The aluminum dust generated during the grinding of metal coffee makers presents a risk of explosion caused by sparks or an increase of temperature. The Nicomax company has installed a new piece of equipment to collect, convey, and precipitate these dust particles.

The task

Nicomax s.n.c. grinds the bodies of coffee makers manufactured by the casting of aluminum alloys. The term "grinding" designates the operation of surface finishing done with a band of which the active surface has abrasive particles fixed by an adhesive. Manually loaded automatic machines perform this operation. It produces aluminum dust of varying size, particularly in function of the kind of abrasive used and of the precision of the grinding under pressure of the item. The dust consists essentially of "corkscrew" shavings between 100 µ and a few µ in size for the invisible particles. The dust particles are collected at the point of emission by means of aspiration equipment that directs the device to a system for central precipitation.

Depending on the lot to be processed (1 to 8 cup coffee makers), a specialized operator adjusts the machine mechanically and electronically by programming the PLC according to the operation to be performed.

The operator loads and unloads the item. The machine automatically performs all the movements and commands. The machines have been designed and fabricated to minimize the man-machine interaction and, particularly, to eliminate all contact between the operator and the abrasive product.

The risk

These operations emit aluminum dust that, dispersed in the atmosphere, forms mixtures that can explode in the presence of sparks or localized temperature increases. The capture of the dust particles at the point of formation as well as their conveyance and collection is done by means of special dry and wet operations. During the wet treatment phase, particular attention must be given to the elimination of hydrogen, which forms by a reaction of the aluminum dust with water.

The solution and the method

The first risk evaluation was done in 1996. This evaluation led to the replacement of the dust-collection installation operated with a dry process with an installation that used a wet process. The latter installation was constructed with the technical assistance of the Health and Safety Agency of Azienda Sanitaria Locale (local sanitary agency).

In 1999, an explosion caused by a construction defect of the cyclone destroyed the installation and injured 11 workers and inflicted considerable damage on the nearby structures of the firm. After this accident, one tried to find a number of solutions without being able to have recourse to information in the literature or to Italian norms on the matter.

The analysis was made of the risks of fire and explosion making use of the indications given in American and German regulations and the control lists prepared by a research group of the technological institute in Turin. This method resulted in the preparation and publication of a manual by the Health and Safety Agency of the ASL (Azienda Sanitaria Locale) 14 of Verbania.

This installation starts with tight housings that enclose the operational units, leaving open only the space strictly necessary for the introduction of the item to be ground. The mixture of air and dust is sucked off at the base of the housings and conveyed by circular tubes with continuously increasing diameter to the washing dome with a Venturi inlet, where it is thoroughly mixed with water jets agitated by the presence of the Venturi cone and then goes into the separator cyclone. The wet and weighed-down dust particles settle in the hopper of the cyclone while the cleansed water, sucked to the upper part of the same cyclone by the ventilator is dispersed into the atmosphere by means of a chimney of suitable height. Via the hopper of the cyclone, the sediment descends into a water-filled tank, settles on the bottom, and is scraped together by a scraper. It is then collected in a vessel provided for this purpose and finally transported for disposal. In order to make the air-aluminum dust mixture more inert, each suction collector has in its inlet an ad hoc device that blows in calcium carbonate powder at intervals of a few minutes. This operation is shown on Diagram A1.

The complete installation, including the operational units, has an EC certificate of conformity with an NFPA 651/98 extension issued by the manufacturer.

All the components of the operational units in contact with aluminum dust are clad in aluminum or an anti-static material across the evacuator inlet situated in the lower part of the housing. There is grid to keep out foreign bodies, which might generate sparks. Each machine is connected to the main collector (or by group of machines) with a vertical pipe of anti-static stainless steel with junctions assembled in function of the flow. The initial segment consists of a flexible corrugated tube of antistatic synthetic material with smooth internal walls and also protected against the accumulation of static electricity by copper wire connected to conductive elements in the vicinity.

The three principal collectors of galvanized steel have a slightly sub-horizontal position, an increasing cross-sectional diameter, and terminate in the dome of the washing group with a Venturi inlet. The connections are end-to-end or simply assembled in the direction of flow (to avoid possible infiltration of water into the outer segment), and the elbows consist of clamped segments that are smooth on the inside in order to prevent the deposition of aluminum dust, which possibility is also reduced by the velocity of the fluid and the presence of calcium carbonate powder. The calcium carbonate is blown in at irregular intervals by the propulsion unit placed in the inlet of each collector and settles first on small irregularities that may be present. The highest points of the collectors have evacuation ports, which are always open, to disperse into the atmosphere the hydrogen molecules that may form when the equipment is shut down, in the presence of condensation. To maintain the aluminum dust safely in suspension, the velocity cannot be less than 20 m/sec, which makes the uniflow assembly of the pipes acceptable and which assists the elimination of water infiltrations relative to the possibility of dust-deposit formation (suggestion of the American norm NFPA 651). However, this is improbable because of the agitation of the flow.

The mixture of air and aluminum dust and calcium carbonate goes from the collector dome to the Venturi cone at the summit of which it is mixed, by agitation, with recycled water pumped from the final sedimentation tank.

This contact, at the passage in the Venturi inlet, is intensified in order to assure adhesion of the liquid to the particles to be precipitated. The depression due to the Venturi effect, also causes partial evaporation of the water, which, by recondensing in the collection and routing chamber, situated below, enhances the collection of the particles that are smaller than the dust. The summit of the dome has an open port for evacuation into the atmosphere of any hydrogen molecules, and the Venturi cone has a trapdoor for inspection and periodical manual cleaning. The shape of the collection and routing chamber is designed so that the washing group can be emptied if the installation should shut down.

From the collection and routing chamber, the air-water-dust mixture is introduced tangentially into the separator cyclone, and the liquid fraction, centrifuged on the walls, collects in the conical base, and falls into the final sedimentation vessel via the submerged outlet.
The atmospheric portion, cleansed, is evacuated by the ventilator at the end and conveyed into the atmosphere by a chimney equipped with a unified sampling system. The separator cyclone is also equipped with a port, which is always open, for evacuation of the hydrogen and a trapdoor for inspection and manual cleaning.

The sedimentation vessel has a large capacity (about 10 m³) and at its bottom the sediment from the cyclone settles. It has a scraper that operates at fixed intervals to extract the sediment that is being deposited and load it into an adjacent open container, where it is ready to be taken for disposal.

The vessel is connected to the water supply system, and the refilling, which is necessary because of the consumption of water by the process, is done automatically by means of a float mechanism. The water level, maximum or minimum, is read by sensors connected to the electric command and control panel of the system that stop the process and turn on visual and audio alarms.

If the float refilling mechanism does not function correctly, in addition to stopping the process and turning on the alarms, a system evacuates the overflow and shifts the excess liquid into an auxiliary receptacle. In the upper part of the vessel is a labyrinthine baffle chamber. From this chamber, the purified water is pumped and sent to the Venturi washing group via a hydraulic circuit of which the interception devices, normally blocked in open position, can be activated manually in the event of an emergency. A pressure-sensitive switch and a flow switch, linked to the electric command and control panel, enable the system to be stopped and the visual and audio alarms systems to be activated in the event of a dysfunction.

The centrifuge-type ventilator (90 kW and 34,000 m³/h) is located at the end of the installation and has antivibration joints at the level of the evacuation and the routing as well as a set of vanes for the evacuation controlled by a pneumatic servocontroller connected to the star/triangle device of the motor switch.

The power supply of the operational units and of the dust-sedimentation equipment is done at IP 55.


The effectiveness of the results

The risk of explosion and fire has been fairly well eliminated.

The solution developed by Nicomax s.n.c. has been made available to other companies in the sector in the Province of Verbania. In some cases, this solution could be applied while, in others, because of the diversity in the working methods, technical or economic studies in function of the interested companies are still in progress.

The participation of the workers

The workers of Nicomax s.n.c participated actively in the implementation of this example of good practice and adopted the procedures included in a text that describes and analyzes the risks associated with the equipment, methods, and the precautions. They use them to perform in complete safety the operations for which the enterprise was created.

The costs and benefits

The study and its implementation resulted in a financial charge of about 500,000 euros for the company. The advantages reside principally in the resumption of the activities of the company, which had closed its doors for about a year, and in the possibility of paying again the wages of the 15 workers of the company.

Entrepreneur’s testimony "We are a craft company in which the workers and the shareholders work in close contact. Our safety and that of our collaborators and of the nearby companies, is our most valuable possession. The possibility of continuing in complete safety an activity that has for decades constituted something typical of the local industrial sector is for us a source of pride".