Salt Contamination with Garnet for Blasting

I knew that salt too much salt was bad for your health, I never knew it was also bad for blasting.

http://en.wikipedia.org/wiki/Garnet

Garnet sand is a good abrasive, and a common replacement for silica sand in sand blasting. Mixed with very high pressure water, garnet is used to cut steel and other materials in water jets. Garnet sand is also used for water filtration media.

If you can't get rid of the salt, can't you sell it then?

"I knew that salt too much salt was bad for your health, I never knew it was also bad for blasting.

But you never knew that

" Not reading the original post carefully before commenting is bad for your reputation"

He can sell salt but,buyers will have to lick the steel surface after blasting!

Sisira ....ha ha ha ha ...you are so funny...i have pissed my pants......

Just joking my friend... wanted to add some humour.. no offense meant to the person

Blasting with sand will do the job but you have to be careful of clay inclusion.

I would suggest changing from garnet to copper slag (we call it black beauty), it is more agressive and much cleaner. You may want to take a look at the SSPC or NACE web-sites and download their preparation standards.

Your other option is to find another source of garnet, because your source is too contaminated with salts. The presence of salt at the level you are getting will cause many problems with adhesion and corrosion after your coating operations.

Cheers,

Bloefeld

This salt can only come from the garnet or from the prewash, so if it is coming off the garnet by breaking off and it remaining on the panel because the tough garnet does not break very much, but the softer salt does break and remains as a fine dust in excess of spec. Is that correct?

If it is from the degrease and wash cycle, look there

Once it is there he only way to remove it is with a grit blast with another grit that is 100% salt free, or wash it in some way? If these are sheets, and oil free air blast might also work

Where does this garnet come from?

Is what you are blasting mild steel which has been exposed to sea water? If that is the case very likely salt you get later is what has been trapped under the layer of corrosion. Initial washing and/or degreasing does not go deep as expected.

One way to this is to do blasting in 2 steps. First you may blast to get the loose rust and then do the final step of blasting.

You also can follow the current procedure followed by water washing. Then you can let the surface dry and do a final flash blast.

If steel you blast has not been exposed to salt, then you will have to find the source of salt and try to eliminate it!

Sisira, that is a very good point. In some steel mills, sea water is used to cool the material while it is being processed. It is normal for some of the mineral content of the cooling water to become embedded into the surface of the steel. This happens as the steel is rolled into shape. You may have to change to a different steel manufacturer if this is the problem.

TT3

Perhaps you have some ionic attraction or static electricity causing it to stick. There is a lot of static involved with this process (I've watched lots of arcing thru the observation window!)(especially on low humidity days).

I forget what voltage and polarity we used to neutralize it, but could find out.(Perhaps not before your experimentation led you to an answer.)

I don't get to check this every day, but will watch the thread.

Good Luck!

Steel is degreased & washed before blasting, this gives us low levels prior to blasting.

What is the value of Micro Siemen/cm in your final rinse water? You may need to add a final rinse with DI water like some car washes and then blow dry to lower the residue on the steel before blasting. Add the contamination level on the current washed steel to the contamination level in the garnet and see if that may be the problem, even though each is within spec individually. The other option is leaching the salt from the garnet, leaving a disposal opportunity. Or change vendors.

The level of salt in your garnet points to an easy solution.

Go to this site http://web.nace.org/Departments/store/department.aspx?ID=43 or here http://shopping.netsuite.com/s.nl/c.579824/sc.17/category.1784/.f to get the standard for the cleaning level you are trying to achieve.

Using the standards from either source will make you bullet-proof if subsequent adhesion issues occur.

If you could respond with some further information about your process that would be helpful. Where are you blasting? What are the ambient conditions while you blast? Are you within a mile of the sea coast? What type of containment are you in? How are you conditioning your blast air? What size nozzles are you using? What is the volume of air? What is the ratio of air to media? What is the velocity of the media? How many lbs or kg of media per unit of area are you using?

It may be that you are over-blasting and that is the cause of the increased retained salts. Lower velocity and lower volume of media will clean your metal and reduce the amount of salt that remains imbedded in the steel.

I have had situations where we had too much clay left in our blast because the operator had not used as much compressor before. He had a No 6 nozzle instead of a No. 8 and the increased velocity and increased media load over-cleaned the pipe and doubled the amount of clay. When we switched him to a number 8 nozzle and trained him on the level of cleaning that was required, the problem disappeared.

Cheers,

Bloefeld

I will try to capture all the above observations. We cannot use Copper slag / steel slag - client specs. The garnet is sourced from India, Australia is a preferred source, but to get a coarse grade such as 16/20 there are only two sources that meet the budget - India and China. We suspect the salt is coming from the garnet, and accumulating. this is probably due to the need to "over blast" as to achieve a blasted surface profile of 80~120 microns, is not easy with garnet. As the next application post blasting is TSA (Thermal Spray Aluminum), a water wash (or other wash after blasting is not an option. We have checked the salt level on the washed steel - they are low / acceptable so it is not the wash water. The end job is 1100m2 of coating so we need an "industrial option" to solve this. Bloefeld - some good point here - but the client specs are tighter than the NACE / SSPC etc specs. (I have all the standards already) Customer (SHELL) level is 20mg/m2, SA2.5~3, with 75 to 120 micron angular profile. We are blasting in Singapore, within a mile of the coast. Ambient condition are hot / humid - about 30 Celsius, 80% RH. Containment is a blasting chamber. Blast air is cooled / dried and filtered. we are running a lean media / air mix, about 1/3 as much media as we would for steel grit. We are using a "new" no 8 nozzle, and about 35kg garnet to 1m2 of steel (as above we need to build a profile). We are looking at Al Oxide as an alternative, but that will double the blasting media price.... (not good)

I wonder if you can run the garnet through a false blast phase onto a strong surface whose blast byproducts are harmless, like a garnet or steel block, to shake the salt loose as a dust at which point a centrifuge and a magnet can be used to remove the dust and blasted off steel. This would add some cost, but it might not be as high as you think, as you can use the existing setup, precleaned, with a small box cabinet which you do the false blasting(not need for blast head to move) and you vacuum extract the used media through the magnetic/centrifugal cleaner into a box.

I have to think the garnet picks up the salt from some stage in manufacturiong, a wash with salty water etc? (ever analyze the salt profile to see if it is sea salt or geo salt?). It is possble that the garnet source has salt.

Will take a look at it, but will run the risk of reducing the size of the garnet and compromise the required profile. you can only generally recycle garnet about 5 times before it turns to "dust", in our use we are not planing any recycle due to cleaning issues, removing dust, contaminates etc...

It may be that you can pre-blast the dust at lower pressure to remove the salt. Salt is a lot weaker than garnet