US3871986A - Joint ramming cement for electrolytic reduction cell cathodes - Google Patents

Abstract

A ramming cement suitable for joining a plurality of adjacent carbon blocks having joint spaces between them to form a continuous cell lining structure for an electrolytic cell, such as an aluminum reduction cell, is prepared from a mixture of a petroleum pitch having a cube-in-water softening point between about 40* and about 85*C, and containing not more than about 1% of quinoline-insoluble material, and an aggregate such as calcined anthracite.

Description

United States Patent 1191 Reamey et al.

JOINT RAMMl NG CEMENT FOR ELECTROLYTIC REDUCTION CELL CATHODES Inventors: Herbert K. Reamey; Jerry F.

Newman, both of Hot Springs, Ark.

Reynolds Metals Company, Richmond, Va.

Filed: Oct. 31, 1973 Appl. No.: 411,537

Assignee:

US. Cl 204/243 R, 204/294, 252/502, 252/510 Int. Cl. C22d 3/02, BOlk 3/08, l-l01b 1/04 Field of Search 204/294, 243 R, 244-247; 252/502, 510, 511

References Cited UNITED STATES PATENTS l/l963 Myers et al. 204/294 X [451 Mar. 18, 1975 3,187,089 6/1965 Cosby et al. 204/294 X 3,576,700 4/l97l 3,666,653 5/1972 3,764,509 10/1973 Etzel et a] 204/294 X Primary E.\'aminer.lohn H. Mack Assistant ExaminerD. R. Valentine Attorney, Agent, or Firm-Glenn, Palmer, Lyne & Gibbs [57] ABSTRACT 5 Claims, NO Drawings BACKGROUND OF THE INVENTION This invention relates to electrolytic reduction cells provided with a prebaked carbon block lining which functions as the cathode, and to an improved carbonaceous ramming cement for binding the carbon blocks by filling the joints between them. More particularly, the invention concerns an improved ramming cement for aluminum reduction cell cathodes.

Aluminum metal is conventionally produced in electrolytic cells by passing a current through a bath of molten cryolite containing dissolved alumina, in a large tank or cell lined with carbon, the lining serving as part of the cathode system. Large carbon blocks presented at the top of the bath function as the anode. Molten aluminum metal collects as a metal pad at the bottom of the cell and is removed periodically. The cell may have a bottom lining made up of cemented carbon blocks in which horizontally disposed steel cathode bars extend from the opposite sides of the cell. These cathode bars are connected to the cathode collector blocks by means of cast iron, and the blocks are anchored in a side lining of rammed carbon.

Most of the aluminum reduction cells in commercial use employ prefabricated carbon blocks as the cell lining and as the cathodic working surface. These blocks provide high operating strength, higher density, lower porosity, and lower resistance than that of a continuous rammed paste type of lining. These blocks are formed into a liquid-tight container surface by filling the joints between the blocks with a rammed carbonaceous cement. This liquid tightness is an important factor in determining the life of a reduction cell, which depends to a considerable degree on the extent and rate of electrolyte penetration into the cell bottom.

The best types of cathode blocks currently in use are considered to be those manufactured from calcined anthracite or calcined mixtures of anthracite with other forms of carbon such as metallurgical coke or graphite.

The prior art ramming cements have usually been prepared by mixing a graded calcined anthracite aggregate with a binder. The binder has been conventionally a coal tar type pitch. These materials are heated together to a consistency such that the mixture can be rammed as a hot melt into the joints between the cathode blocks. Thus, for example, it is know to prepare a joint material for aluminum reduction cell cathode blocks from calcined anthracite fines and a carbonizable binder such as a coke-over pitch having a softening temperature of about 75C.

A characteristic of ramming cements made with coal tar or coke oven pitches has been a tendency for the binder to migrate and be absorbed into the pores of the cathode blocks. This has been considered detrimental in the art, and steps have been taken to offset it, such as precoating the blocks with a separate pitch coating. Coal tar or coke oven pitches are also characterized by a considerable content of material insoluble in quinoline, ranging from about to by weight, which may affect the migration properties of the pitch binder. Another approach taken in the prior art has been to employ relatively high softening point coal tar pitches, e.g. 130 to 160C, in admixture with a polymerizable monomeric plasticizer to improve the properties of the binder mixture.

GENERAL DESCRIPTION OF THE INVENTION In accordance with the present invention there is provided a novel ramming cement for electrolytic cell cathodes which is readily prepared from commercially available materials, and which will ram firmly with good interparticle contact into the joint spaces between the cathode blocks, and which will not shrink when the lining assembly is subsequently baked.

It has been found, surprisingly and unexpectedly, that a certain type of binder pitch possesses the quality of being absorbed by the ramming cement aggregate, such as calcined anthracite, to such an extent that the resulting mixture can contain an excess quantity of pitch binder which will then bleed during subsequent baking after application to the cathode block joints, so that the joint cement and block assembly will form an impenetrable and inseparable mass. It has been discovered that in this relationship, the bleeding tendency of the pitch is not only not detrimental, but in fact contributes to the production of a superior ramming cement.

The essential feature of the improved ramming cement of the invention lies in the use ofa type of binder pitch that is very low in its content of quinolineinsoluble material, preferably less than about l7c by weight. The quinoline-soluble material present in considerably greater quantity in the prior art binder pitches is now believed to block the pore structure of the aggregate and to prevent absorption of the binder into the pores of the aggregate and the cathode blocks.

A ramming paste prepared with a pitch which does not readily absorb into the pores must carry any excess pitch in the voids between theparticles, which prevents ramming firmly with good particle contact and leads to a spongy structure. Upon baking, this type of ramming paste shrinks, causing a poor joint at the interface between the cement and the carbon block, and acting as a source of weakness and potential failure in the cathode structure.

For the purposes of the present invention, a suitable type of binder pitch is a petroleum pitch having a softening point (cube test) between about 40 and about C. (cube-in-water melting point), preferably about 55 to 60 C., and a content of material insoluble in quinoline not greater than about 1% by weight. Petroleum pitch having these characteristics may be prepared from residual aromatic tar from the high temperature cracking of gas oil by steam cracking followed by vacuum distillation of volatile fractions. The resulting pitches have been proposed as binders in the body of prebaked or Soderberg type carbon electrodes, not, so far as known, as a binder between preformed carbon blocks as a ramming cement ingredient, which presents a different problem.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the presently preferred practice of the invention, as illustrated with reference to an aluminum reduction cell, the bottom and walls of the cell hull are lined with preformed carbon wall blocks of conventional type to form a cathode carbon lining. These carbon blocks, in place, are joined or bonded by ramming into the joint spaces between the walls of adjacent blocks, the improved ramming cement of the invention, to form a continuous rammed monolithic lining. The assembled lining is then baked in situ at a sufficient temperature and for a sufficient period of time to carbonize the binder ingredients while at the same time avoiding undue shrinkage.

The ramming cement of the present invention is composed of a mixture of a suitable aggregate and the petroleum pitch having the characteristics previously described, forming a mastic. The aggregate is preferably calcined anthracite, although a mixture of calcined anthracite and calcined metallurgical coke or other car bonaceous material such as graphite, may also be employed. Advantageously the aggregate is 100% calcined anthracite, about 30% of which has a nominal particle diameter of about three-eighths inch, the balance representing material comminuted to a fineness such that nominally about 50% will pass through a 200 mesh Tyler screen. This degree of fineness may be achieved by grinding in a ball mill or other suitable device.

In the preparation of the ramming cement, a preferred procedure, according to the invention, is to melt the pitch in a separate vessel, bringing it to a temperature of about l30C., or within a range of 120 to 140C. This melted pitch is then added to the aggregate in a mixer provided with heating means, in which the mixture is maintained at a temperature between about 150 to 190C. for approximately one hour. At the end of this time, the ramming paste will have attained a consistency such that a coherent ball can be formed in gloved handles, which will be firm and not crumble. If too much binder is used, the paste will be spongy and will not compact firmly upon compacting with a pneumatic hammer. The ramming paste when properly prepared, has a consistency similar to that of hot asphalt mastic used in paving of roads.

The proportion of binder pitch in the ramming paste is between about 13% and about 16.5% by weight, preferably between about 13% and about 13.5%.

The petroleum pitch employed, having a cube-inwater melting temperature between about 40 and about 85C., typically about 55C., is preferably one having a Conradson carbon value above 45%.

In forming a cathode lining, the joints are formed by transferring the ramming cement into the approximately two-inch joint spaces between the prebaked cathode blocks, and compacting it with pneumatic hammers. The joint is formed in several layers to a smooth level surface flush with the top of the cathode block. The sidewalls are formed in a similar manner by ramming the cement between suitably shaped forms to provide the cathodic cavity of the reduction cell.

The cathode blocks themselves are made from graded calcined anthracite, or from mixtures of calcined anthracite, metallurgical coke, lamp black, or graphite. A typical formulation may comprise calcined anthracite, with between about 12% and about graphite flour by weight. The block ingredients are admixed with a suitable binder pitch in a heated mixer and formed into shape by extrusion or molding, according to conventional practice. A typical cathode block may have dimensions of 14 inch length by 14 inches width, and 90 inches overlength, with a suitable slot, for the cathode collector bar which is either molded or machined into the cathode block. These blocks are prebaked at temperatures of about 1200C by packing into carbon baking furnaces. Generally, the cathode blocks also have small grooves molded or machined into the side surfaces where the joints will be formed to assist in keying in the joint cement. The slots are relatively small, about three-eighths inch deep.

After the lining and joints have been properly formed and the cell deckplate and other parts are in place, any suitable method may be employed to place the cell in service and to bake the cathode and joint assembly. A method commonly used is to insert a layer of metallurgical coke of particle size approximately one-half inch diameter in the space between the anode and cathode, and then employ this carbon bed as a resistant heating means for the passage of current between the anode and the cathode. When the elecrodes are not, cryolite and/or molten aluminum are added to start the cell in operation. The process heat of the cell will then serve to complete the carbonization of the joints and sidewalls.

THE DESCRIPTION OF THE PREFERRED EMBODIMENTS The following example will serve to illustrate the practice of the invention, but is not to be regarded as limiting.

EXAMPLE 1 Preparation of Ramming Cement Approximately 15 pounds of a petroleum pitch having a Conradson value of 50%, and a quinolineinsoluble material content of 0.80% were melted at a temperature of 55C and heated further to about C. The melted pitch was added to 100 pounds of calcined anthracite aggregate, of which about 30% had a nominal three-eighths inch average particle size, and the mixture (containing about 13.25% pitch) was stirred to a uniform consistency for about one hour at a temperature of C. The resulting ramming paste was then applied to a cathode block lining of a1 aluminum reduction cell as described above.

What is claimed is:

1. Process for forming the cathode lining of an electrolytic cell, said lining being made up of adjacent carbonaceous blocks having joint spaces therebetween, comprising the steps of introducing into said joint spaces a ramming cement comprising a mixture of pitch and a carbonaceous aggregate, at a temperature sufficient to maintain suitable consistency for compacting the cement in said spaces, and then baking the blocks and cemented joints at a temperature and for a period of time sufficient to carbonize the cement and to form a continuous lining structure, said mixture containing from about 13 to about 16.5% by weight of petroleum pitch characterized by having a cube-in-water softening point between about 40 and about 85C. and having a content of material insoluble in quinoline not greater than about 1%.

2. The process of claim 1 in which said electrolytic cell is an aluminum reduction cell.

3. The process of claim 1 in which said aggregate is calcined anthracite.

4. The process of claim 1 in which said aggregate is a mixture of calcined anthracite and calcined metallurgical coke.

5. The process of claim 1 in which said aggregate is predominantly finely divided material of which about 50% passes a 200 mesh screen.

Claims (5)

1. PROCESS FOR FORMING THE CATHODE LINING OF AN ELECTROLYTIC CELL, SAID LINING BEING MADE UP OF ADJACENT CARBONACEOUS BLOCKS HAVING JOINT SPACES THEREBETWEEN, COMPRISING THE STEPS OF INTRODUCING INTO SAID JOINT SPACES A RAMMING CEMENT COMPRISING A MIXTURE OF PITCH AND A CARBONACEOUS AGGREGATE, AT A TEMPERATURE SUFFICIENT TO MAINTAIN SUITABLE CONSISTENCY FOR COMPACTING THE CEMENT IN SAID SPACES, AND THEN BAKING THE BLOCKS AND CEMENTED JOINTS AT A TEMPERATURE AND FOR A PERIOD OF TIME SUFFICIENT TO CARBONIZE THE CEMENT AND TO FORM A CONTINUOUS LINING STRUCTURE, SAID MIXTURE CONTAINING FROM ABOUT 13 TO ABOUT 16.5% BY WEIGHT OF PETROLEUM PITCH CHARACTERIZED BY HAVING A CUBE-IN-WATER SOFTENING POINT BETWEEN ABOUT 40* AND ABOUT 85*C. AND HAVING A CONTENT OF MATERIAL INSOLUBLE IN QUINOLINE NOT GREATER THAN ABOUT 1%.

2. The process of claim 1 in which said electrolytic cell is an aluminum reduction cell.

3. The process of claim 1 in which said aggregate is calcined anthracite.

4. The process of claim 1 in which said aggregate is a mixture of calcined anthracite and calcined metallurgical coke.

5. The process of claim 1 in which said aggregate is predominantly finely divided material of which about 50% passes a 200 mesh screen.