US3396494A - Removal of protrusions formed on carbon anodes - Google Patents

Description

United States Patent 3,396,494 REMOVAL OF PROTRUSIONS FORMED 0N CARBON ANODES Nolan Earle Richards, Florence, Ala., and James S. Berry, J12, Savannah, Tenn., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Filed May 7, 1965, Ser. No. 454,169 4 Claims. (Cl. 51-319) ABSTRACT OF THE DISCLOSURE Protrusions formed on the working surfaces of electrolytic reductions, such as alumina reduction cells, are removed by applying to the surface of the protrusion a jet of high pressure oxygen gas and then applying an electric arc at a site adjacent to the point of impingement of the oxygen jet to volatilize solid electrolyte films and to initiate combustion of the protrusion; the oxygen jet may include a fiow of suspended fine abrasive particles.

This invention relates to a novel method for the rapid and effective removal of protrusions formed on the working surfaces of carbon anodes of electrolytic aluminum reduction cells. More particularly, the invention concerns the removal of such protrusions by cutting or burning by means of a jet of oxygen gas with the aid of an electric arc to the carbon surface.

In the production of aluminum and aluminum alloys in electrolytic reduction cells, such as, for example, Hall- Heroult type cells, the carbon anodes extend downward toward the cathodic reservoir of metal in the cell. Under normal conditions the surface of the carbon anode is consumed at a uniform rate and remains smooth and horizontal. Under certain circumstances, particularly where the operating anode-cathode distance lies within a short distance of 1 to 2 inches, it has been observed that variations in cell operating parameters, such as tempera ture, bath composition, and cathode current distribution, result in the formation of a ridge, cone, or spike on the anode surface. Such protrusions extend downward through the electrolyte into the metal reservoir cathode, causing electronic conduction through the cell and hence greatly reduced current efficiency. Conventional procedures for removal of these protrusions include raising of the anode and attempting to knock off the ridge or spike Wholly or partially with the aid of a heavy steel bar and an air hammer or a pneumatic drill. This procedure is rarely successful since the anode surface is quite hard and does not provide a point for good leverage. Another removal method conventionally employed to get the anode surface back to normal configuration is to operate the cell with a greatly increased interelectrode distance by reducing the depth of the metal reservoir so that the protrusion is out of the metal reservoir, while increasing the depth of the electrolyte and consuming the ridge at an increased current density. This method is impractical because, although the anode becomes smooth and level again after a period of electrolysis at high voltage for many hours or days, the cell during this type of operation becomes very hot (1025 to 1050 C.), the current efliciency is low, electrolyte losses are high, the life of the cathode may be adversely affected, so that the total operation becomes very uneconomical.

It is accordingly an object of the invention to provide an economical and efficient method for the rapid removal of protrusions from carbon anodes of electrolytic aluminum reduction cells.

It is a further object to remove protrusions from the working surfaces of carbon anodes in such manner that the period of interruption of cell operation and drop in cell temperature can be kept to a minimum, and so that the cell can be quickly returned to full capacity.

These and other objects and advantages will be apparent from the following description.

In accordance with the invention, it has been found that the ridges, cones, spikes, and the like, formed on the working surfaces of carbon anodes of electrolytic aluminum reduction cells can be elfectively removed by cutting and/or burning the carbon of the protrusions by means of a high pressure oxygen gas jet, with the aid of an electric arc struck to the carbon surface. In accordance with another aspect of the invention, the cutting and combustion of the carbon is supplemented by the abrasive action of particles of a refractory material applied to the surface of the protrusion in the oxygen gas jet. The practice of the method of the invention employing these principles will be more fully understood from the following description.

A cell in which the protrusions have developed on the anode surfaces is disconnected from the power lines, and the anode or anodes are raised above the surface of the electrolyte. The anode surfaces thus exposed will be found to be coated with a film of the electrolyte (cryolite) which will impede the start of combustion of the carbon protrusions unless eflectively removed. It has been found, in accordance with the invention, that the cryolite film can be readily removed by vaporization by the heat generated by an electric are applied to the hot carbon surface (1350 to 1700 C.), thereby rendering said surface accessible to the action of the oxygen gas jet.

The method of the invention comprises the steps of applying to the surface of the protrusion a jet of oxygen gas and then applying an electric are adjacent to the site of impingment of the oxygen jet to initiate combustion at said site. Thereafter the fiow of oxygen gas is increased to produce an undercutting of the protrusion until the base of the protrusion has been undercut substantially more than half of its base area, after which the remainder of the protrusion is removed mechanically. The undercutting operation may, as mentioned previously, be assisted by the abrasive action of a stream of hard refractory particles, such as an abrasive carbide or oxide, preferably sintered aluminum oxide.

The total treatment time employing the steps outlined is from about 20 to about 40 minutes. Thereafter the anode is lowered, and the cell is re-energized. The temperature will have fallen to about 945 to 960 C. The cell voltage is adjusted to slightly higher than normal for several hours, but is returned to normal voltage and operation within 24 hours at the most.

The source of the electric arc may be, for example, a portable arc welding machine of any conventional type. One electrode of the Welding machine is attached to an anode flex or pin. The other electrode is attached to corrosion resistant length of pipe, made preferably of a nickel-chromium alloy, such as, for example lnconel (78% Ni, 15% Cr, 6% Fe), fitted with a graphite rod sharpened to a point.

The oxygen gas jet is advantageously applied by means of an oxygen lance. In accordance with preferred practice of the invention, the oxygen gas jet is at first applied using a starter lance placed a few inches away from the place .on the anode protrusion where the electric arc between the graphite electrode and the anode will be struck. The starter lance consists, for example, of a length of lnconel tubing fitted at its working end with narrower length of lnconel pipe serving as a nozzle, and at its handling end with a mixing barrel and valve assembly from a conventional oxyacetylene welding torch, through which high pressure (250 pounds per sq. inch) oxygen is supplied from cylinders. The starter oxygen lance is used to initiate combustion of the protuberance, preferably at the base thereof.

After the flow of oxygen through the starter lance has begun at a moderate rate, the arc is struck, for a few seconds, e.g. from 2 to 5 seconds. The heat generated by the arc vaporizes adsorbed cryolite from the anode, thus allowing the oxygen to react with the clear or unprotected protrusion surface while still hot, and forming a reaction site. Thereupon the flow of oxygen is increased, at a pressure of about 100 to 200 pounds per sq. inch. The operator oscillates the nozzle from side to side to start a horizontal cut, extending the combustion around the base of the protrusion.

In order to expedite the cutting effect, the starting lance may be replaced by a cutting lance, in which the simple nozzle is replaced by a fan shaped head of Inconel having multiple holes therein. A greater flow of oxygen can be employed at this point if desired. Thus a greater rate of oxidation of the carbon of the protrusion is achieved, both at the shoulder and the top of the protusion. Oxidation is continued until the base of the protrusion is 50% to 80% cut through, and complete oxidation can be attained under optimum conditions. Otherwise, the combustion is stopped, the undercut protrusion is knocked off mechanically, as by means of a heavy iron bar. Thereafter the anode is lowered and the cell placed back into operation as previously described.

Removal of the carbon protrusion in the foreign manner may be supplemented by feeding abrasive particles, such as sintered aluminum oxide particles, from a hopper to the oxygen lance via a supply tube. At the end of this supply tube, the reduced pressure zone or Venturi effect of the oxygen stream draws the fine abrasive powder into the oxygen gas jet so that the powder acquires a high velocity and has a strong abrading action ,on the carbon protrusion being oxidized. In this manner, small partially oxidized carbon particles are physically dislodged from the carbon matrix. If desired, compressed air may be introduced to increase the velocity of the abrasive particles.

The following example illustrates the practice .of the method of the invention, but is not to be regarded as limiting.

EXAMPLE An aluminum reduction cell on the carbon anodes of which spikes and other protrusions had developed in operation, was taken .out of operation and the anodes raised clear of the electrolyte. One electrode of a portable arc welding machine was attached to the anode pin, and the other electrode was connected to an 8 ft. length of Inconel pipe, /z-inch inside diameter, fitted with a /2-inch graphite rod sharpened to a point. An 8 ft. length of Inconel tubing, /2-inch outside diameter was welded to the mixing barrel of an oxyacetylene welding torch and the oxygen inlet coupled to a 200 cu. ft. oxygen cylinder. The tubing was fitted at its cutting end with a 6-inch length of /2- inch outside diameter, approximately /a-inch inside diameter Inconel pipe, to serve as a starter oxygen lance. The flow of oxygen was started at a moderate flow rate, and the arc struck, for a brief period of 3 seconds. The oxygen lance was applied about 3 inches from the arc point, and

4 the oxygen flow increased to 100-200 psi, and the nozzl oscillated from side to side to initiate a horizontal cut. At this stage the starter lance was replaced by a cutting lance of similar construction, but with the nozzle replaced by a fan shaped Inconel head with multiple holes. Oxidation was continued until about .of the base had been undercut. The remaining carbon was knocked 01f manually with a 2 inch diameter iron bar. The total time was 30 minutes, and the cell temperature had dropped to 950 C. The cleaned anode was lowered, the cell placed back in series with the line, the cell voltage adjusted to 0.5 volt higher than normal for several hours, and then reduced to normal level.

What is claimed is:

1. Method for removing carbon protrusions formed on the working surfaces of carbon anodes of alumina electrolytic reduction cells, said working surfaces being coated with a film of cryolitic electrolyte material which impedes removal of said carbon protrusions, comprising the steps of first applying to the surface of the protrusion a jet of high pressure oxygen gas and immediately thereafter applying an electric arc to the hot carbon protrusion surface at a site adjacent to the site of impingement of said oxygen jet to vaporize said cryolitic material and to render said carbon protrusion surface accessible to the action of said oxygen gas jet.

2. The method of claim 1 in which the oxygen jet is applied by means of an oxygen lance.

3. The method of claim 1 in which a stream of finely divided particles having an abrading action on the carbon protrusion is discharged against the carbon protrusion surface by means .of said high pressure oxygen jet, to dislodge small partially oxidized carbon particles from the carbon matrix.

4. Method for removing carbon protrusions formed on the working surfaces of carbon anodes of alumina electrolytic reduction cells, said working surfaces being coated with a film of cryolitic electrolyte material which impedes removal of said carbon protrusions, comprising the steps of first applying to the surface of the protrusion a jet of high pressure oxygen at a moderate rate of flow, immediately thereafter applying an electric arc to the hot carbon protrusion surface at a site adjacent to the site of impingement of said oxygen jet to vaporize said cryolitic material and to render said carbon protrusion surface accessible to the action of said gas jet, and then increasing the rate of flow of said oxygen jet while maintaining the oxygen pressure between about and 200 pounds per square inch.

References Cited UNITED STATES PATENTS 968,350 8/1910 Harrison 1489 1,609,859 12/1926 Bond 21970 2,152,145 3/1939 Moss 148-9 2,855,337 10/1958 Holub 1489 3,063,267 11/1962 Shand et a1 51-319 X LESTER M. SWINGLE, Primary Examiner.