NO142974B - PROCEDURE AND DEVICE FOR NUMERICAL ANGLE INSTRUCTIONS - Google Patents

Description

Method for the production of carbon electrodes.

This invention relates to carbon electrodes which have been prepared from a mixture

of petroleum coke produced by delayed coking and coke produced by fluidized coking.

Fluidized coking has been described

in many patent documents, e.g. in the U.S. Patent Nos. 2,881,130 and 2,805,177. This method involves the thermal cracking of a hydrocarbon oil, which. is preferably a residual oil but can also be a lighter fraction, in the presence of a fluidized layer of finely divided inert solid particles, preferably coke, so that lighter products such as e.g. petrol, middle distillates, unsaturated hydrocarbons, normally gaseous hydrocarbons, hydrogen and coke. The nature and quantity of the products obtained depend on the particular materials supplied and on the reaction conditions used. The coke obtained is called fluidization coke in the following.

The electrodes produced according to the present invention are particularly advantageous for the production of aluminium, by electrolytic reduction of aluminum oxide in a molten bath. The carbon electrodes previously used for this purpose have been produced from petroleum coke obtained by means of the delayed distillation method. The coke must be relatively clean, so that the introduction of metal impurities into the aluminum product must be avoided. It has also been proposed to use mixtures of petroleum coke from delayed distillation and coke from the fluidized distillation process, for the production of electrodes, but the electrodes obtained have proven to be not entirely satisfactory. Electrodes made from coke obtained by delayed distillation are too reactive and a large part of the electrode is lost by reaction with the oxygen in the air and by reaction with the carbon dioxide that is formed during the electrolysis of the aluminum oxide, which is carried out at approx. 950°C.

As previously mentioned, it is known to produce electrodes that contain coke from delayed distillation and fluidization coke, but the fluidization coke is used in an amount of 1-50% by weight, calculated on the total coke charge. In contrast to this, the present invention relates to an electrode comprising 60-90% by weight. fluidized coke.

It is known from previous publications that as the amount of fluidized coke is increased from 30-50 per cent, the crushing strength decreases from 400-315 kg/cm2. The state of the art teaches that an increase in the fluidization coke content in the mixture will lower the compression strength. In contrast to this, the present invention teaches that far larger amounts of fluidization coke can be used, and that these will give a higher compression strength provided that the particle size range for the fluidization coke is carefully selected and part of the fluidization coke is ground. Thus, the electrode produced according to the present invention has a compressive strength of 490 kg/cm2 compared with a maximum strength of 400 kg/cm2, which is known from the state of the art. One of the problems when using the previously produced electrodes is that the electrodes emit dust — "dust" — due to different consumption rates of the coke particles and of the coke binder resulting from carbon deposition from the pitch binder during the burning of the electrode. The coke components that are consumed slowly fall down from the electrode and create the so-called "dust problem". This coal dust short-circuits the electrolytic bath's current circuits, and thus means lost coke, i.e. coke that does not reduce aluminum oxide. Another problem is that the electrode shrinks and cracks during use.

Electrodes are usually produced by selecting coke particles, which have been calcined and partially ground, and coke particles of selected particle size are distributed together with a suitable carbonaceous binder. The resulting mixture is formed into the desired shape and is heated to carbonize or coke the binder part, so that what has been called a "pre-burned" or pre-burned electrode is obtained. The mixture of pitch and coke can alternatively be used by continuously supplying it to a Søderberg electrode, which continuously burns the mixture in situ, using the heat from the electrolysis bath.

The coke particles from the fluidized distillation process normally have a size of between approx. 10 and 200 standard stitches or approx. 74-2000 microns, below which approx. 80% by weight of the coke particles have a diameter between approx. 20 and 140 standard stitches or between approx. 105—840 microns.

Some typical distributions of fluidization coke are as follows:

Other size distributions can be achieved, depending on how the fluidisation apparatus is operated, but in general fluidisation coke which is much larger than in ex. II is not achieved, due to clumping of the fluidized layer when attempting to increase the particle size. Fluidization coke particles are generally too small to be used alone, and coarser particles of coke produced by delayed distillation are therefore used to obtain the necessary coarse material.

The petroleum coke particles that are formed by the fluidized distillation process have a laminar structure, and each individual particle can consist of between approx. 10 and 100 layers of coke deposited on top of each other. The real density of these coke particles from the fluidized distillation process lies after calcination at between 1.83 and 2.00.

The calcined particles of coke from delayed distillation have a greater real sp.v. than the particles from the fluidized distillation process, and this sp.v. is from 2.0 to 2.10. After moderate grinding, the coke particles from the delayed distillation have, for example, a size of between approx. 30 mesh (590 microns) and 12.7 mm diameter, where most — i.e. 80—90 wt. — of the particles lie between approx. 12.7mm and 20 standard mesh (840 microns), while the rest are finer than 20 mesh. By varying the measurement method, other particle distributions can easily be obtained. Heavy painting can result in particles whose size is below 325 mesh. For the purposes of the present invention, it is not necessary to grind the coke from the delayed distillation to these very fine particle sizes.

The calcination of the coke particles from the delayed distillation and of the fluidization coke particles takes place in the usual way between approx. 980 and 1650° C in from approx. 0.5 hour to 1-3 weeks, the longer periods being used for the lower temperatures. The preferred calcination temperatures are 1260-1540°C. During calcination, the density of the coke particles is increased and volatile substances are removed.

In general, two types of electrodes are used in industry, namely (1) pre-burned electrodes and (2) self-burning electrodes of the Søderberg type. The present invention can be used in both of these electrode types.

In the drawing, fig. 1 and 2 are diagrams showing how the maximum packing or bulk density varies with different mixtures of calcined fluidization coke fractions.

The present invention thus consists in a method for producing a carbon electrode, where particles of calcined coke produced by delayed distillation with a size of between approx. 20 mesh and 12.7 mm is mixed with particles of calcined fluidization coke, and this coke mixture is then mixed with a carbonaceous binder, after which the resulting mixture is shaped and fired at a temperature between approx. 980 and 1315°C for 8-300 hours, and the method is characterized by the electrode being produced by mixing 10-40 wt. calcined fluidization coke consisting of ground and unground fluidization coke, the ground fluidization coke comprising coke flour with a size finer than 200 mesh in an amount of between 6 and 45% by weight. of the total coke mixture, and the unground flydiserihgscoke particles have a size above 200 mesh.

The ratio between ground and unground fluidization coke is between approx. 0.1 and 1.0, preferably between approx. 0.15 and 0.75, in order to achieve good compaction and low electrical resistance in the electrode. In other words, of the entire fluidization coke mixture, approx. 90-50% by weight consists of coarse or unground coke from the fluidized distillation process and 10-50 wt. of finely ground fluidization coke or fluidization coke flour from the aforementioned process.

The petroleum coke particles are preferably first calcined, and after grinding, if necessary to achieve the desired particle size, and in some cases sifting, they are mixed at a high temperature, i.e. at a temperature above the softening temperature of the added pitch, with approx. 12-40% by weight. lime binder per 100 parts by weight coke binder mixture. The larger amounts of pitch by weight are used when Søderberg electrodes are to be manufactured. The amounts of pitch used are approx. 13-20 per cent for pre-fired electrodes and 25-35 per cent for Søderberg electrodes. The mixture is then shaped, in the case of pre-fired electrodes, at a pressure of approx. 210-700 kg/cms or extruded, and is then annealed for up to 30 days at approx. 980—1315° C. The thus pretreated electrodes are then ready for use. For the Søderberg electrodes, the pitch-coke mixture is stamped directly as an electrode.

Electrodes according to the invention have an electrical resistance of the order of 0.75-1.18 x 10-3 ohm per cm and compressive strengths of approx. 315-630 kg/cm2. They neither crack nor dust. If finely ground fluidization coke is used instead of finely ground coke from delayed distillation, a much cleaner operation is provided when preparing the coke for the manufacture of electrodes. Finely ground coke from delayed distillation is very dusty, while finely ground fluidization coke is cleaner to handle.

The coke from delayed distillation causes cracking and shrinkage in

carbon electrodes. The coke from the fluidization process fills the spaces between the particles of coke from delayed distillation and the electrode acquires great electrical conductivity. The density of the fluidization coke

is closer to the density of the coke formed by the splitting of the lime binder than the density of coke from delayed distillation does. The consequence is that the fluidization coke and the tj ear coke residue are very close

same reactivity, which results in less dust formation from the electrode during aluminum electrolysis.

The pitch binder usually consists of pitch from an aromatic coal tar, e.g. something that has a melting range of approx. 70— 120°C and contains approx. 5 percent or less of hydrogen. The amount of benzene-soluble material in the binder preferably amounts to approx. 20-35% by weight. of the binder, and the nitrobenzene-insoluble material constitutes between 10 and 20 wt. of the binder. A good lime binder will leave over 50 per cent carbon when pre-coked. Other pitch binders, e.g. such as are obtained from petroleum and have the desired and necessary properties can be used instead of ordinary coal tar pitch binders.

Use of 10-40 percent coke from delayed distillation and 90-60 percent fluidization coke provides an optimal mixture that utilizes the delayed coke's ability to prevent cracking and shrinkage and the fluidization coke's ability to counteract dust formation from the electrode. Dust formation is a very serious problem with electrodes that consist of 100 percent coke from delayed distillation. It is permissible to allow a small amount of finely ground coke from delayed distillation of particle size below 20 mesh, even down to -200 or -325 mesh, to be incorporated into the carbon mixture from which the electrodes are to be made, but this amount should be kept as small as mu- if very good electrodes are to be obtained.

The optimum ratio of ground to unground fluidizing coke can be found by preparing different mixtures of the two, and then determining which mixture gives the maximum packing density. The greater the packing density, the lower the electrical resistance of the electrode, when the other factors are kept constant. This ratio between ground and unground coke will depend on the particle size and on the distribution of the unground coke, which varies with different coking methods and devices, as well as on the degree to which the ground portion is pulverized. The ratio must be determined for each type of work used. Fig. 1 and 2 show examples of maximum packing density.

Fig. 1 graphically indicates the results of work that has been carried out to determine the maximum mass density of mixtures of fluidizing coke and illustrates the effects of different particle sizes and size ranges on the relative deposited mass densities.

In fig. 1, the curve with the open circles represents the mixture that has only fluidization coke as base material and the curve with the filled circles represents the mixture with +48 meshes of fluidization coke.

Fig. 1 shows that by adding fine calcined fluidization coke particles (92 percent through 325 meshes) to an initial

portion of only original calcined fluidization coke or to a coarse fraction (greater than 48 mesh) of only original calcined

fluidization coke, you practically get it

same maximum density at less than approx. 30% by weight of the fine coke particles. The one in fig. The maximum mass density shown in 1 is around 20-30% by weight. fine coke particles. The abscissas in fig. 1 represents weight percent. fine coke particles in the mixture of fine particles and of the base material.

Fig. 2 graphically sets forth the results of attempts to determine the maximum deposited or bulk density of calcined fluidization coke mixtures obtained by adding calcined fine coke particle fractions to a starting material. Variations in the maximum mass density are dependent on the particle sizes in the fractions of fine particles.

The curves in fig. 2 shows the results of adding three types of calcined fine particles to a starting material of 48 mesh calcined fluidization coke. The following table indicates the size distribution in the three fractions of fine coke particles.

In fig. 2, the top curve with the filled circles represents the mixture containing "Fine 1", the second lowest curve with the inverted open triangles represents the mixture containing "Fine 2" and the bottom curve, with open squares, the mixture that contains

"Nice 3".

The abscissas in fig. 2 represents weight percent. of fine coke particles in fine plus starting material.

It can be seen from fig. 2 that maximum mass density is obtained at approx. 25 weight percent. fine particles, and with a density improvement approaching 20 per cent. It can also be seen that the greatest increase in the maximum mass density was achieved with the "Fine l" fraction.

In mixtures of unground and ground fluidizing coke and pitch binder, regulation must in some cases take place out of consideration for the fluidity of the pitch, as the pitch, which is still quite viscous at the mixing temperature, to some extent counteracts the movement of the coke particles to the position in the mixture that gives maximum density. This can be compensated for by increasing the quantity of the ground portion of the fluidizing coke by approx.

5 — approx. 50 percent of the present malt

fluidization coke and uses correspondingly less unground coke. How large this regulation should be can only be determined by preparing test electrodes from different mixtures of the particular coke one has at one's disposal and then measuring the electrodes' electrical resistance. The lower this latter is, the better the mixture, but the mixture will fall within the above-specified quantity ratios. The exact quantities used must be determined, as there is a strong dependence between the size and distribution of the coke particles, both with regard to ground and unground fluidizing coke as well as the softening point of the pitch, the amount of remaining carbon from the pitch when the electrode is annealed as well as many other properties of the coke and the pitch. If pitch is used that is relatively easy-flowing at the mixing temperatures, the correction can be made to account for

the fluidity may be very small or even completely omitted. The higher the mixing temperature used, the less correction for fluidity is necessary.

Table II indicates some coke mixtures that fall within the scope of the invention.

As a further special feature of the invention, coarse coke produced by delayed distillation can be mixed with the ground and unground fluidizing coke to determine the maximum packing density. The ingredients are then thoroughly mixed by moderate vibration, until the mixture no longer sinks together. The volume is then measured. The smallest volume found for the different coke mixtures of the same weight represents the maximum packing density.

Example.

A pre-fired electrode is produced by mixing approx. 70% by weight coke

particles from the fluidized distillation process with 30% by weight. coke particles from the delayed distillation process. The last-mentioned particles have a size of between approx. 20 stitches and 12.7 mm in diameter. The particles of fluidization coke account for approx.

49% by weight of coarse particles that have a size between 10 and 200 meshes and the rest, or approx. 21% by weight, consists of finely ground coke or flour with a particle size of less than 200 mesh, all calculated as % by weight. of the total amount of coke used to manufacture the electrode. The coke was calcined at approx. 1315°C for approx. 1 hour. About. 82 parts by weight of

this calcined coke mixture was mixed with approx. 18 parts by weight of coal tar pitch

m.p. 99°C. The resulting mixture was

pressed at 350 kg/cm2 in a mold, after which

the shaped mixture was annealed at approx.

1090°C to coke the binder. It

thus burnt electrode had a compressive strength of approx. 490 kg/cm2 and an electric one

resistance of approx. 1.0 x 10—s ohm/cm.

A good electrode for aluminum production must have a minimum compressive strength

about. 224 kg/cm.2, a minimum mass density of

about. 1.45 and a maximum electrical resistance

of approx. 1.57 x 10—3 ohm/cm. Electrodes that

is produced according to the present

invention can also be used for others

purpose than for the production of aluminium.

Claims (1)

Method for the production of a coal electrode, where particles of calcined coke produced by delayed distillation with a size of between approx. 20 mesh and 12.7 mm is mixed with particles of calcined fluidization coke, and this coke mixture is then mixed with a carbonaceous binder, after which the resulting mixture is shaped and fired at a temperature between approx. 980 and 1315°C for 8-300 hours, characterized in that the electrode is produced by mixing 10-40 wt. calcined coke produced by delayed distillation with 60-90% by weight. calcined fluidization coke consisting of ground and unground fluidization coke, the ground fluidization coke comprising coke flour with a size finer than 200 meshes in an amount of between 6 and 45% by weight. of the total coke mixture, and the unground fluidization coke particles have a size above 200 mesh.