September, 1925
INDUSTRIAL A N D ENGINEERING CHEikiISTRY
895
The Study of Coke Macrostructure’ By Harold J. Rose THEKOPPERSC O M P A N Y LABORATORIES, PITTSBURGH, P A
Sperr and Ramsburg met these difficulties by sectioning HE cellular structure of coke is perhaps its most obvious physical characteristic. Although this structure is full-sized coke pieces with a thin alundum cutting wheel extremely variable, and is characteristic both of the rotating a t high speed. This method has proved so practical coals used and the methods of manufacture, no adequate that it has become routine practice in this laboratory to secmethod of describing and illustrating it has existed in the tion several typical pieces of every coke sample received for examination. The apparatus has been described by past. Thorner?published micrographs3 of thin coke sections which Examination of Coke Surfaces were taken by means of transmitted light. Owing to the thickness of the sections used, the cell valls appeared to be The casual examination of a full-length coke section a t onw gives information as to it3 dense and structureless, and , u n i f o r m i t y and general a wrong impression was con, structure. But when an atveyed which has persisted to The study and reproduction of coke cell stru’cturehas tempt is made to study the the present time. Various proved difficult, because the coke substance is black, structure in detail, either technical articles appearing and much of the surface is highly reflecting. The with the naked eye, or with from time to time have condifficulties due to these conditions can be eliminated or a microscope, a serious difitained macrographs of coke minimized by filling the cells with a white material, by culty is encountered. The surfaces taken by means of polishing a plane surface, or by making an ink print coke substance is black and For the reflected light. from a plane surface. the cells have a glossy surmost part such illustrations Typical results obtained by each method are photoface which strongly reflects have been of unsatisfactory graphically reproduced and the macrostructures of a light. The resultant field is photographic quality, and wide variety of American cokes are illustrated. a black background broken have been of doubtful value by hundreds of dazzling in illustrating the points in points of light. This effect question. Beilby,: in presenting a discussion of coke structure accompanied by several is very trying to the eyes, and the brilliant reflections of the good macrographs, emphasized in considerable detail the source of illumination are responsible for halation and difmany difficulties encountered in working with coke surfaces. fusion disks in a photographic image. Another problem encountered in photomicrographic work is the depth of the Sectioning Large Coke Pieces cells, which makes critical focusing difficult. Beginning work on a systematic study of coke cell structure An important contribution to the study of coke structure was made by Ramsburg and S ~ e r r of , ~The Koppers Com- in 1920, the writer soon found that these difficulties could be pany, in 1917. These writers presented an extensive paper met in three ways: (1) by grinding and polishing the coke on the manufacture and grading of metallurgical coke, in surface; (2) by filling the cells with a white composition; and which they described a method for cutting large sections of (3) by making ink prints from the coke surface. coke for examination. Grinding and Polishing the Coke Surface One of the difficulties formerly encountered a t the outset of every attempt a t systematic coke investigation was that of Since a large part of the confusing effect seemed to be due to preparing large, flat surfaces of coke. It is obvious that large the irregular and broken condition of the cell walls in the surfaces should be used for study, since different parts of the surface plane of the coke piece, the effect of grinding and of same piece of coke have been subjected to quite different polishing the surface was investigated. It was believed that coking conditions, and furthermore, the coals from which a considerable part of the confusing reflections could be coke is made are not homogeneous. eliminated by giving all the cell walls in the surface plane the If a full-sized piece of coke is broken, the initia,l breakage same definite reflecting angle. It was also expected that new occurs along the longitudinal and cross fractures that are details would appear in polished specimens, owing to the varypresent. The surfaces of these natural fractures a,re more or ing degree of polish taken by different constituents of the less covered with a carbonaceous deposit, which obscures the .coke. These expectations proved to be fully justified, and true cell structure. Additional breakage will reveal the such specimens were found to be excellent for microscopic true structure, but such surfaces are usually small a,nd ragged. examination a t any magnification, both with vertical and A hack saw is not satisfactory for sectioning hard, dense oblique reflected light. The method used for preparing coke. Grinding away the outside surface of coke is tedious the specimens will be described later in this article. and produces large quantities of fine dust, which annoys the When examining coke a t medium or low magnifications; operator. especially with a binocular microscope, the cell walls stand out in good relief, and by focusing up and down and by moving Presented under the title “Methods for Studying the LMacrostructure the source of illumination about, one is able to obtain an of Coke” before the Section of Gas and Fuel a t the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8, 1922. accurate three-dimensional conception of the cell structure. Received June 4, 1926. But when coke is photographed under magnification, the reSfahl u . Eisen, 4 , 71 (1586). sults are usually disappointing unless stereoscopic images are 3 By A.S.T.M. standard definitions, a graphic reproduction of any obobtained. As has previously been said, a macrograph of ject magnified more than 10 diameters is a micrograph. If the magnification is not more than 10 diameters, the reproduction is a macrograph. coke is liable to be confusing, since the black coke substance Report of the (British) Fuel Reseaich Board for the Years 1920, 1921. appears to be almost devoid of detail, and the picture consists Second Section: Low Temperature Carbonization. 8 J . Franklin Ins!., 183, 391 (191i). 5 THISJOCRNAI,, 1 6 , 901 (1921)
T
4
A-Lonpitudlnal Coke Section Ground Smooth. By-Product O v e n Coke from 25 Per cent Volatile Matter Mixture of Upper Freeport a n d Kittannin8 Seam Coals. Northeastern W. Va. 1 x
D--Same
08
Plaafer-Filled and Polished. 4 x
8.
C--Same Kind of Coke.
B-Same. Piaster-Filled and Polished. 1 x
K-Same as C.
Macrograph of Ink Prlnf 4x
Print.
F
-Same a8 B and D.
Polished.
1
x
Ink
Plaster-Filled and 25
x
Fieure 1
chiefly of uumlJerless reliected p i n t s of light. I t is diflicult, strncture of coke is by no iiieiins adequately described Iiy t,lie if not impossihle, t,o illustrate with quantitative accuracy by size of the larger cells, since the thickness and porosity of the w c h a photograph the details in which pract,ical men are most cell walls are of as great if not greater importance. Cokes iiitcrested. t,otally unlike in their percentage of cell space (porosity), Coke oven and blast furnace operators are accustomed to sppearance, and in general value may have cells of the sann ilisruss coke cell structnre in terms of the size of cells and the size. This was minted out bv Ilamshure and Snerr. . ,but thev t,liickness and porosit.y of the cell ivalls, arid they attribute did not atteinpdto coinplicate their standards by introducini iwnsiderahle importance to these characteristics. They i i ~ e such factors as the thickness and porosity of the walls of the such terms as “dense coke,” “medium-size cells,’’ “thin larger cells. cell walls,” “thick, but somew1ia.t porons walls,” ctc., in an The whole problem of the nature and imp)rtance of coke iafiort to describe coke. Yet surh descriptions are of very niaerostructure has always existed in a VCTY unsatisfactory limited value, hecause there are IIO accepted standnrds for iwndition, because there has been no method for accurately such terms, arid each individnal uses them in a purely and clearly illustrating the structural details. It has not relative sense after nierit.ally eonipariiig the coke in qnestion lieen possible readily to study and (:ompare structural differcnoes, or to interchange ideas oil this subject, because the with his reeollection of other cokes. Ramsbnrg and Sperr5 greatly simplified iwke terminology strnetiire could not be reproduced iii a simple pictorial way, by describing and ilhistrat,ing a set of four standards for gagFilling Coke Cells with a White Composition ing the averagi‘ size of the easily visihle cells of coke. These standards consist of longitudinal src%ions of different byThe strneture in the surface plitne of a piece of coke niay be prodnct cokes, which itre ninnhcrcd from 1 to 4 with increasing r.cveald with striking distinctness by grinding and polishing size of cells. The average cell size oS a mke sperimen may he coke that has been coated with a hard, white composition. determined at a glance tiy c,omparing it with a naburitl-size The finished surface consists of a continuous black network photograph of t.lie four standard rokes. However, the cell of i d walls, showing up in sharp contrast to thc mhitc filling
of the cells. Such a auriace is ideal for reproducing photo-
xraphically. Since all the visible coke is in the same plane, there is no focusing problem, and there arc no confusiug reflections from the sourec of illumination. This method has proved t,o be of general value for preparing large plane surfaces on various coarsely porous, brittle materials. The filliiig composition supports fragile cell walls, and prevents tliem from breaking during.grinding operations. It mn lat.er be reinovcd witlrout, affcct,ingihe cell structure. The directions given lierrwith should enahle anyone to prepare coke surfaces s a t ctwily by this method. A niunbcr of alternative ~TO~Y!IIU~I!S have beeti irivestigated and Sound to have merit, hut the praeess to be described is a general method dcvcloged for preparing large perfect surfaeea with t.he niinitnuin expenditure of time. If several fullsized coke pieces from a 356-mm. (I4incB) oven-i. e,, pieces approximately 76 mm. (3 inches) wide by 178 mm. (7 inches) long-are being prepared at tlie same time, not more tban 4 hours should be required to finish each piece. Small pieces require much less time, whereas very large or soft (undercoked) pieces may require considerably more work. FILLINGComosITioN-98 per cent “plaster of Paris,” 2 per cent magriesium oxide. A suitable grade of plaster of Paris may be obtained froin dental supply houses. It should be selected for whiteness, fineness, and hardness of set. Only that portio11 which easily passes t.hrough a 200-mesh sieve (0.074 mm. or 0.0029 inch syuare openings) should be used. The magnesium oxide should he extremely light and fluffy. I t is added for several reasom, the most important being to prevent iron stains from appearing in the plaster. Increasing percentages of magnesia iucrease the solubility of the composition in dilute hydroobloric acid, but diminish its hardness when set. GRINDiNG PowDEas-Carborundiim powders are used in \-ariaus sizes, ranging from about 40 mesh down to the h e s t powder obtainable, which is tlie “60 minute” grade. This last-mentioned size should he separated by levigation into
FIDure 2 - P o c a h o n t ~ ~72-Hour Beehive Cuke. Mcnowell County. W. VZI. Cd19 Fllted. 4 X
GRINDING PL~I,.ES-&X or eight pieeaa uf plate glass not less than 9 mm. (S/S inch) thick are required. If coke pieces 178 to 203 mm. (7 or 8 inches) long are being used, plates 254 by 356 mm. (10 hy 14 inches) are suitable. GRINDINGTABLE-This should be a heavy low table or bench, preferably with a stone top which can he ground to a plane surface at one end, to receive the grinding plates. OBTAINING THE CUT SECTION->\typical full-length piew of coke should be selected. The surface sbould be cleanail tlrorouglily with a brush and a forcible jet of water to remove dirt. and adhering coke breeze, and drained well to remow surface water. The entire piwe is coated witb a thick creamy paste of the plaster composition, to a thickness of 3 to 6 mm (I/* to ‘/a inch). The pieces may be sectioned as soon as the plaster has hardeued, but it is prcfcrable first, to dry the pierc t,horoughly and quickly in a warm room. The coke is sectioned uith 8 thin alundum cutting wbed according to the method described by Malleis.6 FILLING THE CxI,I.s-The cut surface it?cleaned thoroughly with a forcible jet of water and a hrush having short, soft bristles, the surface water being shaken or blotted off. A thick creamy paste of the plaster composition is rubbed into the cells. If i t is discolored by the , it must he washed out at Once and a second coating ied. If tho plaster does not have a convenient hardening time, very dilute solutions (1 to 0.1 per cent) of salt or glue may be iised instead of pure water to accelerate or retard the set,. GI~INDING THE CUT SURFACE-If the surface of the Coke is not approxirnately plane, it may be roughly ground true OJI any convenient flat, hard surface, using c.oarse,carborundunr pouder and water. The Burface layer of plaster-filled cells will soon be cut through in places, and judgment must be used in deciding how often it is necessary to wash and refill the coke surface to prevent appreciable breakage of the d l walls. As soon as the surface is approximately plane, it is cleaned and refilled, and wet grinding begun 011 glass plates that havt,
Fleure 3-Coke from Arkmnaa l o r voiatiie coal. P*II ‘reef in 4 6 . ~ ~ . (18*i~-lnch)By-Product Oven. Celb Fllled. 4 X
F W r e 4.- Coke f r o m Illinois Coat NO. 6 Seam. Ftanklln County,lll. Mad: In 46-Cm. (18iia-lnch) By-product Oven Celln Pllled.
4 X
three or OW S W C ~ ~ O J Wirf, u-hirh olrl?. llle last t,wo art! used irt Ieeii triirxl Ily grinding to&ier in pair? with Inrlroruudnm the filial grinding oprratioiis. powdcr a i d water. Po~.~isr~iso I’owDER---A p o ~ v i l ~orr t,hin ( T P ~ I I Iof levigated At fil t,he progress of grilr&>g (‘a11lit: foll(,wed wit11 the a Ina*,ifii.ati t C tf O ~
I?. I*. hIrie,orh,
tilr ~~~~~~~i~ institutu
iii~i: t!w# / h i < mciliod !>e :wi>!icd torokc.
