Abschnitt 3.3 - 3.3 Hazards associated with salt baths and measures to be taken
3.3.1
Requirements for workrooms and work areas
Owing to the properties and particular characteristics of the salts used in heat treatment shops, further requirements upon the workrooms and work areas may need to be observed.
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The specific hazards posed by salt baths and the heat treatment salts used in them arise primarily owing to the temperature of the molten salts and the properties of the salts. The hazards are primarily:
Burns caused by hot salt baths, hot workpieces and ancillary materials, and by hot plant components
Poisoning by the salts
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The protective measures for operation begin with design of the workrooms and work areas. Implement the following measures:
General
Workrooms and work areas must be located in buildings constructed from non-combustible building materials. Should this not be possible, a heat shield must be installed permanently to prevent combustible parts of the building from being ignited by the molten salt or the heat radiated from it.
Workrooms and work areas must be capable of withstanding the mechanical, chemical and thermal stresses anticipated during operation of the baths.
Location and dimensions
Workrooms and work areas in which steel and other heavy metals are heat-treated in baths containing cyanide must not be below ground level on all sides. They must have a minimum floor area of 20 m2 and a minimum ceiling height of at least 3 m.
Exits
Workrooms must have at least two exits, if possible in opposing directions. The exits must be located such that the distance to the nearest exit measured as the crow flies does not exceed 20 m from any point in the room.
Floors, floor gratings and platforms
Floors, floor gratings and platforms must be easy to clean.
Owing to the risk of fire and formation of gases hazardous to health, floors, floor gratings and platforms in the proximity of salt baths must not be made of organic materials. Organic materials include wood, plastic and rubber.
Floors must not have any open joints, grooves or other recesses in which salt residues can accumulate. Smoothed concrete floors impermeable to water have proved effective.
Floor gratings, platforms and their coverings must be installed or erected such that their position cannot change inadvertently.
Inadvertent changes in position are prevented for example as follows:
By sufficient dead weight of the floor gratings and platforms
By recessed installation of the floor gratings and platforms
By the use of fixing elements such as pins, catches or screws
Where pins, catches, screws or similar fixing elements are used to secure floor gratings, platforms and their coverings, they must be of the captive type.
Floor drains and collecting chambers must not be connected to the public sewerage system
(requirement of the German Water resources act). The liquids collected must be disposed of properly.
Washing facilities, eye showers
Washing facilities with running water and a facility for thorough eye washing (e.g. eye showers or a sufficient number of eyewash bottles fit for use) must be available in workrooms and work areas or in the immediate vicinity.
Erection of salt baths and associated equipment
Salt baths and the associated equipment, such as quenching and cleaning equipment, must be erected such that they are easily accessible and can be operated safely.
Salt bath plants containing salt baths with mutually incompatible molten salts must be erected and operated such that unwanted contact between salts that react dangerously with each other is not to be anticipated.
Extraction equipment
Where substances hazardous to health are able to escape from the salt baths or associated equipment into the air breathed by the employees, equipment must be in place to collect the hazardous substances at their point of origin or emission, and discharge them. Suitable equipment for collecting hazardous substances at their point of origin or emission from salt baths include peripheral extraction systems (annular hoods, suction walls) and hoods in funnel form.
3.3.2
Areas in the proximity of salt baths
The area in which a plant is located often gives rise to or exacerbates a hazard. In the case of salt baths, the proximity to liquids such as water and inadequate measures to prevent persons falling into the baths are particularly relevant.
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When operating open salt baths you must take additional aspects into account over and above the provisions and requirements of the ArbStättV:
Owing to the open design of the salt baths, a danger always exists of persons or objects falling into the baths.
Due to the temperatures and properties of the molten salts, salt spatter presents a risk of thermal and chemical burns.
The temperatures of the salt baths, which may be very high, present an increased risk of fire.
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Salt baths must be safeguarded to prevent persons from falling into them; this can be achieved for example by the use of sufficiently high bath sides or railings of adequate height.
Should the rim of the bath be less than 0.9 m above workplace level, additional means of reliably preventing falls into the bath (e.g. railings with a height of 1 m) should be installed.
In derogation from this requirement, a minimum height of 0.7 m of the bath rim or railing is permissible on the charging side of manually charged baths when:
The rim of the bath (outer side of the furnace <-> inner side of the bath) is at least 0.2 m in width
or
the distance between the railing and the inner side of the bath is at least 0.2 m
or
an equivalent measure is taken.
Protection against spatter:
In order for protection to be provided against salt spatter, the crucibles must be closed with a mechanically lockable cover during the melting process.
Work or circulation areas in the immediate vicinity of salt baths must be protected against salt spatter.
Charging and ancillary equipment
The workpieces must be placed safely in the molten salt by means of suitable charging equipment.
Charging equipment must be placed safely and securely on the salt bath.
Charging and ancillary equipment such as ladles must be made of solid material and be suitable for the temperature of the salt bath.
Hydraulic fluids and foreign media
Hydraulic fluids used in conjunction with salt baths must be flame-retardant.
To prevent molten salt from being ejected from the salt bath, you must ensure that no foreign liquids, such as water condensation, quenching agent, hydraulic fluid, etc., is able to enter the salt bath. This can be achieved by the fitting of collecting channels or protective roofs to supply, control and measuring lines or parts of buildings.
3.3.3
Storage of and activities involving heat treatment salts
The use and storage of heat treatment salts places particular demands on the employees and on the intended storage facility for the salts. This section supplements the German TRGS 510 (Technical rule 510 for hazardous substances), which contains provisions for all hazardous substances.
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Heat treatment salts
Risk of poisoning if swallowed
Reaction with other substances causing the formation of acutely toxic gases
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Storage rooms and areas
Heat treatment salts must be packaged such that any undesired leakage of the contents is prevented. Achieve this for example by using packaging that meets the requirements for the transport of dangerous goods.
The storage rooms and areas used for heat treatment salts must be dry. The storage of hazardous substances within circulation or escape routes is prohibited. Observe appropriate hygiene measures for your employees; in particular, ensure that the store is cleaned regularly.
Acutely toxic heat treatment salts (including used salts) must be kept under lock and key or stored such that only skilled and reliable persons have access to them.
Heat treatment salts and other substances capable of reacting dangerously with each other must be stored separately. Examples are cyanide and nitrite/nitrate salts, which may react dangerously with acids. In this context, observe the information on the safety data sheet and the prohibition of combined storage of goods in accordance with TRGS 510.
Label the individual salts such that confusion is not possible.
Signage
Areas in which salts are used or stored must be marked with the "No unauthorized entry" sign (D-P006).
Rooms and areas in which acutely toxic or oxidizing salts are stored must be marked as follows:
With the "No unauthorized entry" sign (D-P006)
In addition with the "No open flame; fire, open ignition source and smoking prohibited" sign (P003)
Rooms and areas in which salt baths are operated must be marked as follows:
With the "No smoking" sign (P002)
With the "No eating or drinking" sign (P022)
Containers with hazardous contents must be marked clearly and permanently, permitting their unambiguous identification. Should permanent marking not be possible or the permanence be uncertain, you must post a suitable and clearly discernible notice of the hazardous content in the vicinity.
Areas in the proximity of salt baths
Ensure that salts that react dangerously with each other are not confused with each other or able to come into contact with each other during operation of the bath. Dangerous reactions may occur when nitrite/nitrate and cyanide molten salts come into contact with each other.
Workplaces must be kept clean, and soiling by salt residues must be eliminated. Larger quantities of spilled or discharged heat treatment salt must be collected and removed immediately.
Activities involving heat treatment salts
Heat treatment salts must be transported and stored in closed and undamaged packaging or containers.
Contact with heat treatment salts must be restricted to skilled persons.
Disposal
Used salts and completely emptied packaging of heat treatment salts must be handled and disposed of in accordance with the German Recycling management act (KrWG). Transport in public areas is governed by the European Agreement concerning the International Carriage of Dangerous Goods by Road (ADR).
Recovery of salts
Used heat treatment salts can to some extent be recovered for subsequent reuse. Recovery is primarily employed for nitrite/nitrate salts.
The following safety measures are recommended for operation of a salt recovery plant:
The general requirements for the handling of heat treatment salts and salt baths apply by analogy.
The hygiene requirements correspond to those for the operation of salt baths.
The work area must be kept free of any other substances which react dangerously with the heat treatment salts to be prepared. This includes for example acids used in the preparation of nitrite/nitrate heat treatment salts.
The fire extinguishing equipment must be suitable for the workplace, the workplace environment, the method of salt preparation and the types of salt to be prepared.
Waste (residues that can no longer be used and melted down) must be disposed of properly in accordance with the provisions governing solid residues of hardening salts.
3.3.4
Handling of molten salts
When handling salt baths, employees are present in the immediate vicinity of the molten salts. Severe accidents may occur should molten salt spatter. Spattering of molten salt must therefore be prevented under all circumstances.
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The greatest hazard to your employees is physical contact with the hot molten salt. This immediately causes major skin burns. Contact with the hot molten salts is often caused by spatter.
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To prevent spatter from the hot salt bath, take the following measures:
General handling of the workpieces
At salt bath temperatures of over 250 °C, the workpieces and the charging and ancillary equipment must be introduced into the bath in the dry state. This can be achieved by adequate preheating. Drilled and blind holes are particularly critical in this context.
Workpieces should be as free as possible from soiling (e.g. with grease, machining fluids) when they are introduced into the salt baths.
Workpieces with closed cavities such as hollow bodies or blocked holes must not be placed in salt baths.
Workpieces may be introduced into and removed from the salt bath only by means of the ancillary equipment intended for this purpose.
Workpieces must be fastened securely to the charging equipment.
Ancillary equipment such as ladles must be thoroughly cleaned between use on mutually incompatible molten salts.
Melting and solidification of the salts
Whenever the salt bath is shut down, it is recommended that it first be desludged. Desludging must be performed in compliance with the salt manufacturer’s instructions.
Precautions must be taken to ensure that when salts are remelted, excess pressure does not build up which could lead to the surface of the salt bath breaking up whilst still solid and thus to spatter of the salt already in the liquid phase. Before the salt solidifies, take precautions such as the following to prevent spatter of liquid salt:
Reduce the content level in the crucibles below the level up to which heat is applied.
Insert a steel wedge or cone; do not remove it again until the salt has fully melted. The cone inserted into the salt serves as a plug which is forced upwards as the salt melts and expands, and thus serves to relieve pressure and prevent spatter of salt.
Surface layers of the salt bath that have solidified or have not yet melted must not be broken up. This does not apply to surface layers of carbon foam in baths containing cyanide.
Mechanically lockable covers must be in place on the baths during melting of the salt baths.
Moving of crucibles
Crucibles may be moved only once the salt has solidified.
Nitrite/nitrate salt baths
Foreign substances containing carbon must not be allowed to enter nitrite/nitrate salt baths. Such substances particularly include solids such as wood, paper, plastics, gloves and cleaning cloths.
Compatibility of molten salts
Ensure that when different salts are combined, the manufacturer’s information on the compatibility of the salt baths is taken into account.
Compatibility is assured when for example the cyanide content of the salt carried over into a nitrite/nitrate molten salt bath does not exceed a mass fraction of 13 % KCN or 10 % NaCN.
Nitrite/nitrate salts must not be introduced into baths containing cyanide.
3.3.5
Draining and cleaning salt baths
Draining and cleaning are among the operations performed on salt baths, but are not a daily occurrence. Precisely for this reason, major accidents repeatedly occur during these activities.
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The greatest hazard to your employees is physical contact with the hot molten salt. This immediately causes major skin burns. Contact very often occurs by spatter from hot salt baths.
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To prevent egress or spatter from the hot salt bath during draining and cleaning, take the following measures:
Draining the baths
Only ancillary equipment that is dry, ideally preheated, and not contaminated by foreign salts or other substances may be used for draining of the baths.
Use and operation of salt pumps
When selecting personal protective equipment, give consideration to the possibility of salt suddenly exiting from the pump system at unanticipated points.
Aluminium-laminated heat-resistant protective clothing has proved effective for this purpose. The specified personal protective equipment must be worn during the entire process.
A procedure must be drawn up for use and operation of the salt pump. Instruction must be provided to the employees based upon this procedure.
The salt pump must be operated by a person specifically assigned to the task who performs no other tasks during operation of the pump.
The operator of the salt pump must immediately report any changes on the plant that impact upon safety and must shut the plant down until operational safety is restored. The person responsible must also be informed of any problems, damage or deviation from normal operating conditions.
Persons not employed by the company and unauthorized persons must be barred from accessing the work area intended for the pumping process. The work area must be kept clear as far as possible of stored goods and in particular of flammable materials. The work area should provide sufficient freedom of movement for the operating process. Escape routes must be created and kept clear.
Whenever the salt pump is put into operation, it must first be inspected visually together with the delivery line and electrical system.
The salt pump may be introduced into the salt bath only when dry, preheated and cleaned.
As the pump is introduced into the salt bath, its outlet must not point in the direction of persons, as steam generated by residual moisture in the pump or the pipe connected to it may otherwise present a hazard.
The salt pump must be fixed such that it remains in position during start-up and cannot be thrown about.
The delivery line should preferably be at a gradient so that the lines drain automatically. The line must be fixed, protected against contact and thermally insulated to prevent cooling. Should the delivery line need to be flexible to permit movement of the outlet, metal hose may be used for parts of the line.
Whenever the delivery lines are put into use, their unions must first be checked for leaks and tightened if necessary. In order for undesired backflow of liquid salt to be prevented, the level of the salt outlet should always lie above that of the molten salt in the salt bath. Should this not be possible, other suitable measures such as pipe ventilation must be taken to prevent the salt from flowing back by itself, and the resulting risk of overfilling.
The containers into which the molten salt is pumped must be leakproof, dry, temperature-resistant, uncoated and free of contaminants.
Where the molten salt is filled into drums for disposal, they must comply with the transport regulations.
During filling, the containers must not stand on combustible surfaces such as wooden pallets or a concrete floor sealed with plastic.
When the salt pump has been switched off, the outlet of the delivery line must be raised above the salt bath level of the full container.
Following use, salt pumps must be cleaned with suitable agents in accordance with the manufacturer’s instructions, and stored clean.
Repair and maintenance work may be carried out only on cold, clean salt pumps.
Cleaning the baths
Salt baths must be desludged regularly according to the workpiece throughput. Where the sludge is heavy, desludging may have to be performed daily. The temperatures of the molten salts during desludging as specified by the manufacturer must be observed.
3.3.6
Maintenance work and tests on salt baths
In the interests of safe working, it is crucial that machinery and plant be intact. Important specific requirements and measures must be observed for the maintenance and testing of salt baths, over and above the general requirements and measures.
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For the testing of power supply, regulation and control equipment, refer to:
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Specific hazards arising during setup and maintenance work and testing on salt baths:
Skin contact with the molten salts and hot parts of the plant
Falling into the salt baths
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Setup and maintenance work
In order for the hazard arising during maintenance work on and above salt baths or containers to be reduced, the risk assessment for these activities is of particular importance. You must set out the necessary safety measures in writing and instruct your employees accordingly.
In addition to the general hazards, take account of the particular characteristics of salt baths and also ensure that the health of your employees is not endangered for example by thermal radiation or spatter.
Pay particular attention to the possibility of employees falling into the baths. Irrespective of the height of the fall, this must be prevented by technical measures such as covering of the baths or the use of work cages. In addition, organize maintenance work such that no foreign objects such as tools, workpieces or work materials are able to fall into the salt baths.
Inspections
Inspections performed before initial commissioning and following major modifications and repairs
As the employer, you are obliged to ensure that salt baths are checked by a competent person with respect to their safe condition and reliable operation, and at least for externally visible damage and defects, before they are placed in service for the first time and following any significant modification or repair. This requirement also extends to their components such as temperature control and limiting devices, safeguards, equipment for media and energy supply, charging, extraction and ventilation, and other ancillary equipment.
Examples of significant modifications in this context are modifications to heating and control systems, and structural changes resulting from a change in the composition of the salt bath.
Periodic inspections on salt baths
You must ensure that temperature limiting devices, unless they are self-monitoring or intrinsically safe, are inspected regularly in order for their serviceability to be ascertained. Inspection of these devices at quarterly intervals is recommended.
You must instigate inspection of the following by a competent person as required and at least once a year:
- 1.
Proper condition and serviceability of safeguards and charging, ancillary and ventilation equipment
- 2.
Serviceable condition and leak-tightness of gas pipes and their fittings
Documentation of results of inspection
Salt baths are not subject to documentation requirements beyond those of other work equipment; as with such equipment, you must ensure that the results of the inspections are recorded. The records must be retained at least until the next inspection.
3.3.7
Process gas furnaces employing salt quenching baths
Operation of process gas furnaces employing salt quenching baths is subject to the general safety rules for process gas furnaces and the general safety rules for salt baths. In combination, the two parts of the plant give rise to new hazards, which are described below.
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The salts used in martempering and austempering are usually a mixture of nitrite and nitrate salts. These salts have a strong oxidizing effect and can therefore react violently with hydrocarbon compounds (e.g. soot deposits). Should such substances accumulate in salt baths or a larger quantity be introduced suddenly, a considerable risk exists of fire or explosive reactions in which the formation of nitrous gases cannot be ruled out.
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The following measures are recommended in order to prevent soot deposits from giving rise to hazards presented by fire or explosive reactions:
The general requirements for salt quenching baths apply by analogy.
The salt quenching bath must feature good circulation. This prevents deposits from forming on the bottom of the bath or crucible, and overheating effects in the heating system. Effective circulation also minimizes local overheating or heat pockets caused by the hardening material (e.g. on small parts with a large surface area). On systems with a drop shaft, an effective barrier curtain of liquid saltis required in the shaft; this supports thermal decoupling of the furnace and quenching bath.
The salt bath should be covered as completely as possible.
A cover reduces both the ingress of foreign substances and the release of harmful vapours.
The process gas furnace must be operated such that the formation of soot deposits in the furnace or on the components is kept as low as possible.
The following measures may be taken to reduce soot formation:
Control of the atmosphere to keep the carbon level below the soot limit
Regularly allowing the furnace to burn out
The furnace chamber, specifically the transition to the quenching bath, must be inspected regularly for soot deposits and cleaned if necessary.
The components should be free of greasy or oily residues.
3.3.8
Particular requirements associated with heat treatment of aluminium or wrought aluminium
alloys in nitrite/nitrate salt baths
A particular application of salt baths is the heat treatment of aluminium or wrought aluminium alloys in nitrate/nitrate salt baths. Exceeding of the permissible salt bath temperature is particularly critical in such salt baths.
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Nitrite/nitrate salt baths have a high oxygen content. The higher the bath temperatures, the sooner oxygen is released; nitrate salts (nitrate = NO3) become increasingly unstable at temperatures above around 600 °C. Should the workpieces to be treated contain magnesium, critical reactions between the molten salt and the workpiece can occur at temperatures even lower than this.
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The following aspects are particularly critical when aluminium is treated in nitrate salt baths:
Magnesium content and temperatures of the molten salt
With consideration for the magnesium content of the wrought aluminium alloy, the following temperatures of the molten salt must not be exceeded:
Table 6:
Maximum temperatures of molten salt
| Magnesium content | Temperature of the molten salt |
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| Up to 0.5 % | 550 °C |
| Up to 2 % | 540 °C |
| Up to 4 % | 490 °C |
| Up to 5.5 % | 435 °C |
| Up to 10 % | 380 °C |
The determining of intermediate values for temperatures of the molten salt at other magnesium contents is permissible only between the stages indicated in Table 6.
Exclusion of certain materials
Ensure that the nitrate salt baths used for the treatment of aluminium or wrought aluminium alloys are not used for workpieces made of alloys of unknown composition.
Testing of the temperature monitoring devices
Proven good practice is for proper functioning of the temperature monitoring devices to be checked by a competent person at regular intervals and at least quarterly.