Preparation of Raw Gypsum and its Conversion into Various Hemihydrate Plaster Forms. Parts I and II

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Preparation of Raw Gypsum and its Conversion into Various Hemihydrate Plaster Forms. Parts I and II

Eipeltauer, E.

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A study of the mode of dehydration of gypsum under various conditions of temperature and water vapour pressure constitutes one of the oontinuing projects of the Building Materials Section of the Division of Building Research.

An active interest is therefore taken in the

large body of literature on this subject. The

pre-sent papers are believed to be a valuable contribu-tion to this literature concerned as they are, with the practical implications of the fundamental princi-ples involved.

The Division is indebted to Mr. D.A. Sinclair of

the Translations Section of the National Research Council for preparing this translation.

Ottawa, June 1960

R.F. Legget,

T1tle:

Author: Reference:

Translator:

Techn1ca.l Translat10n 899

The preparat10n of raw gypsum and 1ts convers1on 1nto

var1ous"hem1hydrate plaster forms. I and II

(Aufbere1tung und Oberffthrung des Rohg1pssteines 1n

se rne verschledenen Halbhydrat-Plasterformen.

I und II) E. E1peltauer

Zement-Kalk Glps, 11 (6): 264-272. 1958 and 12 (8):

351-355, 1959

PART I

1. Introduction

The advantages and disadvantages of calcined gypsum have been

known as long as the material itself. Its defeets lie primarily in

its poor resistance to moisture and its low creep strength as well

as its varying behaviour with respect to setting. Nevertheless,

there is a rather wide field of application for this material in

ｾｨｩ｣ｨ neither resistance to moisture nor creep strength are

re-qui r-ed , However, the varying setting, times still cause trouble and

often lead to processing errors and thence to considerable damage. Before seeking remedies in the form of chemical additives, all possibilities in the production of the plaster itself which might lead to the attainment of constancy in the properties of the binder

were to he exhausted. In this connection the crushing of the raw

gypsum plays a decisive part which is not yet sufficiently

recog-nized in the industry. For this reason we shall devote considerable

space in what follows to the crushing and grinding of the gypsum in the production of hemihydrates.

2. Correct Preparation of the Raw Gypsum

As in many other branches of the industry it is customary in gypsum production to state the fineness of the raw or calcined

products in terms of screen residue data. In the author's experience

this is inadequate. Only in conjunction with the values of the

specific surface areas involved do the screen values give an ade-quate idea of the grain 1uality of the intermediate and end products.

The purpose of crushing the raw gypsum for the production of ｾﾭ

plasters is to provide as uniformly grained a flour as possible in

order to obtain a uniform calcined gypsum. In the production of

there, as is understandable, is to reduce the raw gypsum blocks as

qUickly and cheaply as possible to the finest grain sizes. The

information provided by the scpeen residues on the 1 or 2 mm screen, for example, is considered adequate in order to decide whether the

crushing operation has been successful. Hammer crushing of raw

gypsum, on account of its economy and high degree of crushing, has

recently found wide application. The subject of hammer crushing

in the gypsum industry has ｡ｬｾ･｡､ｹ been dealt with in detail by the

present author in an earlier

ｰ｡ｰｾｲＨｬＩＮ

Here we shall morely

emphasize once more that in hammer crushing the impact strength, which of course is generally less than the compressive strength of

the material to be crushed, is the decisive factor. Accordingly,

the crushing of the raw gypsum in the hammer mill succeeds by

reason of the overcoming of the impact strength of the crystal,

pre-dominantly a Long the ｾｭｴ･ｲ layer planes of the crystal lattice

which are, of course, the weakest point in the gypsum lattice

(Fig. 1). It is clear that this method of fragmentation must

pro-duce an extremely large number of needles and flakes. This in itself

would be of no importance as far as calcining of gypsum is con-cerned if the dehydration rate in the raw gypsum crystal were the

same in all directions, so that the smallest diameter of the needle

and flakes would be the determining factor. Microscopic

investiga-tions have clearly shown, however, that dehydration perpendicular

to the water layer plane, i.e., in the ､ｩｾ･｣ｴｩｯｮ of the b-axis, is

negligible compared with that in the water layer plane.

Dehydra-tion in the direcDehydra-tion of the c-axis proceeds fastest while that in the direction of the a-axis frequently lags behind.

Fig. 2 and 3 illustrate very well the dehydration lag in the

a-axis compared With the c-axis direction. The ｾ｡｣ｴ that not the

slightest dehydration has taken place along the b-axis is eVident from the clear transparency of the gypsum flake in the centre of the crystal.

The above statements are probably proof enough that hammer crushing should never be applied directly after gypsum calcining if

handling capacity, small power consumption and extremely small waste of the hammer mills are certainly economic advantages that are not likely to he clismissed, but the use of a hammer mill can only be recommended for coarse fragmentation if it is to be followed by processing in another mill which fragments by means of a suitable

screen. In this manner the range of grain sizes obtained in the

raw gypsum flour can be suffiCiently restricted. However, if a

still smaller hammer mill were employed after a hammer mill for the fine grinding of the raw flour, as indeed has already been done in the gypsum industry, the result would be an inadmissibly wide range of grain sizes as well as grain shapes that are unfavourable for

calCining purposes. Furthermore the gritty, intermediate grain

sizes (5 to 10 mm) unable to pass through the screen of the second hammer mill (generally 2 mm mesh size) multiply and therefore have to be returned to the second hammer mill, where, however, owine to their small intrinsic weight they can be but little reduced further

in size by the impeller process. In this cycle, therefore, grit

accumulates and unnecessarily burdens the mill system. Eventually

the intermediate bin is bound to overflow unless the feed to the

first impeller is cut off. The overflow of the 2 mm screen in such

cases is 40%, and it may help somewhat to follo\tl wi th a 3 mm screen and return the grits to the first hammer mill where still larger

weights participate in the crushing process. The 3 mm screen, in

turn, again results in too wide a range of grain sizes in the

gypsum kettle, which ought to be avoided. By constricting the slit

and increasing the rpm of the impact roller in the second hammer mill a better fine grinding can indeed be attained, but the wear

increases sharply and the handling capacity is rapidly reduced. It

is just these advantages of hammer mills compared with other types of mill which would be lost; instead of overcoming the impact

strength, severe constriction of the slit reduces the compressive strength as a resir,tance-giving factor.

The calcining times in the gypsum kettle increase with the

second povrer- of the radii of the raw gypsum grains. This means

alongside many coarse grains and a few intermediate-sized grains after normal hammer crushing, would long since have been calcined through, if not over-calcined, while the coarse grain was being converted only in its outermost boundary zone. The result is a heterogeneous product.

From this short sketch it is clear that the crushing of the raw gypsum plays a vital role in determining its subsequent quality

and the behaviour of the calcined gypsum. Fundamentally,

fragmenta-tion by grinding is more favourable for the gypsum quality, although

it is less ･｣ｯｮｯｭｩ｣｡ｬｾ For producing bUilding gypsum in the Harz

kettle, the author believes the following fragmentation system to be the most favourable:

I

hammer mill

ｾｅ

creen _ｾ drying drum _ｾ edge-runner mill

with 1 mm screen

V

ｾ

I

raw flour bi';

I

I

ｫ･ｴｾｬ･

I

finished gypsum bin

--1

sacking

I

For finer types of gypsum, e.g. alabaster, dental, model gypsum, etc., additional screening should be installed after the bin, since the return flow of the screen contains primarily the impurities from the raw gypsum, e.g. limestone and dolomite, and

can be used as an admixture for ordinary bUilding gypsum. From the

above diagram it is clear that there is no further fragmentation

after the kettle. This is intentional, because after calcination

the gypsum flour should be left undiRturbed as far as possible. In

this way a much more stable product is obtained, because after calcination the boundary zones of all grains are of more or less

grains are of cour-se of elec1si ve importance for the setting process. Any post-grinding causes the interior of the grain to come to the

surface and variations are then unavoidable. It should be mentioned,

also, that very few gypsum factories operate on tilis prinCiple, and that opinior.s on the most suitable methods of preparation differ

greatly. TIle main reason for the standard post-grinding of the

calcined material in the gypsum industry consists in the lower

grinding costs for fine grinding (in the above scheme largely taken care of by the drying drum) and the sUbstant1ally lower dust

co-efficlents in the actual calcining process. Today, however, dust

elimination is no great problem, and the great advantage of a sub-stantially reduced calcining time, with fine grinding completed be-fore calcining, must not be overlooked.

3. The Fundamental ｐｨｹｳＱ｣｡ｬＭｃｨｾｭｩ｣｡ｬ Princ1ples of Manufacture ｡ｾ

Hardening of Hemihvclrate Pls.ste.x.

In the foregoing section the method of preparation of raw

gypsum ｜ｾｨｩ｣ｨ in the author's opinion is the most advantageous, was

described. It was explained that a relatively finely ground raw

gypsum « 1 mm) which contains few gritty grains is favourable for

dehydration. This applies particularly to the production of ｾﾭ

hemihydrate plasters. Before the technical dehydration process can

be dealt with it is ahsolutely necessary to discuss the phys1cal-cher.lical basis of "gypsum calcining" and "gypsum setting".

(a) The various hemihydrate modifications

In the fundamental and still important works of J.ff. vanlt Hoff, E.F. Armstrong, ",. Hinrichsen, F. Weigert and G. Just(2) the existence of a dihydrate, a hemihydrate, a soluble and an insoluble anhydrite was established for the range of temperatures (aoe to

lOaOoe) involved in the technology of gypsum. Even today there is

still no difference of opinion as far as the dihydrate and insoluble

anhydrite are concerned. With regard to the hemihydrates, however,

one hand, with respect to its hydrate water content, A. DuhUisson(3)

reports, for example, on a hydrate CaS0₄ •

2h

H

20 that he believes

he has found. Actually, this is again the hemihydrate which was

the subject of van't Hoff's investigations. This hemihydrate is

able both to take up and give off Hater without disturbing its

crystal lattice. Its water content (stoichiometric value 6.21%)

may go as high as 12% without producing any change in the lattice.

Conversely, the water content of the hemihydrate may be reduced to

a few tenths and often to hundredths of one percent. This product

is called the soluble anhydrite, or anhydrite III. No one has ever

succeeded, however, in obtaining this anhydrite III actually free of

water. In thermogravimetric tests the author still found a residual

water content of 0.03% even at 325°C (see Table I).

It is clear from Table I that a so-called "soluble anhydritell

did not form even after 17 days processing at temperatures above

200°C. The end product at 325°C was analyzed chemically and by

X-ray diffraction, and was found to consist of two-thirds "in-soluble anhydrite" and one-third "dehydrated hemihydrates". G. Li.nck and H. Jung( 4) gave what the author believes to be the correct designation in 1954 by identifying the IIdehydrated

hemi-hydrate" with the II s oluble anhydrite" of vanlt Hoff. Keane(5),

L. Desch(6), C. Haddon and M. Brown(7), as well as K. Fill(8), also speak only of an anhydrite modification, so that a certain

clarifi-cation is gradually emerging. P.B. BUdnikoff(9) makes no decision

concerning the existence of a soluble anhydrite. D. Balarew and

A. KOluschewa(lO) attempted to resolve the contradictions in the results of vanlt Hoff and his co-workers as well as those of Linck and Jung, but only introduced additional confusion by claiming to have established the existence side by side of a IIdehydrated

an-hydrite" and a II so1uble anan-hydrite". Their observation that vanlt

Hoffls soluble anhydrite is enclosed in dehydrated hemihydrate

appears to me to he very contradictory. It is not surprising,

there-fore, that these res1Jlts have not been taken into account in the

work of other investigators. The most important work on the

vant t Hoff and his co-wor-ker-s was performed in 1941 by the ｬＮｲ［ｬｾ｟［ｲｊＮ｣ｊ｟ｌ

physical-chemists K.K. Kelley, J.C. Southard and C.'J.'. Anderson(ll). These investigators do of course retain the soluble anhydrite as a

phase 0f i t sown 'tJi th the name IIanhydri te I I I", and even compli ca te

the picture somewhat by assigning to this anhydrite an 0,- and

/3-form obtained by dehydrating the a- and f3-hemihydrates,

respective-ly. The designations a- and f3-hemihydrate wer-e also introduced by

these investigators and are probably a concession to gypsum tech-nology, which in general produces an autoclave gypsum (a-hemihydrare)

and a building gypsum- (/3-hemihydrate). From the physical-chemical

standpoint this nomenclature cannot be justified since:

1. These ｭｯｾｩｦｩ｣｡ｴｩｯｮｳ are entirely identical in lattice

structure.

2. These modifications are not. clearly-occurring forms of the

hemihydrates, since, as already shown in a previous

work(12) based on calorimetric measurements, there are all

the transitions between these two forms. The a- and ｾﾭ

forms with the heats of hydration of 4100 and 4600 cal/mole fOlmd by J.C. Southard, moreover, are not even the end

terms of this series, as will be shown belowOl

Accordingly, the f3-hemihydrate grain, in my opinion, is a

con-glomerate of the very smallest a-hemihydrate crystallites. These

crystallites, of course, have been severely obstructed in their

formation in the dihydrate grain, and furthermore it is ｡ｬｴｯｾ･ｴｨ･ｲ

possible that the hemihydrate water in the ｾＭｨ･ｭｩｨｹ､ｲ｡ｴ･ is partly

present in a different arrangement from that in the comparatively

well developed a-hemihydrate of autoclave gypsum. In short, in the

case of f3-hemihydrate there are very considerable lattice disrup-tions which are responsible for its greater instahility compared

wi th the so-called a--hemj.hydr-a te , It is obvious that all physical

values such as solubility, density, thermal conduction, etc., will

be influenced by this. There is no need, however, to introduce a

new modification and then to extend this to the IIso1 uble anhydrite" which can be produced from these modifications.

To date, therefore, only the existence of a dihydrate, a

hemi-hydrate and an insolu'hle anhyirlte have been definitely proved. From

the professional point of view the "soluble anhydrite" must be

identified as 'tlehydrated hemihydrate" which at standard atmospheric humidity is converted again into the hemihydrate without lattice transformation, and accordingly does not constitute a phase in itself.

Further modifications cannot be set up solely on the basis of

the occurrence of different crystal habit. In the present paper,

however, the a- and p-designation will nevertheless be retained in

order to avoid misunderstandings. The extent to which the crystal

habi t of the gypsum. and. its hemihydrate can be influenced \'lill be shown be Low (see for example Fig. 5 to 10).

(b) Dihydrate-hemihydrate transformation noint

The classical papers of vanlt Hoff and his co-workers deal also with the dihydrate-hemihydrate transformation point in particular and the stability ranges of the calcium sulphate and its hydrates

in general. First of all, it is necessary to deal briefly with the

transformation of hydrate compounds. Sharp transformation points of

the kind known in metallurgy are not generally encountered among the

hydratesa The transformation point of a hydrate compound is defined

as the temperature at which the vapour pressure of the hydrate

reach-es the same value 8S that of the surrounding medium (in the present

special case, therefore, the vapour pressure of the saturated gypsum sOlutlon).

Vapour pressure measurements are carried out by means of a

tensimeter. With the calcium sulphate hydrates a satisfactory state

of equilibrium is scarcely ever attained by this method no matter

how much time is given. Van't Hoff therefore employed an artifice

in which he introduced the dihydrate into a salt solution and de-termined the transformation point corresponding to the external pressure in each case with the aid of bOiling-point measurements at predetermined pressures, and in this way he was able to construct

pressure curve of the saturated gypsum solution corresponds to the vapour pressure curve of pure water, since its activity is but

little affected by the low sOlubility of the gypsum in water. The

pOint of intersection of these two vapour pressure curves is the transformation point of the dihydrate into hemihydrate in tbe system

CaS0₄ - H_20, and ac cor-otng to van't Hoff it occurs at l07.2°C.

According to J.C. Southard(ll) this transformation point is

too high. His dilatometer measurements gave values in the vicinity

of 100°C. By calculatlon alone he comes to a value of 97°C, finding

himself in good agreement with the values of E.P. Partrldge and

(10) (14)

A.H. White and E. Posnjak • Southard was unable to give any

explanation of vantt Hoff's too high value. In my opinion (and the

considerable scatter in J.C. Southard's values about 100°C support this view) the dilatometer is still too inaccurate for the

deter-mination of the dihydrate-hemil1ydrate transformation pOint. Van't

Hoff had these ｭ･｡ｳｵｲ･ｾ･ｮｴｳ carried out only incidentally in order

to confirm his vapour pressure curve intersectlon point. The main

purpose behlnd his vror-k lay in the determination of the vapour

pressure curve of the dihydrate. In order to achieve a state of

equilibrium more rapidly, van't Hoff, as already painted out, used

salt solutions. Once two ･ｸｰ･ｲｩｭ･ｮｴ｡ｬｬｹｾ･ｴ･ｲｭｩｮ･､ vapour pressure

values at different temperatures were obtained, further vapour

pressure values could be calculated by the Clausius-Clapeyron

equation. The correctness of these calculated values, of course,

depends on the accuracy of the experimentally found values (this

applies also to the transformation point of 1070 2 ° C ) . As one of the

basic vapour pressure values van1t Hoff' used '758.8 mm at lOl.45°C,

which he had obtained in R solution of common salt. According to

my investigations this value cannot be correct, slnce no eqUilibrium is obtained here be twe en calcium sulphate dihydrate and calcium

sUlphate hemihydrate, but only a.n equillbrium between dihydrate

and calcium sodium sUlphate hydrate. Chemical and X-ray diffract1Cm

investiga tions have shown conclusively that the resulting heml-hydrate lattice probably contains no chlorine lon, but almost 10%

salt solution is employed, where a calcium sulphate hernihydrate does not even form, but instead a double salt hydrate with an entirely

different lattice. Vanlt Hoff also used a mac;nesium cr.lorlde

solution for low temperatures. The author did not determine the

extent to which magnesium chloride also interferes. As far as I

know, no mixed crystals Or double salt compounds occur here. If

vanlt Hoff had used a calcium chloride solution lnstead of a common

salt solution the results would have been accurate. Investigations

of the author in this direction are under way, but will have to he reported on at a later date.

The dilaton1eter measurements of J.C. Southard gave considerable scatter with respect to the dihydrate-hemihydrate transformation

point. Southard attempted to support his transformation pOint of

97°C + 1, primarily by setting up a solubility diagram (Fig. 4). In Fig. 4 the point of intersection of the sOlubility curves

of the dihydrate and hemlhydrate is exactly at 97°C. However, in

the author's opinion only the solubility values of the dihydrate

can be considered reliable. The hemihydrate curve is based on

measurements by P. Jolibois and L. Chassevent(15), by R.E. Hall,

J.A. Robb and C.E. COleman(16), by 1d.C. Riddell(17) and E.P.

Partridge and A.H.

ｾｾｩｴ･ＨＱＳＩＮ

The curve according to Riddell's

values was drawn, and the point of intersection lies at 97°C.

Hovrever , if the solubili ty curve of the hemihydra tes above 80::JC

were constructed according to the values of L. Chassevent, then VJe would get a point of intersection which would lie above 100°C. The greatly varying values, however, are not the fault of the

re search wor-ker-s involved, but are due to the capricious na tur-e of

the hemlhydrate, which occurs in very different states of energy

and therefore, as the author will show beLow , is found with very

widely fluctuating heats of hydration, and naturally also \vith

dif-ferent heats of solution ｾｮ､ different solubilities. A stability

diagram (Fig. 4) haser'! on solubility values of hemlhydrate plasters can scarcely be useri in support of a transformation point determined by the dilatometer method.

Thus, the only promising way of determining the

dihydrate-hemihydrate transformation pOint accurately still appears to be the

van't Hoff method, but using a calcium chloride solution inste8n of

a sodium chloride one. It is certain, in any case, that the

trans-formation point lies between 97°C and 107°C. According to the

author's observations of gypsum transformation in the autoclave the

transformation point 11es at 103°C. An autoclave test at 102°C

lasting 14 days gave not the slightest trace of a hemihydrate or of

an incipient dehydration.

(c) Inlluencing the ｣ｲｾｳｴ｡ｬ habit

In gypsum technology the crystal habit plays a decisive role, the complete importance of which has hitherto not been recognized. Gestures in this direction are indeed found in the patents of C.L.

Haddon(18) and in the works of J.J. Eberl and A.R. Ingram(19), but

these efforts all tend only in the direction of producing a

hemi-hydrate of 10\'1 coris i s t.ancy , These authors used additives (appr-ox ,

0.2%) of water-soluble alkaline and alkaline-earth salts of the aliphatic polycarboxylic acids, the double bonds of which, if

present, are in ciS-Position while their acid radicals are separated by at least 2 C atoms, e.g. the salts of citric acid.

Quite apart from the production of plaster, however, the

crystal hahi t is now a factor in gypsum t echnoLogy , the Lrnpor-tance

of which can no longer be overlooked. Indeed, it is the foundation

stone of gypsum technology with which one must constantly be con-cerned in order to produce crystals suitable for a given application,

whether in the calcining or in the setting process. All additives,

including retarders, accelerators, strengtheners, etc., depend for

their effect primarily on the formation of a specific crystal

hahit (Fig. ｾ and 6).

The effect of a surface-active substance such as sulphite waste

liquor on the setting of a standard hemihydrate plaster may be shown from an example.

To this plaster aqueous solutions containing 0.1%,

0.5%,

1%,

added and the setting process was observed microscopically and wi th

respect to time. Flexural tensile strength and compressive streneth

were also determined (Table II).

The values in the table reveal that up to 1% sulphite waste liquor additive the loss of streneth is still acceptable, but the

setting is definitely and effectively retarded. Consideration of

Fig. 7 to 10 shows clearly that these changes are due to altera-tions in the crystal hahit.

With increasing amounts of sulphite waste liquor added to the mixing water the dihydrate needles turn into rodlets which get

progressi vely shorter and tht.cker-, Until a crystal habit of the

type shown in Fig•. 7 is reached (0.5% sulphite waste liquor) the retardation of the plaster by sulphite waste liquor is still

accept-able. In Fig. 8 very wide rodlets can be recognized, which in

Fig. 9 are already so short and thick that some of them are now

standing upright. Fig. 10 sho\'ls this folding of the crystals even

more clearly, since here the c-axis is no longer predominantly in the plane of the picture, but is oriented perpendicularly thereto. With a 5% addition of sulphite waste liquor no setting in the normal

sense took place at all. Only a drying-out with shrinkage took

place; the strength values were practically zero.

What is bad in this case, however, may be favourable in othe.r

technical fields. It may well be imagined, for example, that in

the manufacture of boric acid by the interaction of calcium bOrate with hot dilute sulphuric acid, the filtration difficulties

en-countered from accumulation of gypsum would be completely e11minated if the crystal habit of the gypsum were made to correspond to

Fig. 8 and 9.

The reasons why very small quantities of solution additives

influence the crystal habit are not ･ｾｴｩｲ･ｬｹ clear. O. Knacke and

I.N. Stranski(20) made a valuable contribution to this important

question in a paper that has just recently appeared. A close

re-lationship exists between adsorption and nucleus formation. In the

course of adsorption ccnditions of local supersaturation occur which

different crystal faces ls of varylng lntenslty, the formatlon of nuclel and the crystal hablt assoclated therev:lth ls lnfluenced by thls fact.

(d) Calorlmetrlc measurements of hemlhydrate plasters

Calorlmetrlc measurements are very lmportant ln the productlon

and testlng of ｾｹｰｳｵｭ because they can be carrled out falrly

quick-ly and glve the englneer lmportant lnformatlon_ In a previous

paper the author has already dealt fully wlth the problem of a phase analysls of lndustrlal calclned gypsum based on calorlmetrlc

meas-(21 )

urements as suggested, for example, by J.C. Southard and

A. Kruls and H.

ｓｐｾｴｨＨＲＲＩＮ

At that tlme lt was found that thls was

posslble at best for mlxtures of calclned and autoclave gypsum

when thelr heats of hydratlon were known. As already stated in

sectlon (a) of thls chApter, there ls strlctly speaklng no a- and

f3-hernlhydrate dlstlnctlon but only one hemlhydrate wht ch can show more or less dlstlnct lattlce dlstortlons and dlfferences of

sur-face texture. The dlfferent heats of dlss01utlon and hydratlon are

assocla ted '.'11 th thl s , It 1 s not surprlslng, therefore, that the

heat of hydratlon flgures for hemlhydrate glven by dlfferent

In-ｶ･ｳｴｬｾ､ｴｯｲｳ dlffer strongly, slnce they all have studled different

hemlhydrates. In Table III heat of hydratlon values are clted from

the work of J.C. Southard.

The author's calorlmetrlc investigations have shown that

hemlhydrates obtained from calclum chlorlde salt solutlons have R

hydration value of 3600 cal/mole and are therefore more stable t.han

autoclave gypsum. In the case of the so-called B-hem1.hydl'ate 1t

was also shown that the hydratlon value depends very strongly on

the manner ln whlch this !3-hemihydrate ls produced. For exarnpLe ,

Schott\"11ener alabaster gypsum was dehydrated Ln a test ca'l crner,

and after the first ca LcLnat Lon "las heated agaln to 150°C and Vlrt

of the calcined gypsum VlBS t hen removed. Its water content HcU:';

6.05%, the heat of dissolution 1060 cal!mole. The remainder was

subjected to further calcination at 220°C and thereby almost fully

atmospher1c hum1d1ty. At certa1n 1ntervals of t1me the water con-tent of the plaster was neterm1ned and the heat of solut1on 1n

2N hydrochlor1c ac1d was measured. The course of th1s process 1s

ev1dent from F1g. 11. The plaster ult1mately atta1ned a content of

6.1%, wh1le the heat of solut10n was now 1250 cal/mole. Thus the

heat of hydrat10n was 4450 cal/mole lower and the product was more

stable. Th1s expla1ns what the gypsum 1ndustry has d1scovered

emp1rically, that 1t 1s favourable to heat the gypsum beyond the

second b01l1ng po1nt and then to store 1t 1n b1ns. Table IV e1ves

some heats of hydrat10n found by the author for hem1hydrate plasters.

For pract1cal purposes the heats of hydrat10n of the

hem1hy-drates are between 0600 and 4600 cal/mole. The value depenns not

only on the manner of product1on, but also on the water content of

the hem1hydrate. It 1s not correct to speak of an a- and ｾＭｨ･ｭＱｨｹﾭ

drate w1th heats of hydrat10n of exactly 4100 ann 4600 cal/mole,

respect1vely. The facts are consonant w1th the d1scovery of a

metastable hem1hydrate w1th heat of hydrat10n vary1ng accord1ngly. Therefore, even though calor1metr1c measurements cannot be used for a phase analys1s, for the s1mple reason that str1ctly speak1ng there are no d1fferent phases, nevertheless th1s phys1cal

ｾ･ｳｴＱｮｧ method 1s extremely vaLuab Le for the test1ng and study of

gypsum, s1nce 1t prov1des 1nformat10n wh1ch would otherw1se elude

us. In Table IV, for example, cons1der the substant1ally h1gher

heats of hydrat10n shown by the autoclave product w1th h1gher water

content. When dLhydr-at e 1s present only a very small part of th1s

gap 1n 1nformat10n can be closed. It must be concluded, therefore,

that the last d1hydrate water has much greater 1mportance as far as the reduct10n of the heat of solut1on 1s concerned than the water f1rst removed, whence 1t may be concluded that the res1dual

d1-hydrate water 1s bound much more strongly 1n the latt1ce. It 1s

well known to enG1neers that gypsum y1elds poor strength w1th even

s11ght quant1t1es of water above the sto1ch1ometr1c. In the f1eld

of gypsum research, nowever-, espec1ally w1th reference to the crystal hab1t, calor1metr1c measurements y1eld informat1on about

the adsorption capacity of the hemihydrates. Calorimetric test results miGht indeed be just the means by which we might arrive at numerical values for the film thicknesses of crystal surfaces, i.e., the absolute value of the adsorption on the crystal surface.

4. The Production of a-plasters

There is hardly any other product which is RO varied with

re-spect to its calcining. This indicates that on the one hand much

of the gypsum industry is very conservative and clings to ancient, rather uneconomic processes, while on the other hand new processes may have been intronuced in many places but have not been

satis-factory, so that eLseviher-e still another calcining process has been

tried. In the more recent calcining processes the influence of

the cement industry is clearly being felt, suggesting that the manufacturer would be very happy to see the economy achieved there in the calcining processes transferred to the production of gypsum. This has to do mainly with gypsum calcining in internally-heated rotary furnaces, in grinding and calcining installations and on

the sintering grate. However, what is best for the calcining of

the cement by no means applies to the calcining of gypsum. As far

as thermal economy is concerned these methods have unquestionably

resulted in a great advantage, but the gypsum ｾｵ｡ｬｩｴｹ is not

gener-ally satisfactory. Thus direct calCining is not as favourable for

the production of plaster, modelling, dental gypsum. etc., as for the calcining of clinkers in the cement industry.

ｾＭｰｬ｡ｳｴ･ｲ is often classified according to the method of

manu-facture, as kettle gypsum, rotary furnace gypsum, blast furnace gypsum, etc., or according to its application as dental gypsum,

modelling gypsum, lump gypsum, etc. A simple classification

consists in the follovling two groups:

Directly calcined ｾＭｰｬ｡ｳｴ･ｲ

(a) Direct calclnlne

The manufacture of these plasters in the rotary furnace or in grinding and calcining plants has become widespread, psrtlculc\rly in Germany, but also to some extent in France and throuGh the

limited sale of rotary furnaces in overseas countries as well. It

is only natural that the manufacturers of rotary furnace gypsum plants, which, however, unlike the cement tubular rotary kilns, work on the direct current principle, should be full of praise for

them. The owners of such plants and the users of these gypsums, on

the other hand, are not always so enthusiastic. Undeniable are the

great economical advantae;es, since the direct transfer of heat to the calcined material makes for high efficiency, a pOint which will

be taken up in detail later on. The product itself, however, does

not at the present time at least come up to the standards of

kettle gypsum as far as quality is concerned. In particular, it is

still insufficiently uniform. It remains to be seen whetter

chemi-cal additives, which have the effect primarily of regulating the setting, can by themselves overcome these disadvantages.

The raw gypsum is generally fed to the furnace in 25 to 35 mm

lumps. These lumps are kept constantly moving and rolling around

during their passarre to the rotary furnace by the quadrant equipment present in the whole furnace, and are subject to constant abrasion

accompanied by progressive dehydration. The resulting fines are

carried through the furnace by the flow of hot gases more rapidly than the lumps, which is a desirable circumstance, of course, since the finer erains require a considerably shorter calcining time. In Stokes' law for the steady rate of fall of spherical particles in viscous media

,

where v - speed of falling particles in cm per sec,

g - acceleration due to gravity (981 cm per sec2 ) ,

S2 - speci fic vieight of particles,

S1 - specific wei8ht of medium,

ｾ - frictional resistance,

we can recoenize the quadratic dependence of the rate of fallon the

radius of ｴｨｾ particles. Since the calcining time increases vlith

the square of the radius, the basis is hereby provided for a classi-fication of calcining in the rotary furnace by the heatine gases

in the calcining of gypsum. It mllst not be oV8rlooked, however,

that beginning with a certain grain size the transport of the 8YPsum grains by rotation and inclination of the furnace becomes

decisive, Thus the classification of the middle and coarser grains

becomes progressively unsatisfactory_ It is obvious, moreover, that

at the end of the rotary furnace very fine Grains of gypsum will be

present alongside coarser grains up to very small lumps. Since a

very definite water vapour partial pressure prevails in the space due to the mixture of vapour in the heating gas, an equilibrium is

struck on the surfaces of the individual ｾｰｳｵｭ grains and causes

a certain water content to be present in the boundary zone. If

dehydration on this water content has not taken place in the

in-terior of the lumps r'lurinr: passage through the rur-nace , then after

fine Grlnding a pron.uct of uniform behaviour can never be obtained. A certain over-calcining of the finer grains is also unavoidable and these then remain structurally different to a considerable

extent from the mass of residual lumps. Under these circumstances

it is extremely difficult to produce a unlform plaster in an

in-ternally-heated rotary kiln. The main reason lies, as already

indicated, in the wide range of gr-a t n sizes.

The manufacture of 0-plasters in grinding and calcining

plants would scarcely be possible, since on the one hand the plants themselves are costly, and the separation of the vapours and the calcined gypsum flour requires considerable outlays for machinery, and, among other things, the temperature must not fall below the

condensation pOlnt of the vapours. It is very difficult to produce

a mortar binder in such plants, but the rapid binders whi ch are

The use of 8 mu l t LpLe-cs t.a.re roasting kiln, when charged wlth

more or less un r f'orrnLy g:rRinecl raw gypsum « 10 rnm ) yields a [3-hemihydrate plaster which is uniformly calcined throughout ｾｮ､

con-stant in its setting hehaviour. For a daily output of 50 to 55 T:1etric tons a multiple staT3 furnace of about 4 m diameter with 7 hearths is neeclecl. The gypsum temperature on the hearths is 110

to 125°C. The heating gases (generator gas is preferable) enter the multiple stage furnace at about 720°C and the exhaust gas tempera-ture is about 260°C. There are great many other methods which employ direct calcinlng. AmontS these are the venerable blast furnace method and the modern fluidized bed method. All of these employ direct calcining with its aclvantages and disaclvantages. nle advantages are primarily economic. The di s adv ant.age s are a frequent-ly variable setting behaviour and relrrtivefrequent-ly poor strength

properties.

(b) Indirectly calcined ＰＭｰｬ｡ｳｴ･ｾ

The best ｫｮｯｾｬｮ indirect method of calcining is probably that employing the Harz kettle ｜ｉｾｩｴｨ closed top. The considerably wider, open kettles· in which the gypsum flour charge is, however, only about 30 em high, are still frequently encountered in France. The gypsums produced in kettles and (Harz) calciners are superior in quality to the directly calcined gypsums, so that the principle of pr-oduct i on must not be changed in any ""Tay. However, the economy of kettle calcining must and can be considerably increased, on the one hand by using the exhaust gases from firing for the pre-drying of the gypsum rock to ahout 12 to 14% water content in a blast

furnace, or in the broken state in a drying drum. And in connection with this we find grinding must take place before calcining in the kettle. Other improvements are possible by proper insulation of the furnace block and by modifying the kettle floors so as to in-crease the area of the floor on the combustion space side with the aid of ribs and the like, Rnd also by the use of a better cast iron material which is more resistant to scaling, with the aid of nickel

*

Translator's Note: Aside from this sentence or where ｯｴｨ･ｲｷｩＸｾ

stated, the ｣｡ｬ｣ｩｮｾｲ discussed (however d.esignated) is the Harz device.

or aluminium alloying. Automatic heating is an obvious necessity. If the stirring equipment of the kettle is also improved (planet stirring mechanism), it may be said that more or less all of the technical possibilities will have been exhausted in order to

in-crease the efficiency of a Harz calcining ｰｬｾｮｴＮ There remain,

however, the chemical possibilities. With the aid of additives of

both organic and inorganic mixed salts the author was able to avoid

the ｾ･ｮ･ｲ｡ｬｬｹ accepted over-calcining of 180 to 190°C and to drain

off the kettles at 150°C (Fi8. 12) without altering the setting

times and the strength properties in a disadvantageous manner.

The calcinine; curve of Fi8. 12 is notable in that the first

bo.iLrng point during the brief calcining \lIas 6°C lower than for the

earlier gypsum ｣｡ｬ｣ｩｮｩｮｾＬ and mOreover the kettle mOre qUickly

reaches a temperature of 150°C. The 150 to 184°C calcining

dis-appears altogether.

Theoretically, in order to produce one metric ton of

hemi-hydrate, adding the separated amounts of heat necessary for

evapora-tion of the quarry moas tur-e (about 0.5/0 and of the water of

crystal-lization split off, a total of 170,000 kcal are needed for the

heuting of the GYPsum up to a transformation pOint of about 120oC*

and for the cherm ca L transformation from dihydrate to hemihydrate. The actual calory consumption per ton of hemihydrate and the

efficiencies of the calcining installations in five different plants are shown in Table V.

In the second plBllt, showing an efficiency of ＴｾＮＲＥＬ an oil

burner had been installed and the kettles could be well filled. It

may be said, therefore, that the efficiency of a well-conducted

Harz calcining plant is 43%. If chemical combustion aids are used

53 to 54% efficiencies are attainable, making the Harz kettle the

equal tn this r-espect to the continuously operating rotary kiln.

Unfortunately, the Harz kettle eats up wages, since for four kettle3,

•

There is sharp transformation point, as already mentioned

for example (of 2 tons hemihydrate capacity each) at least two, if

not three worker's arc required. Conver-si on to large kettles of

the American Ehrsam type, for example, wit}} 20 tons hemihyctrate

capacity is one means of ｳｾｶｩｮｧ wages. The GYPsum produced,

more-over, is much rilore uniform, since the calcining process in such

large kettles ｲ･ｾｵｩｲａｳ a longer combustion time and therefore the

gypsum is pr-odu ce d inq !I101'e gentle manner in an atmosphere vlell

saturated with vapours. Hhen it is taken into account that in this

case 20 tons of gypsum are handled on a floor area of 7 m2 this

becomes tantamount in each case, particularly for the lower layers of gypau!1l, to a kind of autoclave treatment, which because of the

stirring, of course, does not r-ernai n the same, but is nevertheless

effective in the long run. On emerging from the water of

crystal-lization, therefore, the gypsum grains are not so disrupted, so that the heats of hydration(17) come closer to those of the

80-called ｾＭｨ･ｭｩｨｹ､ｲ｡ｴ･Ｎ The gypsums produced in the Ehrsam kettle

are therefore characterized by higher strength values and a more stable setting behaviour.

In stationary kettles, including the Ehrsam type, heat transfer a Lways remains something of a problem owt ng to the inadequate

motion of the material, a probJem that can probably never he solved

sa ti s fact.or-iLy in the ke t t Le , so that one has to reckon with

con-stantly recurring floor damage, etc. The author therefore considers

the best method for the production of ｾＭｨ･ｭｩｨｹ､ｲ｡ｴ･ plasters to be

that employing externally-heated rotary furnaces of the type that

are being build and used, ｾｳ far as he knows, only in England. The

externally-heated rotary kiln combines the advantages of a more gently calcining- process, as in the larger kettle types, and an excellent motion of the ":Ypsv.m, since the kind of stirring needed in the kettles, and which is always inadequate, is replaced by the

rotation of the eontainer itself. A modern gypsum factory for

p-hemihydrate production is accordingly constructed on the following plan.

The preliminary crushing equipment is located at the quarry. It Gomprises a roll and hammer crusher or a duplex SWing-hammer

cr-usher- and furnishes a raw gypsum of 5 cm maximum erain size to

the gypsum factory. The raw gypsum is now fed to a two-stage

Grind-ing and dryGrind-ing apparatus whi ch car-r-Led out rned Ium {7indlng in the

first stage and fine rrrinding in the second stage accompanied by

drying and partial deb ydr-at.Lon , The drying gases employed are the

exhaust gases or the rotary furnace. which in the case of the

ex-ternally-heated rotary kiln are free of vapours. In practice the

rotary kiln is simply a turned-over, rotating kettle wh1ch is

as-sailed from all sides by hot gases. The calcining conditions are

thus similar to those found in the kettle plant; its advantages con-sist in the continuity of the precess and the elimination of

stir-ring. In the author's opinion the future belongs to the

externally-heated rotary ｫｩｬｮｾ

In conclusion we offer Table VI in which some comparative data

for kettle and rotary lci.Ln plants with respect to efficiency are

shown.

References

1. Eipeltauer, E. Silikatechnik, (1): 27, 1956.

2. Van1t Hoff. J.H., Armstrong, E.F., Hf.nr-f.ohsen , VI., l.Jeie;ert, F.

and Just, G. Z. phys. Chem. ?57, 1903.

3. Dubuisson, A. Revue des ｾＱ｡ｴ･ｲｩＸＮｵｸ de Construction, Edit. C,

No. 418, p.22S/232; No. 419, p. 259/264; No. 410, p. 282/287;

No. 421, p. 313/315.

4. Linck, G. and Jung, He Z. 8TIOrg. allg. Chern. 137: 407, 1924.

5. Keane. J. Phys .. Chern. 20: 701, 1916.

6. Desh, L. Tr8,ns. Ceramic Soc. 18: 1, 1918/19.

7. Haddon. C. and Brown, M. J. Soc. Chern. Ind. 43 (3): 11, 1925.

8. Fill, K. Diss. Berlin, 1931.

10. Ba1arew, D. and Ko1uschewa, A. Kolloid-Z. 70: 289, 1935. 11. Kelley, K.Y., ｓｯｵｴｨ｡ｲｾＬ J.C. and Anderson, C.T. Technical

Paper No. 625, Bureau of Mines, Berkeley, Calif. Washington, 1941.

12. Eipe1tauer, E. Zement-Ka1k-Gips, 9: 501, 1956 (CSIRO Australia Trans. 0662).

13. Partridge, E.P. and ｾｦｵｩｴ･Ｌ A.H. J. Am. Chern. Soc. 51: 360, 1929.

14. Posnjak, E. Am. J. Sci. 35A: 247, 1938.

15. Jo1ibois, P. Rnd Chassevent, I.. ｃｯｾｰｴＮ rend. 178: 1543, 1924. 16. Hall, R.H., Rohb, J.A. and Coleman, C.E. J. Am. Chern. Soc.

60: 1647, 1938.

17. Ridde1, W.C. Rock Products, 53: 68, 1950.

18. Hadnon, C.L. British Patent 582 749 (1946); A.P. 2 448 218 (1948); A.P. 2 460 266 (1949); A.P. 2 460 267 (1949). 19. Eberl, J.J. and Ingram, A.R. Ind. Eng. Chern. 41: 1061, 1949. 20. Knacke, 0, and Stronski, I.N. Z. f. E1ektrochemie, 60 (8):

816, 1956.

21. Southard, J.C. Ind. Eng. Chern. p.442, 1940.

Dehydrat1ng of powdered selen1te to the " a nhvdr1te" stage (measured thermograv1metr1c test values)

Durat10n of ther- Water content

Temperature mal treatment of the sample

%

by weight

---19°C 0 20.7 Increased to 40°C 1 day 20.67 II 50°C 1 day 20.66

"

60°C 1 day 20.64

"

65°C 1 day 20.58

"

70°C 1 day 20.41 II 75°C 1 day 20.08

"

80°C 1 day 18.92

"

80°C 1 day 16.17

"

80°C 1 day 11.56

"

BOoC 1 day 8.76 II _{BOoC 3 days 6.65}

,

80°C 1 day 6.43 80°C 1 day 6.38 85°C 3 days 6.35 90°C 3 days 6.21 95°C 3 days 6.16 100°C 2 days 6.04 105°C 2 days 0.94

,

_{105°C 3 days 0.78} II 110°C 2 days 0.65

"

120°C 8 hours 0.63

"

140°C 16 hours 0.38

"

150°C 16 hOU1'S 0.25 II 160°C 16 hours 0.17

"

165°C 1 day 0.126

,

_{170°C 16 hours 0.11} 170°C 2 days 0.08 200°C 2.days 0.06 220°C 2 days 0.05 250°C 7 days 0.045 280°C 3 days 0.045 310°C 1 day 0.04 325°C 2 days 0.03

Table II

The effect of sulphite waste liquor on the setting behaviour of gypsum

Sulphite Quantity Start of End of Flexural COr.lpressive

waste added setting setting strength strength

liquor kg/cm2 kg/cm2

%

gm min. min. 0 144 10 26 45 95 1 148 25 43 39 90 3 152 28 52 22 44 5 174 260 380

-

-Heats of hydration of the hemihydrates of calcium sulphate

Probable form present

ｾＭｨ･ｭｩｨｹ､ｲ｡ｴ･ a-hemihydrate Mixture of a- and ｾﾭ hemihydrate a-hemihydrate a-hemihydrate f3-hemihydrate Preparation Heated 3 to 4 hours 8t lloac in dry air

Heating in a water vapour atmosphere Heating of dihydrate in a covered cruci-ble at l45-l50 aC Heating of dihydrate

for 3 hours in satu-rated steam at 8 atm.

Heating of powdered selenite in a stir-ring autoclave at l50 aC continuously for several days Dehydration of sel-enite in the vacuum at looac followed

by ｨ･ｾｴｩｮｧ for

sev-eral days at looac

Heats of hydra-tion in cal/mole 3860 3850 3600 4100 4100 4600 Calorimetric method Dissolution of hemihydrate and dihydrate· in water Dissolution of hemihydrate and dihydrate in water Direct hydra-tion Dissolution of hemihydrate and dihydrate in hy-drochloric acid solution Dissolution of hemihydrate and dihydrate in hy-drochloric acid solution 1) Direct hy-dration 2) Dissolution of hemihydrate and dihydrate in hydrochloric acid solution Investigator de Forerand (23) Chassevent (24) Chassevent (25) Newman and l";ells (26) Southard (21) Southard (21) I N ｾ I

Table IV

Heats of hydration of hemihydrate plasters

Hemihydrate from common salt solution (containing Na) ,

Hemihydrate from calcium chloride solution (pure)

Autoclave gypsum (commercial product) Schottwiener alabaster gypsum

Autoclave gypsum (produced by the author at 147°C)

Autoclave gypsum (produced at 127°C,

5.3% H₂₀₎

Autoclave gypsum (produced at 127°C,

6.89% H₂₀₎

Hemihydrate (calcined up to 150°C,

H₂₀

=

6.05,%)

Hemlhydrate (calcined up to 200°C and stored for one week, H

20

=

6.10%) 3565 cal/mole 3600 cal/mole 4380 cal/mole 4620 cal/mole 4000 cal/mole 3970 cal/mole 4250 cal/mole 4640 cal/mole 4450 cal/mole

Table V

kcal/ton hemihydrate Efficiency

1. plant 489,000 34.8 2. plant 394,000 43.2 3. plant 540,000 31.5 4. plant 535,000 31.8 5. plant 483,000 35.2 Table VI

Efficiencies of various gypsum calcining plants

Harz kettle, antiquated (2 ton hemihydrate) Harz kettle, with automatic heating

Harz kettle, with automatic heating and combustion assistance

Ehrsam kettle (20 ton hemihydrate) Rotary kiln, internally heated

Rotary kiln, externally heated (exhaust gases for pre-drying and partial

nehydration of the raw ｾｰｳｵｭＩ

30 - 35% 40 - 45% 50 - 55% 54% 49 - 56% 54 - 58%

• •

o

0 0 0

(] [) 8- -8

<J

I>

8- -8

<Jf)

8--8

. 1

<11>

g- -8

-

-(J,f>

ＮＺｾｾ

8-:-8

(]!)

8-:-8

<Ji> -g- -8

-(JI>

8- -8

ｾｴ＼ｊ

[)

8--8

Ｈ｝ｾ _0 ..f"1t-\.

o

0 0 ﾰｯＢＧｾｯ

-

\'

ｾ

ｾ

-

ｾ

ｾ

"

,

"

,

"

ＮｾＧｾｾＭ

,

,

ｾ

Ｂ ｾｾｾ

" \ - 6.S'!. " \

ＭｾＢＭｲｳｬＭ

Fie. 1

Structural pattern of gypsum according to W.A. Wooster(37)

Top: Projection of the structure on the plane a x sin ｾ - b,

c-uxi s perpendicular to the drawing plane. SQq. -tetrahedrons

seen slanted towards the front

Bottom: Construction of a single Ca-S04 layer. b-axis

per-pendicular to the drawing plane. S04 tetrahedrons seen from

above. Two edges run parallel to the drawing plane. The

Fig. 2 and 3 ｮ･ｨｹｾｲ｡ｴｩｯｮ of raw gypsum b-axis: c-axis: a-axis: No dehydration;

Rapid advance of dehydration; Dehydration lagging behind that

t.o .:t-O en III .8 u ｾ ..c: Ｎｾ .6 (\J :l >. .0 .4 0

.

•

.

1\

.

ｾＭ ｾＭＭ -._._..•._--

-\:

\

ao.o.s.J.lI'eJtt ｾＭ 8 ｾ. . ,ＱｉｯｾｾＮ ' ...n _

\

• till 0 ... " AlIon oPmtt"'ido','-'hit• ｣ｾ • ｐｏｩｾ o R,ddell • Tl)f"IU""",Wor'"o _

.

ｾ

_ｾ

_• t--- _•

I

o 50 400 ISO Tl:MPERAl\JR£, "c. 100 250 Fig. 4

Solubility curve of dihydrate (curve A - B) Solubility curve of hemihydrate (curve E - F)

F1g. 5

Hemihydrate plaster set

Add1t1ve: Pure water

(Magn1f1-cation: 125 times)

Fig. 7

Hemihydrate plaster set

Additive: ''later + 0.5% sulphite

waste liquor (Magnification: 125 times)

Fig. 6

Hemihydrate plaster set

Additive: Water + 0.1%

sul-phite waste liquor

(Magnif1-cation: 125 t1mes)

F1g. 8

Hemihydrate plaster set

Additive: Water + 1%

sul-phite waste liquor

F1g. 9

Hem1hydrate plaster set

Add1t1ve: Water +

3%

sulph1te

waste l1quor (Magn1f1cat1on:

l2f; t ime s

I

F1g. 10

Hem1hydrate plaster set

Add1t1ve: water + 5% sulph1te

waste l1quor (Magn1f1cat1on:

Fig. 11

Heats of solution in 2N hydrochloric acld

(Quantlty lntroduced: 10 gm, in 600 cc 2N hydrochloric acld) Left of figure: Anhydrite III (Water absorptlon ln air under normal

pressure). Deslgnatlon of curves, top to bottom:

I} ｾｔ

=

+ O.lOOC, 0.6% H2 0 , - 908 cal/mole; 2} ｾｔ

=

0.05°C, ＱＮＲｾ

H20 , - 456 cal/mole; 3} ｾｔ

= -

0.1050C, =3.4% H2 0 , + 980 cal/mole;

4} ｾｔ - - 0.12°C, 4.7% H₂ 0 , + 1137 cal/mole; 5) ｾｔ = - 0.130C,

6.1% H₂0 , + 1250 cal/mole

Right of figure: Selenite and Schottwlener alabaster gypsum. Identlficatlon of curves: top: ｾｔ

= -

0.498°C, 20.8% H₂0 , +

5677 cal/mole; bottom: ｾｔ

=

0.114°C, 6.05% H_{2 0 ,} + 1060 cal/mole

301 'C 110 ISO flO ＭＭｾ ｾ - -ｦＭＭｾ

-...

I JI • ｾ :UV \, y 50 , /irs. Fig. 12

Transltlon ln an Austrian gypsum plant from the usual gypsum cal-clning to the author's rapld process

PART II

Abstract

The determination of the calcium sulphate

dihydrate-hemihydrate transformation point ｵｳｩｮｾ concentrated salt

solutions as dehydrators, as first introduced by van't Hoff and his cOllaborators, is an elegant and relatively

rapid method of determination, but one which ｾｵｳｴ not be

used with just any salt solution. With a calcium

chlor-ide solution as the dehydrating agent a transformation

point of 103.loC _{is found in the present case.}

In the course of carefully conducted dehydrations and radiographical examination of these dehydration

products C.W. ｆｬｾｲｫ･Ｇｳ discovery of a soluble anhydrite

is' confirmed. Introduction

In Part I of this paper(l) I dealt with the dihydrate-hemihy-drate transformation pOint of calcium sulphate which is important among other things for the industrial production of hard gypsum, and showed why van't Hoff and his cOllaborators(2) obtained such a

high value for it. I also emphasized that I nevertheless consider

this van't Hoff method, which had been used almost 60 years earlier for the determination of this transformation point, to be especially advantageous when modified by the use of other salt solutions.

I therefore continued these investigations, using a calcium chloride solution as a medium for removing water of crystallization. For comparison purposes a sodium cllloride solution was again used and finally also a potassium chloride solution, since the latter shows particularly clearly that just any salt solution cannot be employed for determining the dihydrate-hemihydrate transformation pOint of calcium sulphate.

It was through a communication from Prof. Haaz " that I Lear-ned of O.W. FIOrke's paper "Kristallographische und rOntgenometrlsche

Untersuchungen tm System CaS04 -CaSO • Ｒｾ 0" (Crystallographic and

radiometric investigation in the CaS0

4-CaSO • 2H20 system), which

deals with the investigation of the fine structure of calcium

sul-phate hemihyorate 8nn soluble anhydrite. In what follows reference

will be made to these communications and to my previous and current results.

The Dihydrate-Hemihydrate Transformation ｐｯｾｾｾ

This transformation point may be defined as the temperature at which the vapour pressure of the dihydrate of calcium sulphate

reaches the vapour pressure of the saturated gypsum solution. If

the temperature is increased then a vapour pressure gradient occurs

that will be compensated by dehydration of the dihydrate. The

com-pensation may take place via the gaseous phase, water of crystal-lization evaporating from the dihydrate while 'dater vapour

con-denses and dilutes the saturated gypsum solution. If this process

proceeds via the gaseous phase then the dehydration is comparatively

81m\1. It can be accelerated, however, by reducing the surface

tension of the enveloping aqueous solution, thereby facilitating its penetration between the double layers of calcium sulphate dehydrate. This can be done, for example, merely by the addition of salts.

The reduction of the surface tension results in a more intimate con-tact between the dihydrate and the surrounding aqueous medium, and

the vapour pressure ･ｾｵ｡ｬｩｺ｡ｴｩｯｮ proceeds more rapidly. The

"dehydrator" thus penetrates the water layer planes of the gypsum and very lorgely eliminates the gaseous phase as an intermediary •

•

Prof. Raaz, Director of the Institute of Mineralogy,

Crystallo-graphy and Applied PetroCrystallo-graphy of the Technische Hochschule,

Vienna. I thank him very much for having drawn my attention to

this important paper. It is unfortunate that this paper has not

been abstracted in Chemisches Zentralblatt, as it presents new results for constructional chemists as well.

It is also ev1c1ent here that both the physical and chemical changes of the calcium sulphate dihydrate take place by preference along the water layer planes of the calcium SUlphate dihydrate. An example of a chemical transformation of the gypsum is given by

Fig. 1 and 2, which provide an excellent illustration of this

principle. It is clear that the ammon1ur.l fluoride attack is much

more rapid than that of the Br.lmon1uI:l carbonate. This can be

attri-buted to the smaller radius of the fluorine ion. The attack occurs,

for practical purposes, only along the water layer planes. The

fact that no compor-ab Le attack takes place perpendicular to the layers is ev1nent from the continuing transparency of the grain

centre. In principle these observations hold also for the

dehydra-tion of gypsum. Even 60 years ago V21I1't Hoff was applying the

ex-pedient (after failing to construct the vapour pressure curve of

calcium sulphate dLhydr-at e by means of a tens1meter owing to the

lack of an equilibrium adjustment) of dehydratll1E; the dihydrate in

aqueous media, wi thout however- explaining why equt Li.br tum now had to

set in much more rapidly.

Van't Hoff and his collaborators found a calcium sulphate

d1-hydrate-hem1hydrate ｴｲ｡ｮｳｦｯｲｾ｡ｴＱｯｮ point of 107.2°C. However, the

fact that almost 10% sodium sulphate is incorporated in the

reSUlt-ing hem1hydrate was overlooked, and this, of ｣ｯｵｲｳｾＬ affects the

transformation temperature.

After I had discovered the reason for this too high trnnsforma-t10n point it was an obvious step to try a calcium chloride solution

as a dehydrator in place of sodium cblor1de. The results of the

investigation are given in \'lhat f'o Ll.ows , In order to gain a further

insight into this interesting matter potassium chloride was also included in the investigations.

ｄ･ｳ｣ｲＱｰｴｾｯｮ of the Experiments

The experiments were carried out in B Beckmarm vessel of about

100 cc capacity. A Peckmann thermometer and a reflux condenser

completed the setup. The entire apparatus was protected against air

water, taking into account" the prevailing atmospheric pressure. In order to prevent any superheating of the water small, porous,

ceramic hlocks wer-e added until no further change in boa Ling point

occurred, even when the supply of heat was varied somewhat. The

bOiling point remained constant within about O.OloC. After

deter-mining the boiling point of the pure water and taking the ｢ｾｲｯｭ･ｴ･ｲ

reading, 40 gm of finely ground pure plaster stone containing 20.8%

water of crystallization was added to 80 cc of the boiling water slowly, so that the solution boiled continuously.

Sodium chloride was added in I gm portions every 30 minutes

(first series of tests), and the bOiling point was measured. The

water of crystallization content was determined analytically, while the appearance and changing of the crystals were observed under the microscope.

The calcium chloride (second series of tests) was also added

in I gm quantities, but at intervals of 4 hours, since the

dehydra-tion was considerably retarded by the use of calcium chloride. This is probably due to the lower solubility of calcium Gulphate

in a calcium chlorine solution.

In a third series of tests potassium chloride was added instead

of sodium chloride. The procedure was the same as for test series 1.

Calculation of the ｔｲ｡ｮｳｦｯｲｭ｡ｴｾｯｮ Point

The temperature is determined mathematically at which the

vapour pressure of the water of crystallization of calcium sulphate

dihydrate is equal to that of the saturated gypsum solution. Since

the solubility of calcium sulphate in water is slight, the activity of the water is lowered insignificantly so that the vapour pressure of a saturated gypsum solution can be equated to that of water. The temperature dependence of the heat of vaporization of the water and the heat of transformation of the dihydrate can be disregarded, since the boiling point of water and the dihydrate-hemlhydrate

f'o LLowt.ng; formulae therefore hold

., ;1n, (1 1 )

log P" - log Pg " = -1:1 .4-57--4-- T

-, g T,

where To - bOiling temperature of water

Tg - temperature at which the vapour pressure of the water of crystallization equals the external at-mospheric pressure

T₂ - transforoation temperature of the dihydrate

6Hw - heat of vaporization of water

6Hg - heat of hydration of the hemihydrate

P_w - vapour pressure of water

P_g - vapour pressure of the dir.ydrate

Pgo - vapour pressure of the dihydrate at Tg

If we put Pw

=

Pg (at the transformation point) it follows that

.iI HI\' ( 1 1 " .,. <1 n, (' 1 1 ) I 4,574 r;;- -1';"""") = -/:l' 4,574 Tg - 1';" + og Pg o -- LI H\\ + 21" L1 Hf' 4,574 4,574 Te =

2/, AH" ;:jHI\' _{+ log P"o}

4,574 4,574

For further determination of the transformation temperature T2

of the dihydrate it is necessary to know the heat of hydration. As

I have already stated in a previous paper(4), very different values

are found for this in the literature. My own measurements of the

heat of hydration of a hemihydrate obtained by dehydration of

plaster stone in a 30% ｃ｡ｃｾ solution, in which the hemihydrate

could crystallize out without difficulty, showed a heat of hydration

of 3600 cal/mole at room temperature. This value increases at

100°C to 4055 eal/mole. Substituting this value in the above

point in a

(a) sodium chloride solution ••..•..•..•.•.•• l06.9°C,

(b) CaC1₂ solution •..•••...••..•.•• l03.1oC,

(c) potassium chloride solution ••... l05.4°C.

From ｴｾ･ｳ･ results it is evident that the value given for an

NaCl solution agrees well with the result obtained by van't Hoff et al., but when a calcium chloride solution is used the

transforma-tion point is l03.1oC. _{Since the resulting hemihydrate in this case}

is founn chemically pure - containing only a few hunnredtts of one

percent Cl - the figure of l03.1oC _{is the more accurate.}

The transformation point l05.4°C, obtained by using a potassium chloride solution as the dehydrator, cannot be the dihydrate-hydrate transformation point, since a pure calcium sulphate

hemi-hydrate is not obtained here either. Instead we find almost

entirely a wholly different compound of composition 5CaS0₄ • K_QS04 •

H2 0 . This compound was also prepared synthetically and checked by

radiographic comparison in ｄ･｢ｾＭｓ｣ｨ･ｲｲ･ｲ pictures (Fig. 4 to 6).

As these investigations show, when gypsum is dehydrated with a KCl solution actually no hemihydrate occurs, but instead the

com-pound 5CaSO_{4 2 4}• K SO • H O.₂

The Structure_.9f the ..9_alcium SulP.-hate HemihYdrate and the Soluble Anhydrite

In Part I of this paper(l) I reported on some dehydration ex-periments with gypsum using a thermobalance and X-ray diffraction

studies of the stages of 1ehydration with the aid of Debye-Scherrer Pictures, wbich led me to the conclusion that the "soluble

an-hydrite" would be better described as a "dehydrated hemihydrate", since this soluble anhydrite still contains water of crystalliza-tion, whereas "anhydrite" would refer to a compound with no water

of crystallization at all. When the water of crystallization was

completely driven off then insoluble anhydrite was already obtained. The Debye-Scherrer pictures showed no difference in the line arrange-ment and the intensity of the lines for hemihydrate and soluble

anhydrite, but the change in lattice dimenGions was clear. I con-cluded from this that the transition from hemihydrate to soluble anhydrite takes place without any· lattice transformation.

From thorough X-ray diffraction structural investigations,

however, O.W.

ｆｬｾｲｫ･ＨＳＩ

found a hexagonal lattice fOr the soluble

anhydrite, which he prepared in the vacuum at 50°C over phosphorous pentoxide, whereas for the hemihydrate, which he obtained and kept at a temperature above 45°C, he found a rhombohedral lattice.

O.W. ｆｬｾｲｫ･Ｇｳ results led me to conduct further dehydration

tests at standard atmospheric pressure with the aid of a

thermo-balance and taking special care to eliminate the effect of

atmospher-ic humidity. In my earlier experiments I had used thin glass rods

as substance supports and it was not impossible that despite the organic adhesive employed a partial rehydration of the soluble

an-hydrite had occurred. For this series of tests, therefore,

Lindemann tubes were used. The samples were placed in these as

ｱｕｩ､ｾｬｹ as possible at the given dehydration temperature and were

ｴ･ｾｰ･ｲ･､ in another furnace at the same temperature for an

addition-al two hours. Then the ends of the tubes were melted off in the

furnace. The X-ray diffraction results of these experiments is

re-vealed in the remaining Debye-Scherrer pictures (Fig. 7 - 10). It is evident from Fig. 7 to 9 that up to a water content of 0.2%, at least under my experimental conditions and with the X-ray diffraction means available to me, there is no indication of soluble

anhydrite. Fig. 10, compared with Pig. 7 to 9, shows some

differ-ences in the intensity and arrangement of the black lines. Thus

O.W. Flerke's findings (continuing the work of H.B. Weiser, W.O.

Milligan and \l.C. Ekholm(5» with respect to the existence of a

soluble anhydrite with a lattice differing from that of the

hemi-hydrate, must he accepted as correct. It is obvious, however (see

Fig. 9 and 10) that hemihydrate and soluble anhydrite must have very similar lattices.

In conclusion I should like to mention another important point