GB2168074A - Embedding material for sacrificial anodes - Google Patents

Abstract

An embedding material for anodes of stray current corrosion protection installations is based on calcined petroleum coke. In order to provide a low specific resistance and good gas permeability in such an embedding material, even in loose fill form, the petroleum coke contains 93-99% by weight carbon, 0.2 - 1.8% by weight sulphur, 0.3 - 1.2% by weight nitrogen, 0.1 - 2.5% by weight ash, 0.1 - 1.0% by weight volatile hydrocarbons and the balance being other impurities, and has a substantially cubic grain structure of fractured stress-free and crack-free grains. Anode erosion is also rendered uniform and reduced when using such an embedding material.

Description

SPECIFICATION Embedding material for sacrificial anodes The invention relates to an embodying material for anodes of stray current corrosion protection installations, based on calcined petroleum coke.

It is conventional for the sacrificial anodes of stray current corrosion protection installations, for example, for underground pipelines, containers, industrial plant, power plant etc. to be surrounded in the ground by a conductive embedding material.

The function of the conductive embedding material which surrounds the sacrificial anode, particularly when situated in ground with a high specific resistance, is to increase the contact area with the ground. In addition, such embedding material is intended to prevent direct contact between the ground and the sacrificial anode so that any corrosive constituents of the ground cannot prematurely damage the anode. In the event of direct contact between the sacrificial anode and the ground, there would also be a possibility that, in those parts where electrical contact with the ground is particularly good the sacrificial anodes would be eroded to a greater degree because of the greater current density which occurs at such locations.The embedding material interposed between the anode and the ground is a good conductor and makes contact uniform over the surface of the anode so that the anode is eroded in a regular fashion. In addition, the embedding material is generally permeable to gases so that gases which are produced at the sacrificial anode in the operation of the stray current corrosion protection installation may be dispersed without forming insulating gas bubbles in the ground.

It is conventional practice in the art for a sacrificial anode, which generally comprises iron silicon (FeSi), graphite, scrap iron or magnetite, to be embedded by introducing the embedding material to a previously prepared receiving cavity (bore hole, pit or the like), or pumping it mixed with water thereinto, after the anode to be embedded has been fixed in the correct position. In some cases, the sacrificial anode may also be surrounded by embedding material in perforated containers, bags or the like, and then put into the ground together with such containers.

Hitherto little attention has been paid to the actual embedding material. In the Federal Republic of Germany, the embedding material used is usually metallurgical or blast-furnace coke or coke-oven coke, with a grain size of from 10 to 40 mm. Because of its low carbon content (80 to 90%) and its high proportion of non-conductive constituents (sulphur, nitrogen, ash, liquid hydrocarbon, etc.), the material has a relatively high specific resistance (up to 100 n.cm). In addition, this material can be pumped only with difficulty so that it is generally not suitable for embedding anodes e.g.

in deep bore holes which may be up to 200 metres in depth.

An embedding material based on a petroleum coke has also been proposed. This material is a socalled fluidised petroleum coke, which is produced in a fluidised bed process, and differs from other petroleum cokes in that it comprises fine spherical particles which, when mixed with water, can be easily pumped and which are highly permeable to gas in their bulk form. However, the known fluidised petroleum coke also has a comparatively high specific resistance in its bulk form. This may be attributed on the one hand to the fact that it has a comparatively high specific resistance in its bulk form. This may be attributed on the one hand to the fact that it has a comparatively small amount of carbon (92%) and a relatively high proportion (8%) of secondary constituents (sulphur, ash, volatile hydrocarbons, etc.), and on the other hand to the fact that it comprises spherical particles.More specifically, the particles only have a point-like contact with each other so that there is a comparatively high resistance to current transmission from one particle to the next, particularly when the material is loosely filled.

We have now found an embedding material which, even when loosely filled, has a low specific resistance and is adequately permeable to gas. Anode erosion is further made uniform and reduced by the new embedding material.

In accordance with one aspect of the present invention, there is provided an embedding material for anodes of stray current corrosion protection installations, based on calcined petroleum coke, wherein the petroleum coke contains.

3 - 99% by weight of carbon 0.2 - 1.8% by weight of sulphur 0.3 - 1.2% by weight of nitrogen 0.1 - 2.5% by weight of ash 0.1 - 1% by weight of volatile hydrocarbons the balance being other impurities and incidental ingredients, and wherein the coke has a substantially cubic grain structure comprising fractured stress-free and crack-free grains.

In comparison with previously proposed embedding materials, the embedding material according to the present invention has a specific electrical resistance which is up to four times lower, even as a loose fill. This may be partly attributed to the fact that the purer petroleum coke has better conductivity. In addition, it is important that the embedding material additionally has a substantially cubic grain structure comprising fractured, stress-free and crack-free grains. With a cubic grain structure, compared to spherical grains, the contact area between individual grains is considerably larger.

Since the grains are generally stress-free and crack-free, the conduction of current between the grains is virtually unimpeded. In addition, because of its grain structure, the material has an advantageous, low bulk weight or apparent density, with a good gas permeability. Taken overall, the use of the embedding material according to the invention provides that anode erosion is uniform and reduced.

The petroleum coke is preferably obtained from fractured carbon anode residues. Carbon anode residues, as a waste material, are available at comparatively low cost.

in order further to reduce electrical resistance, the petroleum coke may conveniently be graphitised by heating to about 2800"C, in the absence of air. At that temperature, amorphous carbon contained in the petroleum coke is at least partially converted into the graphite crystalline form, thereby giving a considerable improvement in conductility.

In order to reduce the electrical resistance of the embedding material, graphite grains andlor graphite fines comprising broken graphite electrode residues may also be advantageously added to the petroleum coke. Graphite has a very low electrical resistance. By adding graphite grains or graphite fines it is surprisingly possible further substantially to reduce the electrical resistance of the embedding material. When graphite fines are added, a particularly advantageous effect is achieved since this material remains clinging intensively to the heavily structured surfaces of the grains of the petroleum coke, so that the mixture of the two materials does not separate when introduced into the bed. Using graphite electrode residues as a starting material for the mixture means that that waste material is also put into a meaningful use.

In order positively to provide for given resistance values, the above-indicated materials may possibly also be mixed as required.

In a further aspect, the invention provides a method of embedding an anode of a stray current corrosion protection installation by feeding, pouring or pumping an embedding material of the invention. When embedding anodes in vertical boreholes, there is generally a problem that the specific resistance of the embedding material used is heavily dependent on the pressure applied to the fill of the embedding material. If the embedding material is under high pressure, it has a comparatively low electrical resistance. With a loose fill however, the electrical resistance is often much higher. Thus, a loose fill, in comparison with a fill which is under a pressure of e.g. 20 bars, has an electrical resistance which is greater for example by a factor of 100.In a deep borehole, the material will often be subject to such a pressure differential i.e. the material at the bottom of the hole will be under relatively high pressure while loosely filled at the top. A variation in electrical resistance over the length of the borehole obviously has disadvantages with regard to uniform distribution of anode current and uniform erosion of the anode. In order to counteract such a change in resistance over the depth of a borehole, we propose that, when embedding an anode in a vertical borehole, the specific resistance of the embedding material used decreases in an upward direction. In that way, by selection of the embedding material, it is possible to achieve a specific resistance value which is more uniform over the depth of the borehole, in the sheathing or outside shell portion comprising embedding material.

When embedding anodes horizontally, the embedding material is desirably compacted by stamping or ramming in order also to produce a lower electrical resistance compared with material which is under a low static pressure.

Because of the lower static pressures in the horizontal embedding mode, a material having a lower electrical resistance is advantageously used for horizontal use, for example the above-mentioned graphitised material or a petroleum coke to which graphite grains and/or graphite fines from broken graphite electrode residues have been mixed.

As indicated above, the embedding material according to the invention has a substantially cubic grain structure comprising fractured, stress-free and crack-free grains. Such a cubic grain structure of fractured stress-free and crack-free grains is preferably achieved by breaking up a coarse starting material in a two- or three-stage process in impact pulverising mills, and sieved off in a twostage operation. The breaking process in a multistage impact pulverising installation preferably produces grains with a cubic grain structure, which are stress-free and crack-free. Such grains are produced since the grains which are produced from the coarse material in the first crushing operation and which still have stress and crack regions, in the next following crushing stages, are only split in those regions.This generally presupposes that the speeds of rotation of the impact pulverising mills, in the following crushing stages, are matched to the grain size which is produced in the first crushing operation.

The following non-limiting Examples serve to illustrate the invention: Example 1 A calcined petroleum coke, in the form of coarse lumps and comprising 95 to 97% carbon, 0.6 to 1.8% sulphur, 0.5 to 1.2% nitrogen, 0.3 to 0.7% ash, 0.6 to 0.8% volatile constituents (the balance being other constituents), is broken up in a three-stage crushing process in a multi-stage impact pulveriser installation with intermediate sieving, in such a way as to give a cubic grain structure with stressfree and crack-free grains. That grain structure is achieved by matching the peripheral speeds of the rotors of the impact pulverising mills in such a way that individual grains, beginning with the starting grain size of up to 150 mm, are broken down into a final grain size range (up to 5 or up to 8 mm), with an approximately equal amount of kinetic impact energy. That is achieved by the rotor speed in the primary stage being about 20 to 24 m/s, and the ratio of the rotor speeds in the primary mill, the secondary mill and the tertiary mill being 2 : 3 : 5 to 3 : 4 : 7. Downstream of the primary mill, a presieving operation, with a 15 mm grain size, is carried out; grain sizes of up to 15 mm are passed into the tertiary mill and grain sizes over 15 mm are passed into the secondary mill.

The material which is produced in that way, in loose fill form, has, depending on its grain size, a specific resistance of 9 - 20 Q.cm which, at a pressure of 20 bars, falls to 0.01 to 0.07 Q.cm. Depending on the respective grain size, in loose fill form, the bulk weight or apparent density is 0.79 - 0.9 kg/ dm3 while with a pressure of 20 bars, it is 1.035 kg/ dm3.

With the anodes arranged horizontally or vertically, this material may be used where the ground resistance is comparatively high so that, in order to increase the contact area with the ground, the embedding layer may be of great thickness.

Example 2 Coarse-lump carbon anode residues comprising 93 - 95% carbon, 0.8 - 1.5% sulphur, 0.6% nitrogen, 1.3 - 2% ash, 0.6 - 1% volatile constituents and the balance being other constituents are crushed as described in Example 1. The embedding material produced in that way, in loose fill form, with a grain size of up to 1 mm, has a specific resistance of 15 Q.cm while with a grain size of 1 - 5 mm, it has a specific resistance of 8 Q.cm. At an increased pressure of 20 bars, the specific resistance of the material falls to 0.04 - 0.085 Q.cm. Irrespective of the grain size, the bulk weight or apparent density, with a loose fill, is between 0.84 and 0.89 kg/dm while at a pressure of 20 bars, it is 0.94 - 1.2 kg/ dm. The bulk weight increases approximately linearly with pressure.The grains of this material are extremely hard. Irrespective of the grain size, that material is suitable, under low ground resistance conditions, for horizontal or vertical anode installations.

Example 3 A coarse-lump petroleum coke comprising 93.5 98.8% carbon, 0.2 - 1.6% sulphur, 0.1 - 0.6% nitrogen, 0.8 - 2.5% ash, 0.3 - 1% volatile constituents and the remainder being other constituents, is heated to about 2800 C in the absence of air. In this operation, amorphous carbon contained in the petroleum coke is partially converted into graphite.

The material is then broken up, as described in the above Examples. The specific resistance of the material is very low and is 4 - 8 n.cm, in loose fill form. At a pressure of 20 bars, the specific resistance of the material falls to 0.02 - 0.04 Q.cm. The bulk weight, depending on the grain size of the material, is between 0.71 and 1.075 kgidm3.

This material is predominantly suitable as an embedding material for the horizontal installation of anodes, with extremely low ground resistance, in thin embedding layers. The specific resistance may be further reduced by compacting (stamping, ramming), without the porosity being detrimentally affected. The grain size is not important with this material, and may be up to 15 mm.

Example 4 Graphite grains or graphite fines from broken graphite anode residues may also optionally be mixed with the petroleum cokes in accordance with Examples 1, 2 or 3. Graphite anode residues contain 96 - 99.5% carbon, 0.09 - 0.2% sulphur, 0.03 - 0.1% nitrogen, 0.1 - 1% ash, 0.01 - 1% volatile constituents and the balance being other constituents. They have a very low specific resistance, namely 3.5 - 5 Q.cm in loose fill form and 0.01 0.02 Q.cm at a pressure of 20 bars. Depending on grain size, the bulk weight is between 0.8 and 1.25 kg/dm3. By mixing the material with petroleum cokes in accordance with Examples 1, 2 or 3, lower specific resistances may be obtained.Surprisingly, it has been found that, by admixing 5 - 10% of graphite with a petroleum coke, the specific resistance of the petroleum coke in the critical pressure range of up to 4 bars may be reduced to about a third. The use of graphite fines in the mix has the advantage, particularly in regard to embedding materials which are to be pumped with water, that the graphite fines have a lubricant effect in the pumping operation and thereafter clings strongly to the surface of the cubic grains without any mixture break-down phenomena occurring in the embedding material.

Example 5 To particularly obtain given specific resistance values in the embedding material, it is possible for the materials in accordance with Examples 1, 2, 3 and 4 to be mixed as desired. Mixing with constituents in that way makes it possible, when embedding anodes in vertical boreholes, to adjust the specific resistance of the respective embedding material used in such a manner that the specific resistance of the material is at its greatest at the deepest part of the borehole and decrease in an upward direction. That makes if possible substantially to level out the differences in resistance which occur by virtue of the pressure in the borehole increasing in a downward direction.

Claims (13)

1. An embedding material for anodes of stray current corrosion protection installation based on calcined petroleum coke wherein the petroleum coke contains 93 - 99% by weight carbon 0.2 - 1.8% by weight sulphur 0.3 - 1.2% by weight nitrogen 0.1 - 2.5% by weight ash 0.1 - 1.0% by weight volatile hydrocarbons the balance being other impurities and incidental ingredients, and wherein the coke has a substantially cubic grain structure comprising fractured stress-free and crack-free grains.

2. An embedding material according to claim 1 wherein the petroleum coke is obtained from broken carbon anode residues.

3. An embedding material according to claim 1 wherein the petroleum coke is graphitised by heating to about 2800or, in the absence of air.

4. An embedding material according to claim 1 wherein graphite grains and/or graphite fines from broken graphite electrode residues is mixed with the petroleum coke.

5. An embedding material according to any one of claims 1 to 3 wherein the material comprises a mixture of materials according to any one of claims 1,2 and/or 3.

6. An embedding material according to any one of the preceding claims substantially as herein disclosed.

7. An embedding material for anodes of stray current corrosion protection installations substan tially as herein disclosed in any one of the Examples.

8. A method of embedding an anode of a stray current corrosion protection installation wherein the anode is embedded in the ground using an embedding material according to any one of the preceding claims.

9. A method of embedding an anode of a stray current corrosion protection installation using an embedding material according to any one of claims 1 to 7 wherein the anode is embedded in a vertical borehole and the specific resistance of the embedding material used decreases in an upward direction.

10. A method of embedding an anode of a stray current corrosion protection installation using an embedding material according to any one of claims 1 to 7, the anode is embedded horizontally and the embedding material is compacted by stamping or ramming.

11. A method according to claim 10 wherein a material according to either of claims 3 and 4 is used for the horizontal bedding.

12. A method for the production of an embedding material according to claim 1 wherein coarse starting material is crushed in a two or three-stage breaking process in impact pulverising mills and sieved in a two-stage operation.

13. A corrosion protection installation wherein an anode is embedded in the ground using an embedding material according to any one of claims 1 to 7.