The change of supplier was crucial for our customer, a manufacturer of alloy metals and high-performance special steels, specialized in the processing of nickel matte in nickel metal and high-purity nickel salts. The new matte being richer in magnesium and with a different mechanical structure, a complete overhaul of the pretreatment was needed.
To effectively recycle liquids effluents (nickel hydroxide sludges loaded with magnesium), CMI Proserpol was asked to both define and implement a process to comply with current environmental standards: a second stage of physicochemical treatment for magnesium and nickel selective precipitation was added.
CMI Europe Environnement has besides developed a turnkey gas decontamination solution in order to treat harmful and corrosive fumes from chemicals used for nickel refining (chlorine/Cl2 and hydrochloric acid/HCl).
The former process was mainly producing chlorine releases treated with a dedicated scrubber. This phase remains in the new process, but corresponds to the etching phase of the matte after griding. With the new pretreatment, the nickel matte is processed in pulp, in particular with an acid phase that generates acid gas discharges.
Due to the presence of a new acid compound (HCl) to be treated, this new installation was designed to operate in 3 modes:
- Mode 1 consists of treating only acid discharges coming from the acid gas collection network containing hydrochloric acid at a 2200 m3/h airflow (this mode corresponds to the start-up phase of the site; during this period, the former scrubbing column continues to treat chlorinated gases)
- Mode 2 is used to process all gases (Cl2 + HCl), that are driven to and treated in the new air treatment unit with an airflow of 4000 m3/h;
- Mode 3 is for maintenance only, it allows the new air treatment unit to withstand a chlorine peak during a short period of time at an airflow of 10 500 m3/h. During this operating phase, the scrubber column can be filled with concentrated soda at an instantaneous flow rate above 100 m3/h.
This is a turkey service: design, construction, assembly and commissioning of the equipment.
The centerpiece: an air scrubber in ECTFE (Halar)
Gases are neutralized in a packed scrubber in which a soda solution is sprayed. The high temperature of the inlet gases (≈80°C) combined to the strong exothermic reaction of the absorption of chlorine by the soda ash required to implement a heat exchanger of nearly 5 MW in order to lower the temperature in the scrubber and thus avoid the formation of chlorates during operation in modes 2 and 3.
The specific features of the customer’s process compelled to use an extremely chemical resistant material. That is why the inside part of the scrubber is made in ECTFE (Ethylene ChloroTriFluoroEthylene), also known as Halar®. Due to the costs of such a material and its technical processing difficulties, ECTFE is used in thin layer: it forms the internal coating of the scrubber to ensure its chemical barrier. The external part of the scrubber is coated with fibreglass/ resin (Fibreglass Reinforced Plastic or FRP) to ensure the mechanical resistance of the scrubbing column, whose total height is near 10 m (the exchange surface height: 5,7m).
Under these conditions, mechanical reinforcement is all the more essential as the scrubbing column works vacuum, which that can reach -40mbar.
This equipment has entirely been designed and built in our workshop at Aspach-Michelbach.
The ECTFE welding: a whole know-how!
The material requires very stringent working conditions: on the one hand, the welding of ECTFE must be carried out under inert gas (nitrogen), and on the other hand, because of toxic fumes during the welding process, the wearing of specific personal protective equipment (PPE) is essential, such as the air-breathing mask.
Other equipment that makes a difference…
Critical part of the air treatment system, the gas scrubber is equipped with
- a 15m-height FRP chimney with the metal support structure
- a fretted PPh radial demister placed at chimney outlet to collect acid-filled condensation
- 2 x 2 high pressure fans, fitted with turbine in PDVF and a volute casing in fretted PPh
- a scrubber and fans access platform including a ladder with guardrails
- the acid gases collection piping network in fretted PPh or in FRP for the PVDF connections regarding the temperatures of the reactors connected to the scrubber
- the chlorinated gas collection piping ductwork in massive FRP
- all instrumentation to operate the installation
Assembly and commissioning
CMI Services, certified MASE, handled the assembly of the whole air treatment unit. The installation was started after the production shutdown last July. The final acceptance will take place at the end of this year.
Performance guarantee of the treatment
Except for the maintenance mode (3), the standard mode (2) allows to treat 1000 kg/h of Cl2 and 33 kg/h of HCl. The soda consumption is then around 350 l/h. The chlorine release after treatment remains below 1 mg/m3 and the HCl one below 1 ppm, which is much lower than the usual regulatory requirements.