Manufacturing process

Impervious graphite for process equipment applications is manufactured in two main steps. First synthetic graphite is manufactured. Then this material is impregnated with phenolic resin to make impervious and enhance it mechanical characteristics.

GAB Neumann uses exclusively graphite from western origin (United States of America, Europe or Japanese suppliers only). Our process uses a homogenous fine grain graphite with a compact grain structure, an optimal grain and pore sizes distribution and exempt from structural irregularities such as cracks, large pores, or impurities.

GAB Neumann has developed, in close collaboration with a renowned European resin manufacturer, a phenolic resin that is optimized for the type of graphite it uses and for its manufacturing process.

Snapshots

Synthetic graphite manufacturing process

The synthetic graphite manufacturing process includes numerous steps namely the powder preparation, the shape forming, the baking, the pitch impregnation or densification, the rebaking, and the graphitization.  

1. Powder and paste preparation

Raw materials for synthetic graphite fabrication (petroleum coke, pitch coke, carbon black, natural graphite and secondary graphite scrap are loaded and stored in raw materials silos. First, the raw materials are ground in crushers and ball mills. The resulted powder is then conditioned according to the particles size distribution. Finally, the powder is blend with a binder to produce a paste. Coal tar pitch or petroleum pitch are used as binders.

2. Shape forming

The resulting pasty mixture may be compacted by one of the shape forming techniques: extrusion, vibro-molding or cold isostatic pressing.

  • Extrusion consists in forcing the pasty mixture through a die with an opening. Extrusion results in a long product such as rods, bars, long plates, or pipes of regular cross-section, which may be cut into pieces of required length. Extruded graphite materials are isotropic. The properties in the extrusion direction differ from the properties in other directions.
  • Vibration molding is a discontinuous shaping method for large dimension products. A mold is filled with the pasty mixture and a heavy metal plate is put on top of it. Then the material is compacted by vibrating the mold. The formed bodies show a higher degree of isotropy compared to extruded materials.
  • Cold isostatic pressing is the powder compaction method conducted at room temperature and involving applying pressure from multiple directions through a liquid medium surrounding the compacted part. A flexible mold immersed in a pressurized liquid medium is used. Materials with uniform anisotropic structure are prepared by isostatic pressing method.

3. Baking

The compacted parts are heat treated in a baking furnace at temperatures between 900 and 1200°C (1650 and 2200°F) in the absence of air. The carbonization process occurs during the baking. It results in the thermal decomposition of the binder into elementary carbon and volatile components. Carbon formed in the carbonization process binds the powder particles. The volume of the binder is higher than the volume of the formed carbon, therefore carbonization results in formation of pores, total relative volume of porosity is determined by the binder quantity.

4. Pitch impregnation

At this point in the process the carbon parts may be impregnated with pitch and re-baked to reduce its porosity. Impregnation is typically performed using a pitch lower in viscosity than the original binder pitch. Low viscosity is required in order to affect more complete void filling. Petroleum pitch is normally utilized for this function.

For some high-density graphite grades, the carbon parts may go through the baking, pitch impregnation, rebaking cycle several times. For the manufacturing of our GPX 1 graphite, we use a raw graphite material that has been densified with pitch and rebaked. For the manufacturing of our GPX 2 graphite, we use a raw graphite material that has been densified with pitch, rebaked, re-densified with pitch and rebaked again.

5. Graphitization

At this stage the shaped, baked, pitch impregnated, and rebaked parts are heat treated under exclusion of oxygen at extremely high temperature 2700 to 3000°C (4900 to 5450°F).

The graphitization process results in the crystallization of amorphous precursor carbon, which transforms into crystalline graphite. Under the influence of temperature, the crystallites grow and rearrange in an ordered pattern of stacked parallel planes. This transformation is accompanied by a change in the physical properties of the material. During this high temperature treatment graphite is also purified since most its impurities (binder residues, gases, oxides, sulfur) vaporize.

Graphitization is performed in an Acheson-type furnace. This furnace consists of a central chamber surrounded by external walls made of some refractory material such as firebrick. The chamber is roughly rectangular in outline. The top is open. The Acheson furnace is nothing more than a room without a ceiling, designed to keep in the heat generated by electrical resistance heating of the carbon charge.

The end walls of the Acheson furnace are fitted with graphite buss bars. These buss bars extent to the outside wall of the furnace where they are coupled to large caliper copper buss bars that are water cooled.

Loading of the empty furnace proceeds by placing the baked carbon blanks in some pre-determined configuration. Carbon articles may be loaded parallel or perpendicular to the power supply buss bars located on the furnace end walls.

Since graphitization process temperatures are expected to reach 2800°C (5070°F) or more it is of utmost importance that oxygen be excluded from the furnace. This is accomplished by covering the carbon articles with some oxygen scavenging material.

Once the articles to be graphitized are placed in the Acheson furnace and covered with the appropriate packing material a direct current of low voltage and very high amperage is applied to the furnace charge. The furnace load heats up due to its own electrical resistance. As the heating progresses the furnace resistance goes down due to the increase in conductivity that results from the formation of graphitic carbon at the expense of amorphous carbon.

Furnace conditions are constantly monitored, this includes the power consumed. At the point where the operator determines that the proper furnace temperature has been achieved, or that the charge is fully graphitized, power to the furnace is cut. The furnace is allowed to cool and the graphitized articles are removed.

Carbon & graphite structures

 

 

With heat and time the amorphous carbon structure (left) turns into the crystalline graphite structure (right).
Amorphous carbon is hard and very difficult to machine. It has low electrical and thermal conductivities.
Oppositely graphite is crystalline. It is easy to machine. It has very high electrical and thermal conductivities.

6. Quality checks

The graphitized articles go through a series of tests and inspections before being shipped. At this stage, the resulting graphite material has already an excellent corrosion resistance and an outstanding thermal conductivity. However, it is highly porous and therefore not at all impervious.

Phenolic resin impregnation

For process equipment applications, synthetic graphite shall be made totally impervious. For that purpose, it is impregnated with resin. Phenolic resin is selected because of its superior corrosion resistance and mechanical strength.

GAB Neumann has developed, in close collaboration with a renowned European resin manufacturer, a phenolic resin that is optimized for the type of graphite it uses and for its manufacturing process.

The phenolic resin impregnation process includes the three steps namely the drying of the graphite material, its impregnation with phenolic resin, and finally its curing.

 

1. Drying

The raw graphite bars a put in drying chambers, where the temperature is set at about 110°C, during sufficient time to allow them to release all their moisture.

2. Phenolic resin impregnation

The dry graphite bars are then put into large autoclaves. The autoclaves are sealed and then deep vacuum is applied in order to remove the remaining traces of moisture in the material and to remove the air contained inside of the material’s porosities.

Then phenolic resin is introduced in the autoclave through the bottom valve. The liquid level shall be raised substantially above all the graphite blocks since the level will decrease once the resin penetrates into the blocks. High-pressure is applied to force the resin to penetrate to the very core of the graphite bars.

Once the impregnation is completed, after several days, the excess of resin is transferred back to the resin tank and the pressure is released. GAB Neumann impregnates thoroughly its graphite parts to their core in one single step. 

3. Polymérisation

Ensuite, les composants imprégnés de résine sont transférés dans une autoclave de polymérisation. Les pièces sont chauffées selon un profil précis jusqu'à ce que la température de polymérisation de la résine phénolique soit atteinte. Des groupes éther sont tout d'abord formés. Puis des groupes méthylène sont formés. Le polymère formé est réticulé tridimensionnellement, solide et dur.

 Après la polymérisation, des bulles de vapeur minuscules, dispersées de manière homogène, sont encapsulées dans la résine. Tous les pores sont entièrement remplis de résine phénolique et il n'y a aucun écart (mouillage parfait) entre la résine et les parois de graphite. Les matériaux GAB Neumann qui résultent, le GPX 1 et le GPX 2 de, présentent une résistance exceptionnelle aux produits chimiques les plus courants, en particulier les acides, ainsi qu'une résistance mécanique et une stabilité élevée sur le long terme.

4. Quality checks

The phenolic resin impregnated graphite pieces go through a series of tests and inspections before the machining and manufacturing of process equipment parts.

GAB Neumann GmbH

Alemannenstrasse 29

79689 Maulburg

Germany

Tel: +49 (7622) 6751 0

Fax: +49 (7622) 6751 20

info@gab-neumann.de

GAB Neumann GmbH | Alemannenstrasse 29 | D-79689 Maulburg | Phone +49 (7622) 6751 0 | Fax +49 (7622) 6751 20 | E-Mail info@gab-neumann.de | www.gab-neumann.com

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