Top
top banner

 

 

3.General Description of the Technological Process

 

3.1. Main Stages of the Process

3.1.1. Preparing of the Racks with Items

With the help of a special electric bridge crane with turnaround device the empty racks are collected and then put on the raise/lower powered lifting stations.

There are 2 powered lifting stations with adjustable lifting hoist and 2 automatic racks storage systems, each for 4 racks, which are placed one at the beginning of the shed for HDG, as well as the other at the end of it.

The items are transported in the loading area by means of a lift-truck. Once the rack is prepared with the items to hot-dip, it is placed on a racks storage system, on which the racks are transferred automatically.

All the bridge cranes are operated through a radio control.

3.1.2. Chemical Pre-treatment Department

This department includes:

  • 1 Degreasing Tank
  • 5 Pickling Tanks
  • 1 Rinsing Tank
  • 1 Fluxing Tank
  • 2 Cooling Tanks (for post-treatment, one tank is back-up )

The chemical pre-treatment process is necessary to prepare the surface of items to hot-dip. All the impurities coming off, which are lubricating oils, rust, corrosion, welding slags, etc. should be removed in order to obtain an indefectible zinc coating. The accuracy and thoroughness one applies in pre-treatment process would predetermine the final product quality.

degreaser used for pre-treatment process is an acid one, which does not need water rinsing. Once the items are being degreased, they would undergo the pickling inside the pickling tanks with a hydrochloric acid solution.

After being pickled the treated items are immersed into the rinsing water and then, into the fluxing solution, which is heated up to 60C. The last stage is necessary to create a protecting film on the metallic surface before hot-dipping.

The heating of the tank is provided by some heat-exchanging units using the hot water, which are down-dropped into the tanks. They are made of polypropylene and polyethylene pipes which are resistant to temperatures up to 80C. The hot water is produced by a heat economizer, recuperating the waste gas from the furnace fire chamber.

The acid vapors, generated during the immersion of steel items into pre-treatment tanks, are absorbed and filtered in a wet acid-vapor scrubber, where the polluting discharges are reduced until the HCl concentration values in the atmosphere.

The flux treatment unit should be considered an integral part of the chemical treatment. The flux treatment is automatic with the use of hydrogen peroxide and ammonia, which helps to obtain the values of Fe, dissolved in liquids, around 3-4 g/l. A filter-press removes the dross.

3.1.3. Drying of Items

Once the pre-treatment stage is completed, the racks with items are transferred into the dryer. The dryer represents a chamber which is veneered with fire-brick and has 4 horizontal stands. The racks are placed inside the chamber by a bridge crane and being transferred by an automatic pull system. The heating is obtained by the hot air coming from a heat generator that keeps the temperature fixed between 80C and 100C through an air-to-air exchanging unit sand, which is most important, without any carbon dioxide exhalations. A strong ventilation guarantees the perfect drying.

3.1.4 Hot Dip Galvanizing Department.

The HDG department is mainly composed of HDG furnace and the white fumes enclosure. The racks are collected from the last stage of the dryer and dipped into the melted zinc which is contained in a zinc kettle.

The white fumes enclosure allows to absorb the highest rate of white fumes generated from the surface of the galvanizing kettle. These white fumes are absorbed and treated in a baghouse fumes filter which diminishes the polluting content according to the current local regulations. The galvanizing furnace represents the most important part of the plant. Nowadays the major research activity in the field of galvanizing is focused on the development of new technologies which help to achieve the maximum efficiency with the least consumption of energy resources. The brickwork of longitudinal and lateral walls of the furnace's steel structure is made of firebrick and ceramic fibre.

The high-velocity burners are "on/off" type and are placed on the lateral walls. The so called "pulse" operation allows to save further gas, as the burners switch on only in order to reach the temperature of set-point, whereas their functioning is limited to few starts per hour in a stand-by condition. The new type of burners' regulation and control allows to obtain the suitable zinc temperature with a possibility to adjust it in 4 different levels.

The process control is performed by PLC (Program Logic Controller) and is displayed on a touch-screen panel. A great number of different alarm systems, which equips such furnaces, permit to use it safely and with no risk.

Furthermore, a remote monitoring system thru online can be established in order to monitor the furnace functioning and troubleshooting.

3.1.5. Unloading of Hot-Dipped Items.

After hot-dipping, the racks are collected from the furnace and then they pass through the cooling tank .
The racks with ready items arrive to the automatic racks storage and move to the unloading lifting station. The items are then prepared for shipment while the empty racks are sent to the loading area by a transfer cart.

3.1.6. Recycling Plants.

All waste treatment systems of the plant are necessary to avoid the environmental pollution, as well as to reach the values of emission into the atmosphere within the limits provided by the current local regulations.