Tweet

---

June, 1924

---

The extensive use of silica bricks in coking plants in America has attracted considerable attention from coke-oven owners in Great Britain within the last few years, and a close study of American practice by the Consett Iron Co. Ltd. decided this firm to use silica instead of ordinary English refractories in the construction of their new coke works. Difficulty in obtaining a silica brick of sufficiently high quality further decided this enterprising company to lay down their own brick-making plant and to turn out a product to meet their requirements.

The advent of the silica brick in America has been brought about by the use of higher temperatures in coking batteries than the ordinary class of refractory will withstand. In a paper read by Mr. A. H. Middleton* before the Society a silica brick is defined as "one made from a quartzite or ganister rock containing 97 per cent. of silicon dioxide to which nothing is added except the necessary fluxing or bonding material." In the same paper a brick containing from 77 per cent, to 85 per cent. of SiO2 — the type in general use in this country — is defined as a "quartzite brick." The latter term appears to us to be a somewhat unfortunate one since quartzite has a well established geological meaning. Although it is by no means perfect for the purpose, the term "quartzose" is here suggested as a more accurate alternative, and this will be adhered to throughout this article.

* Trans. Ceramic Society, vol. xxii, 1922-23.

Advantages of Silica Bricks.

Briefly, bricks made of silica may be said to be more efficient thermally, to last longer under ordinary conditions and to resist better the corrosive effect of salty coals than the ordinary quartzose varieties. A further superiority lies in the fact that it is possible to vary the heat treatment when it is required to coke different classes of coal. The paramount advantage, of course, is that an increased rate of output is obtainable from a silica construction, a fact which is very apparent from even a casual study of the results obtained from American ovens.

Thermal Efficiency of Silica Bricks.

The extent to which the higher thermal efficiency of the silica affects the output of the battery is difficult to estimate since it is impossible to say to what degree an increase is due to the higher working temperature.

It has been suggested that while there may be little difference in the conductivity of silica and quartzose bricks at temperatures below 1,000° C. a study of American practice would appear to indicate that some factor, other than the increased temperature alone, tends to reduce the carbonising period. Or, in other words, since the increased temperature in itself is not sufficient to explain the greatly reduced coking time it seems reasonable to assume that this is attributable to the higher conductivity of the silica material at temperatures above 1,000° C. A similar conclusion was deduced by Dr. Erzenauer, a German coke oven manager, who observed that in ovens built of silica and quartzose bricks respectively, working side by side, the temperature of the burnt gas in the horizontal flue of the silica oven was 42° C. more than in the case of the quartzose oven. It was also noticed that the temperature in the silica oven at the end of the coking period was 104° C. higher than the corresponding temperature (804° C.) taken in the oven walled with quartzose bricks. Without being absolutely conclusive this evidence is indicative that silica is a better conductor of heat than the quartzose material. It has been questioned whether any advantage accrues from having oven walls of any higher conductivity, but the fact that this quality would enable the oven to recover its working temperature quicker after the reduction due to a fresh charge, is surely a sufficient answer.

Lasting Qualities of Silica.

Evidence of the greater durability of silica under ordinary working conditions is at present purely a matter of accepting the opinion of those who have made a first-hand comparative study of both types of oven walls. In making the comparison it should be remembered that as the coking period of the silica construction is of shorter duration — in common practice a silica oven is pushed 100 times while one of quartzose brick is pushed only from 60 to 70 times — the life should be correspondingly increased if an accurate estimation is to be obtained. In other words, the life of the walls should be calculated on an output basis rather than merely taking the actual working existence into consideration. It is necessary, therefore, to consider American data before it is possible to draw conclusions, and when this is done, opinion is emphatic on the superiority of the silica brick.

Corrosive Effects of Salty Coal.

Particularly in districts where salty coal is mined a silica construction would appear to offer very material advantages to the owners of coking plants. The corrosion due to the presence of sodium chloride has on occasions caused the oven walls to be renewed within a period of nine months and, even at the best, the life of walls made of ordinary material rarely exceeds three years. A British example may be cited as a case in point; it is estimated that from 0.35 to 0.5 per cent. of sodium chloride is normally present in the coal, and as the moisture ranges from 10 to 14.5 per cent, the conditions are exceptionally severe. The temperature of the side walls in this case is about 1,100° C., following the usual English practice. Two batteries of ovens are at work, each being operated under precisely similar conditions. The one battery is walled with silica bricks and has now been in existence for six years whilst the other, of quartzose brick, has had to be re-walled at periods ranging from three to nine months. The resistive powers of silica against corrosion are further exemplified by a peculiar phenomenon which was observed in a third battery of the same plant. Originally, quartzose bricks were used in the construction, but when the walls were repaired stretcher bricks of silica and quartzose header bricks were used. Very soon the headers were again eaten away by the corrosive action of the salt and as the silica bricks remained more or less intact the walls presented a most peculiar appearance, the surfaces being a series of hollows and mounds, rendering pushing operations difficult and finally impossible.

Possibility of Varying the Heat Treatment.

American practice has shown that it is feasible to coke varieties of coal containing relatively high precentages of volatile constituents. The results would certainly appear to be encouraging so far as the coke production is concerned. This practice is made possible only by the use of silica bricks, which enables ovens to be worked at temperatures capable of coking, with satisfactory results, a coal which would entirely fail to coke properly at temperatures considered normal in Britain. The utilisation of silica bricks in regard to this particular point would certainly suggest a means by which many colliery owners would solve a very difficult problem. If confined to this sphere alone the silica brick would still appear to have a wide field of utility.

Difficulties of a Silica Construction.

It is a curious and depressing fact, intruding as it does into every conceivable province of human activity — and especially the province of engineers — that a series of advantages cannot be secured without corresponding drawbacks. In this respect silica bricks are by no means an exception to the rule. The chief drawback to the use of a silica construction, as often made, is its liability to undergo "permanent expansion," this being especially the case when ovens are worked at the temperatures common to American practice. Fortunately, however, it is possible to reduce this permanent expansion to a degree which need not give rise to any anxiety. Whereas, in ordinary English practice, with oven walls composed of the quartzose variety of bricks and a working temperature of, say, 1,100° C., the expansion rarely exceeds 1 or 2 in., the expansion of a silica oven, of the same length but worked at a temperature in the neighbourhood of 1,300° C., would be at least 4 in. Even a high-grade silica construction would expand this amount, and with a badly burnt variety the expansion might be expected to approach 10 in. or more. While an expansion of from 4 to 6 in. can be accommodated in the construction of the oven, an expansion of twice this amount would, obviously, present insuperable difficulties in design to prevent the walls becoming seriously distorted. The explanation of this high degree of expansion in silica bricks when the temperature is raised much above 1,100° C. is a somewhat complicated one, and it is necessary to inquire carefully into the changes which take place in the material of which they are composed as it passes from the raw state to the finished brick.

Allotropic Forms of Silica.

Silicon dioxide (SiO2) occurs natively in many different forms, each having a distinct crystalline peculiarity and a different specific gravity. The three particular forms which enter into and complicate brick-making are quartz, cristobalite, and tridymite. In the making of a silica brick which will successfully resist a coke-oven temperature of, say, 1,500° C., to take a peak temperature liable to occur, a certain amount of the material actually passes successively from the first to the third of these stages. As these three forms have different specific gravities — 2.650, 2.233, and 2.270 respectively — it will readily be understood that the conversion from one relative density to the other will be accompanied by a corresponding increase in volume. Should this process not be completed during manufacture but take place to a greater or less degree in the finished oven walls, it is clear that the consequences would be disastrous. At the temperatures attained in ordinary English practice practically no conversion takes place, since the process is exceedingly slow at temperatures below 1,350° C. Between this temperature and 1,500° C., however, the rate of conversion increases rapidly but varies with different samples of quartz. With some varieties the conversion is very slow even when the temperature verges on fusion point. In Fig. 1 the results of a series of experiments, on thermal expansion by Le Chatêlier are shown graphically. The changes which take place during heating are clearly demonstrated by the curves. It will be noticed that the quartz expands more or less equal amounts for equal increments of temperature until 575° C. is attained, when an inversion takes place due to the unstable nature of this particular allotropic form at this temperature. The cristobalite form of silica shows a similar but even more striking inversion at a temperature of about 220° C. On the other hand, the tridymite curve of expansion, while it gives evidence of a small amount of inversion about 130° C., afterwards expands steadily until a temperature of 800° C. is reached, when, curiously enough, a contraction takes place.

Importance of Correct Heat Treatment.

An imperfectly burnt silica brick consists almost entirely of quartz; a brick that has been indifferently well burnt will contain a fair proportion of cristobalite, whilst a perfectly burnt brick, if initially composed of the right material, may be expected to contain a considerable amount of silica in the tridymite form. As this is the least subject to sudden increases in volume when heated, it is evident that the more tridymite the finished brick contains, the better it will be. Unfortunately, it is not a practical commercial proposition to manufacture a brick consisting entirely of tridymite. When 30 per cent, is present, however, excellent results are obtained in actual practice.. In Fig. 2 the solid curve shows the thermal expansion of such a brick. It will be noticed that while the rate of expansion is rapid below 600° C. it falls off suddenly when this point is reached, the remainder of the curve indicating that little expansion takes place between 700° C. and 1,400° C. At the latter temperature a slight contraction is observable. The underburnt brick, represented by the dotted line, does not expand quite so quickly under about 590° C., but once this temperature is reached its behaviour becomes erratic and from 1,100° C. onwards the rate of expansion is extremely rapid. Also it must be remembered that should the heating be continued over any length of time the expansion would become permanent.

Commercial Quality.

It has already been remarked that the different allotropic forms of silica dioxide have different specific gravities. When one form is converted to the other in the kiln it is evident that the final specific gravity of the brick will be altered in proportion to the amount of conversion that has taken place. It will thus be seen that the specific gravity of the finished product will bear a definite relationship to the degree to which the original quartz has been changed into cristobalite and tridymite. Its specific gravity thus gives an excellent indication of the quality and suitability of the brick. The specific gravity must, however, be taken by a special method or an apparent and erroneous result will be obtained, due to the porosity of the brick and the phenomenon of surface tension in the interstices.

The Templeton Brickworks.

In view of these many considerations it will be obvious that a supply of good quality ganister is imperative to the successful manufacture of silica bricks. The Consett Iron Co. Ltd. have been fortunate in their new venture in acquiring a large royalty of proved extent containing an excellent quality of ganister, the usable strata of rock averaging 6 ft. in thickness of 98 per cent. pure silica. The quarries are situated at Buttsfield, within a few miles of the works. After a preliminary sorting at the quarry the rock is loaded into trucks and brought to a stocking ground at the works. Here it is washed and undergoes a further sorting, the different grades being stacked separately. To facilitate the washing of the stone, the stocking ground is fitted with hydrants spaced at convenient intervals.

Samples of raw material are examined microscopically and are subjected to various physical tests and the hardness, resistance to impact and to crushing stresses are ascertained. Analyses for chemical purity are also conducted, but, in reviewing the various results obtained, experience has determined that more reliance is to be placed upon comparative trial-and-error conclusions than upon a rigid adherence to the physical and chemical properties.

Drawings and Photographs accompanying the article