slag and 100
grammes of peat were mixed together in a litre of water, and it was
found that, after standing for fourteen days, 79 per cent of the
phosphoric acid contained in the slag was rendered soluble.
In the above experiments it was found that the _fineness of grinding_
had a marked effect on the solubility of the slag, and that the finer it
was ground the greater was its solubility. This has been further
demonstrated in Professor Wagner's practical experiments. From these it
was found that finely ground slag has an action _four times_ as quick as
coarse slag; but that, as far as practical results were concerned, there
seemed to be a limit to the fineness to which it was advisable to grind
the slag, as slag above a certain fineness did not give better results
than a coarser slag. At any rate, he found that slag of a fineness so
great that it all passed through a gauze sieve, gave no better results
in his experiments than slag which left 17 per cent behind. We may say,
however, that the _finer the slag is ground, the greater will its
activity as a manure be_; and that a certain degree of fineness is
absolutely necessary to constitute it an active fertiliser. As
Professor Wagner's experiments are among the most valuable and complete
carried out on basic slag, we shall give a somewhat detailed account of
them.
_Darmstadt Experiments._
Professor Wagner's experiments were carried out on such different kinds
of crops as flax, rape, wheat, rye, barley, peas, and white mustard, and
the object of the experiments was to ascertain the comparative activity
as fertilisers of superphosphate, basic slag of different degrees of
fineness, Peruvian guano, damped bone-meal, and very finely ground
coprolites. In order to obtain a correct estimate of the relative value
of these different forms of phosphatic manures, it was necessary to
render the nitrogen in the bone-meal and the nitrogen and potash
contained by the Peruvian guano inactive--_i.e._, to limit the test
strictly to phosphoric acid. This was done by adding to the super, basic
slag, and coprolites, quantities of nitrogen and potash equal to those
contained by the other manures. There was further added to all the
experiments (the unmanured ones, of course, as well) an excess of
nitrogen and potash. In this way the increase in returns could only be
due to the phosphoric acid.
The general results obtained from these experiments may be summed up as
follows: Taking the activity of "super" to be represented by 100, then
the relative activity of--
Basic slag of No. 1[236] fineness is 61
Basic slag, No. 2[237] 58
Peruvian guano 30
Basic slag, No. 3[238] 13
Bone-meal 10
Coprolites 9
From these results the value of the commercial article has been
attempted to be ascertained. As it contains 80 per cent or thereby of
fine meal and 20 per cent of coarse, its activity may be stated to be
50, or half as active as super. Thus 2 cwt. of basic slag is equal to 1
cwt. of super. This only refers to the first year's effect. Professor
Wagner has made further experiments as to the after-effects of the
different manures, with the result that he has found that the
after-effects of the basic slag are even _better_ than those of the
"super." This stands to reason, for if twice as much phosphoric acid be
added in the form of basic slag as is added in the form of "super," and
the effect of the first year is similar--that is, the same quantity of
phosphoric acid is assimilated by the plant from the soil in both
cases--there is naturally more phosphoric acid left behind in the soil
manured with basic slag than in that manured with superphosphate of
lime. For example, if 100 lb. of super has the same effect in the first
year as 200 lb. of basic slag, and it is found that only 60 lb. of the
super and the basic slag have been assimilated the first year by the
plant, it is only natural to conclude that the remaining 140 lb. of the
basic slag will have a better after-effect than the remaining 40 lb. of
super. This has been actually proved to have been the case in
Professor's Wagner's experiments. The following are the results of some
experiments which Professor Wagner has carried out on the after-effects
of different manures:--
Out of 100 parts of phosphoric acid, there was removed by the first
year's crop--
Super 63
Peruvian guano 22
Bone-meal 7
Coprolites 6
Thomas-meal--
No. 1 fineness 39
No. 2 " 43
No. 3 " 15
Out of 100 parts of phosphoric acid left by the first crop, there was
removed by the three succeeding crops--
Super 30
Peruvian guano 9
Bone-meal 13
Coprolites 6
Thomas-meal--
No. 1 fineness 14
No. 2 " 29
No. 3 " 24
Numerous other experiments have been carried out by various
experimenters in different parts of Germany which it is unnecessary to
cite here. None, however, are so complete as those of Professor Wagner.
_Results of other Experiments._
In this country experiments have been carried out at Rothamsted,
Cirencester, Downton, Bangor, and by Dr Aitken at the Highland and
Agricultural Society's stations, as well as elsewhere. The results of
these various experiments naturally differ considerably, this being
owing to the difference in the nature of the soils upon which the
experiments were carried out, as well as the different degrees of
fineness of the slag used. They all, however, serve to confirm Professor
Wagner's general results. The results obtained in Scotland by Dr Aitken
at the Highland Society's stations were especially favourable to basic
slag as a phosphatic manure. The experiments were carried out on
turnips, and it was found that the Thomas-slag was, weight for weight,
superior to superphosphate. It may be added that the slag used in these
experiments was rich in phosphoric acid, and was in an unusually fine
state of division. Experiments carried out by the author have proved
slag to be, on various Scottish soils, one of the most economical
phosphatic manures to apply to turnips.[239]
We will sum up, in conclusion, the deductions which may be fairly drawn
from the results of all the above-mentioned experiments as to the value
of basic cinder as a manure.
_Soils most suited for Slag._
Although its action is undoubtedly more favourable on some soils than
others, it may be broadly stated that generally its phosphoric acid is
_half as valuable_ as that in soluble phosphate. The soils on which it
will have the most marked effect will be those of a _peaty_ nature,
_poor_ in lime, but _rich_ in _organic matter_. The beneficial results
obtained by an application of lime to peaty soils are well known. As the
slag contains a large percentage of free lime, it thus performs on such
soils a double function. On meadow-lands, all kinds of pasture-lands (if
not of too dry a character), and clay soils poor in lime, its action has
been shown to be especially favourable. Of different kinds of crops,
those best suited to benefit from the slag as a phosphatic manure are
those of the leguminous kind. This arises from the fact that their
period of growth is longer than that of most other crops.
_Rate of Application._
As to the rate per acre at which the slag ought to be applied, there
will naturally be a difference of opinion. Professor Wrightson, of
Downton Agricultural College, recommends that it should be applied at
the rate of from 6 to 10 cwt. per acre. This, of course, is very
liberal manuring. We must remember, however, that phosphatic manures,
unlike nitrogenous manures, and to some extent potash manures, may be
applied in even excessive quantities without any risk of loss. It is
impossible to measure out our phosphate manures in the same accurate
manner as we measure out our nitrogen. It is safer, therefore, and on
that account more economical in the long-run, to apply our phosphate in
excessive quantity than the reverse. The reason of this may be shortly
explained. The phosphoric acid which is naturally present in most soils
is with difficulty soluble. Only a small quantity is yielded daily to
the plant. This quantity may, under favourable climatic conditions, be
sufficient; but these favourable influences never last very long at a
time.
For three weeks, perhaps, the plant may experience drought, and during
this period it takes up no phosphoric acid, and its growth practically
comes to a standstill; but this period of drought is followed by rain
and warm weather, and the plant, if it is to be ripe by harvest-time,
must make up for lost time. It must grow as much the next few days under
these favourable climatic conditions as it would have grown under normal
conditions in double or treble the time. In order to do so, however, it
must be able to obtain plenty of phosphoric acid, and this is only
possible where there is a decided excess of phosphoric acid present in
the soil.
The richness of a soil, therefore, in phosphoric acid, must be such
that it is not only able to supply the ordinary wants of the plant, but
to provide an excess when such an excess will be needed; for one must
remember that the amount of plant-substance formed in the course of a
few days under favourable conditions is very great, and that the amount
consequently of phosphoric acid which plants assimilate during that
period must also be very considerable.
_Method of Application._
In conclusion, as to the method of application of the slag,
agriculturists must be _warned against mixing it with sulphate of
ammonia_; for if this is done, a _considerable loss of ammonia_ will
ensue, set free from the sulphate by the action of the free lime which
the Thomas-slag contains. With nitrate of soda and potash salts it may
be freely mixed. Such mixtures, however, are apt to form themselves into
little balls, which soon become very hard. They should therefore only be
mixed shortly before use. To overcome this difficulty, Professor Wagner
recommends the mixture of a little peat or sawdust with the slag.
FOOTNOTES:
[233] See Appendix, p. 417.
[234] _Vide_ paper on "Basic Slag: Its Formation." By Stead and
Ribsdale. 'Journal of the Iron and Steel Institute,' 1887, p. 230.
[235] _Vide_ Professor Wagner's pamphlet, 'Der Duengewerth und die
rationelle Verwendung der Thomas Schlacke,' Darmstadt, 1888.
[236] No. 1 fineness was such as passed entirely through a fine gauze
sieve of 250 wires to the linear inch.
[237] No. 2 fineness was such as passed entirely through the regular
standard sieve--_i.e._, containing 120 wires to the linear inch.
[238] No. 3 was what would not pass through the standard sieve.
[239] 'Transactions of the Highland and Agricultural Society,' 1891;
'Chemical News,' 1893.
APPENDIX TO CHAPTER XIV.
NOTE (p. 404).
For those more particularly interested, we append a full analysis of the
slag, taken from Messrs Stead and Ribsdale's paper in the 'Journal of
the Iron and Steel Institute,' 1887, vol. i. p. 222:--
Lime 41.58
Magnesia 6.14
Alumina 2.57
Peroxide of iron 8.54
Protoxide of iron 13.62
Protoxide of manganese 3.79
Protoxide of vanadium 1.29
Silica 7.38
Sulphur } .23
Calcium } .31
Sulphuric anhydride .12
Phosphoric acid 14.36
-----
99.93