Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
11-27-2006, 05:42 AM,
#1
Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Gordon,

I have a question pertianing the matter above, as you know the electrode that we use in for the air plasma e.g: for 9MB contains Thrioated Tungsten.

I have got a few paper produced by The National Radiological Protection Board in England that talk in detail about the material. But I do not know how to attach them to you. (Let me know if you are interested).

This paper appears to be addressed primarily to the welding industry, which I know little about but it suggests that grinding of thoriated tungsten is practiced. The paper makes clear that thoria is radioactive and states that internal exposure is likely to exist during grinding. In addition to the risk of inhalation, it further states that dust arising from grinding can settle on surfaces and contaminate body and clothing. As you know, the electrodes that we use contain thoriated tungsten and this can be hazardous to the health of workers (You may also note that, without any grinding, there exists a very low exposure to external radiation from our electrodes but this amount is considered negligible).

The reason for the regrinding which I have tested is an initiative of lean event to reduce hardware cost. We manage to salvage atleast >50% consumption of electrodes in an annual basis without affecting any quality of the spraying and coating integrity which I have verified. An electrode can be salvaged max 2 times before it is scrapped.

This is more on sharing and to seek your advice on this practice if this is advisable to continue by putting all the safety measurement in place to eliminate the risk? and if the safty measure is verified by a professional team of EH&S personnel after their have reviewed them.

What 's your opinion on this?

Regards,
Alex
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11-28-2006, 01:38 AM,
#2
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Hi Alex

I read through quite a few documents related to thoriated tungsten electrodes, most seem to get their information from the document I think you refer to in your post.

Normal use of plasma spray torch electrodes should not be cause for concern, particularly when using health and safety measures normally associated with the thermal spray process.

The regrinding of plasma spray gun electrodes is not a general or recommended practice. But like welding electrodes, the grinding operation will produce fine particles that can potentially be inhaled, ingested and contaminate skin and clothing. This obviously increases the risk to health over that of normal use (no grinding). The risk is still claimed to be fairly low, but the underlying statement "Nevertheless it is considered prudent to observe certain precautions in the grinding of thoriated tungsten electrodes so that exposure to ionising radiation is kept as low as reasonably practicable" should always be considered.

I've never considered regrinding worn electrodes. This is mainly because you are not able to adjust the position of the electrode tip relative the anode nozzle. The electrode/anode spacing is an important factor in the plasma characteristics. Thinking about it, my first thought would be shortening the potential overall life by removing extra material.

A few questions:

Have you reground a worn electrode that you know for sure gave poor results and then found it to give good results?

Do you notice any voltage change using standard plasma gas flows between the various electrode states (new, worn, reground etc.)?

What are your criteria for deciding when to scrap an electrode?
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11-28-2006, 09:14 AM,
#3
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Gordon,

Yes, I believe this practice is not very encouraged, but the initiative has yielded a significant saving. However, I also would not want to post any health risk or danger to our workers if the practice has severe EH&S concern. To balance up the cost saving and at the same time guarantee all workers well being, we have involved our EH&S team to review the project.

The followings are the facts and actionable items that are to be uphold to minimize dust exposure, if we were to perform the regrinding of electrode:

1.????Thoriated tungsten electrodes used for TIG, SWEPT and Plasma Spray contained Thorium oxide, which is radioactive and carcinogenic. However, based on study conducted by National Radiological Protection Board, Southern centre, Didcot. It was estimated that a welder holding an electrode for the whole of a working year (2000hrs) may receive a radiation dose to the skin of 4mSv. This is a small fraction of the maximum permissible dose recommended for skin of 500 mSv per year. The exposure of the whole body due to the continuous use of a single electrode has been assessed to be less than 100 uSv per year. Again, this is a small fraction of the maximum permissible dose. In conclusion, radiation hazard is not a health issue when exposed to the thoriated tungsten electrodes.

Nevertheless, EHS has consulted Health Science Authority for input and any licensing requirement. Will keep the team posted when we get a reply. In the meantime, the team has agreed that welders should not keep in possession with them more than 5x thoriated tungsten electrodes at any one time.

2. As Thorium oxide is carcinogenic, whihc is similar to nickle and chromium, we need to minimize dust exposure to the welders and others working in the vincinity. The following measures are to be adhered to by the welders at all times:-

????a. SWEPT/TIG tungsten electrodes should not be grinded using floor bader machines. Electrode grinders that were brought in for this purpose shoud be used.

????b. The electrode grinders' dust discharge location should be wrapped with plastic bag to collect the dust and minimise dust exposure into the work place. Each month, designated welder with mask shall sealed the bag and disposed off as general waste as per National Radiological Protection Board recommendation. Attached photo for compliance. (Note: disposal requirement might change if HSA has different view)

????c. Plasma's electrode should not be manually grind using sand paper due to potential direct skin exposure. The electrode grinder for the plasma electrode should be designed with enclosure to minimize dust exposure or LEV. As the dust from the grinding is highly flammable, limit the number of electrodes to be grinded continuously per day.

????d. Welders should be reminded to maintain good personal hygiene at all times.

Another info we need to have is the activity concentration (Bq/g) of Thorium in the electrode which I am aquiring. If it is more than 1Bq/g, licensing is required.

To answer your question:
Quote:1. Have you reground a worn electrode that you know for sure gave poor results and then found it to give good results?
As I did not perform manual regrinding, we have designed a fixture that hold the electrode at a fix angle(same with the original electrode)to facilitate the grinding process. And the re-grinding is done at a minimal parent material removal. It's more to polishing infact. So the quality of re-groundd electrodes are performing ok for the first re-use. And most of the time, I only re-grind once, or max twice. As we find that 2nd time re-grind will affect the spraying quality. See answer # 2.

Quote:2. Do you notice any voltage change using standard plasma gas flows between the various electrode states (new, worn, reground etc.)?
Yes. the new one ofcourse has no issue. The lifespan is very much depend on the usage, the number of ignition, duration of spraying and etc. Continuos spraying and ignition causing electrode worn and result in voltage drop. That is wehn we change a new one. As for reground ones, the 1st time regrround electrodes will give a minimum voltage drop of ~5%, but this does not affect my process, I still can get the flame outcome, and the coating quality is verified by sprayed specimens passing all the stringent metallurgraphy evluations and tests criterias of aviation standards.

But ard ~70% of 2nd re-ground electrodes will give a voltage drop of more than 10%. I suppose this is due to the shortened length and angle of the electrodes which impact on the electrode/anode spacing as you mentioned.

Quote:3. What are your criteria for deciding when to scrap an electrode?
See answer # 2

Thanks for the patience to finish my reply.
Pls comment further on the unclear portions.

Thanks

Regards,
Alex
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11-28-2006, 09:23 AM,
#4
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Gordon,

To add on to my last reply, the fixture that we design not only facilitate the grinding process, but we also modify and add on a plastic sheet to capture the dust generated during the process. and the process is carried in a small booth with suction to prevent dust expsoing to the ambient. Hope this has clarify the method of regrinding shd you have query.

Anyway, I hope I am not sharing some bad practise here to the other forum members. Pls do not follow if you do not feel comfortable and have not done thorough risk review on this. I apology if this would cause some controversy to any hardware supplierSmile Txs.

Regards,
Alex
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11-28-2006, 08:20 PM,
#5
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Hi Alex

Quote:I apology if this would cause some controversy to any hardware supplierSmile
Happy0193

I would not recommend regrinding electrodes as general practice, unless full evaluation and monitoring are done carefully. Health and safety aspects will only be a problem if people don't do their risk assessments and make no efforts to minimise exposure.

Increasing the arc gap by grinding back electrodes would by itself increase voltage, but the extra space around the electrode increases and the velocity of plasma gas decreases and I think this is the reason for the reduced voltage. So we need to be careful that these changes to plasma characteristics do not adversely effect the coatings. Plasma spray is complex enough without throwing in even more variables Rolleyes

Alex, one more question: When taking a worn electrode and regrinding it, do you actually see any improvement in performance? Say voltage drop on worn electrode is 10%, what voltage do you get when it is reground? Or put another way: a worn electrode starts producing poor coatings, does regrinding make it good again? Really, what I trying to tell here is whether the effect may be an aesthetic feel good factor of a newly ground component compared to worn out one, which may in fact perform equally well.
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11-29-2006, 01:56 AM,
#6
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Gordon,

Of course, as what have been said, this is not a recommended practice without having the detailed evaluation and risk assessment on the area. Yet, I have been working on this for sometime. We can review this from 3 main areas, the 1. productivity improvement(cost saving), 2. coating/process quality, and 3. EH&S aspect.

1. For the fisrt part is a definite " shd go ahead". (when the initiative yield > 50% in consumption)

2. Second area, I have been recording every single re-ground electrodes performance, and the data does not show performance nor quality defect on all the 1st time reground electrodes which I am pretty convinced on data analysis. Before we implemented the regrinding fixture, defect was at ~25% as we were using manual hand grinding which result in alot of variation on the outcome. So, I only stick to one time regrind practice using the fixture at the moment. In terms of process performance, voltage fluctuates during the process and thus we allow tolerance limit in the operation. e.g: spraying of Metco 204C-NS (YZ powder), the voltage established at 80V +/- 5. So the range fall between 75 ~85V. this is consider ok. The new electrode could easily give a voltage reading of 80~85V, whereas the 1st time reground electrodes are performing at 78~80. Taking the advantange of this tolerance limit and by getting the sprayed coating outcome examnied, evaluated in the lab and having result that passed the criterias. Thus I do not see any reason why I shd not do this.

3. EHS and risk assessment which is the most important area, as we evaluated in my previous post, we concluded that the risk is at very minimum, if the regrinding is performed under the recommended action: limiting the re-grinding quantity/day, grind in a vacumm booth and collect the dust for proper disposal which we have the facility, and observe all the safety measure and personal hygiene. I'm waiting for some doc/proof on the activity concentration of Thrioum Oxide in the electrode before finalizing the project.

Having these areas assessed and evaluated, the team would be convinced that this project shd be a "Go-Ahead". However, we are finalizing by searching for more info and proof.

But the best way towards this problem which personally in my opinion is to source for alternative electrodes that has no thrioum oxide/ Thrioated tungsten content electrodes if we were to perform regrinding. . As there are already replacement material in the industries for substitution due to its hazardous nature.

Do you have any idea why this material is used?

Regards,
Alex
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11-29-2006, 04:38 AM,
#7
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Hi Alex

More reading on the subject, if you have not already read them.

(link dead)

(link dead)

Pro-Fusion data sheet

(link dead)

(link dead)

Substituation of thoriated tungsten in Switzerland

Quote:But the best way towards this problem which personally in my opinion is to source for alternative electrodes that has no thrioum oxide/ Thrioated tungsten content electrodes if we were to perform regrinding. . As there are already replacement material in the industries for substitution due to its hazardous nature.

Do you have any idea why this material is used?
Thoriated tungsten has significant advantages over pure tungsten (see links for further info). I'm not so sure on the pros and cons with the alternative doped tungsten products (other than lower ionising radiation).
May be a good question for companies like Sulzer Metco and the like. On second thoughts, they will probably tell you not to grind themHappy0193
Although Sulzer Metco are Swiss (see link above Switzerland).

I'm pretty much in agreement with all of your above posts. I just have this one niggling doubt. If you have an electrode that falls out of your voltage range or produces unacceptable coatings, can you correct it by grinding (assuming this is first regrind and we don't change the nozzle)? If you can then I would certainly go for it. Probably a shame that these electrodes are not adjustable, then regrinding would not be a problem (technically).

Wish you all the best.
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11-29-2006, 07:15 AM,
#8
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Gordan,

Thanks for your advice and the information. Have read some of the material or similar ones before during analysis and assessment phase. Thanks for enhancing the subject.

Yes. You are right!. They are trying their best to convince us not to do that. Big Grin

Will update you more if there shd be any further progress or show stopper that is of major concern, else we implement it at the same time observe and monitor the process under the control phase of the project. The 6 sigma way.

Thanks.

Regards,
Alex
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12-12-2006, 04:01 PM,
#9
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
My husband is a welder. He primarially uses tungsten. Last year he had a lung infection that would not go away in his right lung. It would infect, scar over, infect etc. We tried for 8 months but by then it had spread upward and they took out half of his lung. He returned to work and although he feels fine, a new infection has started. The doctor said it was a new infection because they had gotten it all in surgery. Do you think this is because of grinding the Tunsten and could have been inhaled? He said he grinds them all day long. What symptoms would inhaling this stuff be? He welds alluminum such as kitchen equipment for schools and jails. The doctors cannot understand why this is happening either.
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12-13-2006, 06:59 PM,
#10
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Hi Nancy133

Welcome to the Surface Engineering Forum.

Quote:My husband is a welder. He primarially uses tungsten. Last year he had a lung infection that would not go away in his right lung. It would infect, scar over, infect etc. We tried for 8 months but by then it had spread upward and they took out half of his lung. He returned to work and although he feels fine, a new infection has started. The doctor said it was a new infection because they had gotten it all in surgery. Do you think this is because of grinding the Tunsten and could have been inhaled? He said he grinds them all day long. What symptoms would inhaling this stuff be? He welds aluminum such as kitchen equipment for schools and jails. The doctors cannot understand why this is happening either.

I'm sorry to hear of your husband's condition and the stress that it must be causing your family.

It will almost be impossible to determine whether grinding tungsten or thoriated tungsten or for that matter inhalation of welding fume is the cause or even a contributory factor to your husbands medical condition.

Below, I put together snippets of text from various articles and material safety data sheets indicating effects of overexposure related to electric arc welding and thoriated tungsten electrodes. Please be careful how you interpret this information (consider worse case scenario), for instances take a material safety date sheet for the materials in stainless steel cutlery. Effects of overexposure to nickel and chromium are quite frightening, but the risk to our health by using cutlery in the normal way would be considered extremely low.

Quote:The most significant routes of over-exposure to these products is inhalation of fumes generated during welding operations or of dusts generated by grinding operations. Other possible routes are skin contact and ingestion.

Short-term (acute) overexposure to welding fumes may result in discomfort such as metal fume fever, dizziness, nausea, or dryness or irritation of nose, throat, or eyes. May aggravate pre-existing respiratory problems (e.g. asthma, emphysema).

Long-term (chronic) overexposure to welding fumes can lead to siderosis (iron deposits in lung) and pneumoconiosis (disease of the lungs) and may affect pulmonary function. Bronchitis and some lung fibrosis have been reported.

Although the inhalation of Tungsten has the potential for causing transient or permanent lung damage, it is generally considered to exhibit a low degree of toxicity. With the exception of two Russian studies that found early signs of pulmonary fibrosis in some workers exposed to tungsten trioxide, tungsten metal and tungsten carbide, most studies have shown tungsten to be toxicologically inert.

Thorium is a naturally occurring radioactive element. Its primary hazard lies in inhalation of dust/fumes. Normal handling of these electrodes is not expected to result in any significant radiation exposure. Considerable experience in refining and use of thorium has not revealed any adverse effects from industrial exposure.

Thorium Oxide, which is a low-level radioactive material. Studies performed by the International Institute of Welding have shown that these electrodes do not present any radiation risks during normal use, storage, welding or disposal of residues of these products. However, during grinding of electrode tips there is generation of radioactive dusts, which present a hazard through inhalation.

Carcinogenic Assessment (NTP Annual Report, IARC Monographs, Other): Thorium Dioxide has been identified as a carcinogen
by the NTP and IARC. Evidence for its ability to cause cancer has come solely from its internal medical use.

It would be foolish to dismiss a possible connection between your husband?s illness and his profession as a welder, but you will need to also consider very many other possible contributory factors from life.

Hoping things work out well for you and your family.
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08-10-2010, 11:48 AM,
#11
RE: Hazard of Thoriated Tungsten Dust Arising from Re-grinding of Electrode
Tungsten electrodes are used when arc welding with the Tungsten Inert gas (TIG) process or when plasma welding. In both processes the electrode, arc and weld pool are protected from atmospheric contamination by an inert gas. A tungsten electrode is used because it can withstand very high temperatures with minimal melting or erosion. Electrodes are made by powder metallurgy and are formed to size after sintering.Thorium (Th) is slightly radioactive with a long half life and emits mainly alpha ( alpha) particles, but occasionally some beta ( beta ) and gamma ( gamma ) radiation is emitted. Alpha particles cannot penetrate skin or even paper. However, they are harmful if released inside the digestive tract, or inside the lungs, where they act as a carcinogen..Thorium oxide is, therefore, a low level radioactive material which may give rise to both a small external radiation hazard and an internal hazard from ingestion or inhalation. The external hazard estimated for a welder holding an electrode for a whole year is a very small fraction of the maximum permissible radiation dose and it is concluded that the external radiation hazard is likely to be negligible.
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