Edison Portland Cement - 5

Edison Portland Cement - 5

 In cement-making, generally speaking, cement rock and limestone in the rough are mixed

together in such relative quantities as may be determined upon in advance by chemical

analysis. In many plants this mixture is made by barrow or load units, and may be more

or less accurate. Rule-of-thumb methods are never acceptable to Edison, and he devised

therefore a system of weighing each part of the mixture, so that it would be correct to a

pound, and, even at that, made the device "fool-proof," for as he observed to one of his

associates: "The man at the scales might get to thinking of the other fellow's best girl, so

fifty or a hundred pounds of rock, more or less, wouldn't make much difference to him."

The Edison checking plan embraces two hoppers suspended above two platform scales

whose beams are electrically connected with a hopper-closing device by means of

needles dipping into mercury cups. The scales are set according to the chemist's weighing

orders, and the material is fed into the scales from the hoppers. The instant the beam tips,

the connection is broken and the feed stops instantly, thus rendering it impossible to

introduce any more material until the charge has been unloaded.

The fine grinding of cement clinker is distinctively Edisonian in both origin and

application. As has been already intimated, its author followed a thorough course of

reading on the subject long before reaching the actual projection or installation of a plant,

and he had found all authorities to agree on one important point--namely, that the value

of cement depends upon the fineness to which it is ground.[16] He also ascertained that in

the trade the standard of fineness was that 75 per cent. of the whole mass would pass

through a 200-mesh screen. Having made some improvements in his grinding and

screening apparatus, and believing that in the future engineers, builders, and contractors

would eventually require a higher degree of fineness, he determined, in advance of

manufacturing, to raise the standard ten points, so that at least 85 per cent. of his product

should pass through a 200-mesh screen. This was a bold step to be taken by a new-comer,

but his judgment, backed by a full confidence in ability to live up to this standard, has

been fully justified in its continued maintenance, despite the early incredulity of older

manufacturers as to the possibility of attaining such a high degree of fineness.

[16] For a proper understanding and full appreciation of the importance of fine grinding,

it may be explained that Portland cement (as manufactured in the Lehigh Valley) is made

from what is commonly spoken of as "cement rock," with the addition of sufficient

limestone to give the necessary amount of lime. The rock is broken down and then

ground to a fineness of 80 to 90 per cent. through a 200-mesh screen. This ground

material passes through kilns and comes out in "clinker." This is ground and that part of

this finely ground clinker that will pass a 200- mesh screen is cement; the residue is still

clinker. These coarse particles, or clinkers, absorb water very slowly, are practically inert,

and have very feeble cementing properties. The residue on a 200-mesh screen is useless.

If Edison measured his happiness, as men often do, by merely commercial or pecuniary

rewards of success, it would seem almost redundant to state that he has continued to

manifest an intense interest in the cement plant. Ordinarily, his interest as an inventor

wanes in proportion to the approach to mere commercialism--in other words, the

keenness of his pleasure is in overcoming difficulties rather than the mere piling up of a

bank account. He is entirely sensible of the advantages arising from a good balance at the

banker's, but that has not been the goal of his ambition. Hence, although his cement

enterprise reached the commercial stage a long time ago, he has been firmly convinced of

his own ability to devise still further improvements and economical processes of greater

or less fundamental importance, and has, therefore, made a constant study of the problem

as a whole and in all its parts. By means of frequent reports, aided by his remarkable

memory, he keeps in as close touch with the plant as if he were there in person every day,

and is thus enabled to suggest improvement in any particular detail. The engineering

force has a great respect for the accuracy of his knowledge of every part of the plant, for

he remembers the dimensions and details of each item of machinery, sometimes to the

discomfiture of those who are around it every day.