Acid Cleaning

Acid Cleaning

Filter Press Maintenance and Repair

ACID CLEANING

Acid cleaning of a filter press can be a highly effective means for restoring full flow capabilities to that unit providing that the material fouling the media and under drains is soluble in the acid used or is cemented or bound in place by such an acid soluble material and providing that this procedure is carefully and properly undertaken.Acid Cleaning - Filter Press Maintenance and Repair

It must be understood that the acid used for such a cleaning operation, even though inhibited to retard attack on metals, is a highly aggressive chemical and that its use presents a potential hazard to personnel in the area.

It must also be recognized that no inhibitor is totally effective in preventing reaction of the acid with iron and steel and that a product of such a reaction is hydrogen gas.  Some of this hydrogen is absorbed on the surface of the metal and does not present a problem as long as the filter is thoroughly flushed and a high pH fluid is introduced immediately after the cleaning operation.

The balance of the hydrogen, however, can collect as gas pockets in the unit creating a potentially explosive atmosphere in the unit.  There have been a number of recorded hydrogen explosions in boilers immediately following acid cleaning using the same techniques required to clean a filter and several of these explosions have resulted in fatalities.

Personnel performing an acid cleaning operation should be provided with and required to wear suitable protective clothing such as rubber gloves and aprons and should also be required to wear goggles.  Water hoses should be available with water at the nozzle to instantly shower anyone coming in substantial contact with the acid and to flush surfaces such as concrete, tile, etc. where spillage may occur.

Because a substantial generation of heat occurs during the dilution of an acid of the type required and because this heat can cause spattering back in the face of anyone in the immediate area, acid should always be added slowly to the dilution water.  Water should never be added to the acid.

Filter Press Maintenance

Acid strength used for cleaning operation should be the minimum required to affect the required end result.  This should be determined by swabbing acid onto a piece of the media or under drain, as appropriate, and simply observing the result.  Normally a strength of 2-5% will be effective in removing any deposits that are capable of being removed by the procedure.  It should be noted, at the point, that the strength of the acid required will also be a function to some degree of the method of cleaning used.  The recirculation method will require a somewhat lower strength than will the “fill and soak” method.  More will be said on this point in later paragraphs.

Because deposits in a filter will generally include calcareous material, sulfuric acid should never be used for cleaning the filter.  This would produce calcium sulfate or gypsum as a reaction product and this material could have a blinding effect as bad or worse than that which existed prior to the cleaning.

The acid most normally used for this purpose is hydrochloric acid.  The reasons for this are that chloride reaction products are soluble and will be removed as the acid is drained from the filter and there are a number of inhibitors available which are reasonably effective in retarding the attack of this acid on metal surfaces.

Inhibitors for use with hydrochloric acid are generally arsenic compounds, barium salts, quinolin, pyridine or other similar materials.  A number of commercial inhibitors are available under various trade names and still others are made and provided by companies offering complete acid cleaning service including prepared solvents.  If cleaning is to be performed by plant personnel as opposed to a commercial organization specializing in this type work, it is strongly recommended that commercial inhibitors be used as supplied by reputable chemical suppliers.

Regardless, keep in mind that all inhibitors are not effective with all acids.  It is only common sense to observe the reaction of the inhibited acid on a sample of steel before introducing this into the filter.  Obviously if attack on the steel occurs, this material should be used.

Upon completion of a cleaning operation, a volume of partially spent acid will exist at strength somewhere between 1 and 5%.  This acid cleaning will generally also contain a significant amount of suspended amount of material which is not particularly soluble in the acid but which has been freed from the filter surfaces by the removal of soluble material acting as a binder.

In some plants this spent acid, having a volume somewhat greater than that of the filter, is stored and refortified, as required for future washing operations.  In others, it must be disposed of immediately.  Unless acid washing is intended to be accomplished on a regular and frequent basis, storing and refortification is not generally recommended because of questions regarding the status of the inhibitors.  In both cases, this acid must be ultimately disposed of and consideration must be given as to how this will be accomplished.

If it can be stored and if lime is used for conditioning, it should normally be possible to bleed it into the flow of high pH filtrate from the plant at a rate that will not cause a problem.  If it cannot be stored, this becomes more of a problem as the rate to get it out of the plant will be higher and even with multi-filter plants with the remaining units in service, the filtrate flow/rate will be variable.  In some cases, neutralization of the spent acid will be required prior to its disposal.  Lime can be used for this purpose.

Since the equivalent weights of calcium hydroxide and hydrochloric acid are approximately the same (37.1 vs 36.5), it will take approximately 1 lb. of 100% Ca (OH) ² to neutralize each pound of 100% HCL present.  This neutralization reaction will produce a great deal of heat and lime should, therefore, be fed slowly and carefully to the spent acid solution.

Commercial hydrochloric acid is normally provided in 13 or 12.7 gallons carboys in either of two grades: 18 degrees Baume or 20 degrees Baume.  Principal characteristics for each grade and the dilution required to achieve a 3% and a 5% solution are summarized below:

  • Degrees Baume                       18                    20
  • Specific Gravity                      1.142               1.160
  • Acid Strength, %                    28.0                 31.5
  • Carboy Volume, gal.               13                    13
  • 3%-gal. H20/gal. acid             9.5                   11.0
  • 5%-gal. H20/gal. acid             5.3                   6.2

 

If the filter to be cleaned contains 100, 64” plates having a chamber thickness of 30mm or 1.18”, the total chamber volume would be 1.88 cubic feet per chamber x 100=188 cubic feet or 1406 gallons.  Adding 25% to this to cover the volume of piping etc., the total volume of acid required would be approximately 1750 gallons.

Assuming that a 3% acid strength has been determined to be approximate, 1 gallon of 18 degree Baume acid would produce 10.5 gallons of 3% acid and a 13 gallon carboy would, therefore, be capable of producing 136.5 gallons of acid at this strength.  A total of approximately 13 carboys of acid would, therefore, be required.

It should be noted that nylon is subject to attack by any of the mineral acids such as hydrochloric acid and that the life of filter media made of nylon will be perceptibly shortened if such material is subjected to repeated acid washing.

There are two approaches which can be taken to acid washing a filter: the recirculation method and the “fill-and-soak” method.  As the name applies, the recirculation method involves circulating acid through the filter on a continuous basis.  The fill and soak method simply involves pumping the acid into the filter until it is full and letting it stand in this condition for a period of time.

The recirculation method offers a number of advantages over fill and soak.  These are:

  1. By continuously circulating acid over the deposits to be removed, stratification is minimized and the deposits are constantly being exposed to fresh acid. The consequence of this is the ability to use weaker acid (generally 1-2% lower concentration) and required exposure is generally less.
  2. Reaction of acid with carbonates which are normally present in deposits in a filter produces a great deal of carbon dioxide gas and foam. By circulating flow through the unit this foam can be broken down as it forms and there is less tendency for this to impede reactions in some areas of the filter.
  3. With continuous flow through the filter, there is more of a tendency for insoluble materials broken loose from the filter surfaces to be flushed out of the filter and consequently there is less tendency for the small passages from the under drain areas of the plates into the filtrate eyes to become plugged.
  4. Through sampling of the circulating stream from the filter, positive control of the washing operation can be established. As acid cleaning is initially introduced into the filter, its reaction with deposits in the unit will cause a measurable reduction in strength.  As the washing operation proceeds, less and less acid will be consumed and when the acid has accomplished all that it is capable of accomplishing, no further reaction will occur and the strength of the acid will drop no further.  By periodically running a simple titration on samples of acid from the filter, this equilibrium point can be established and the acid washing operation can be terminated on a timely basis.
  5. By continually removing liquid from the top of the filter there is less opportunity for hydrogen gas to build up in this unit, this gas being removed a little at a time as it is being formed with the following stream.

Inasmuch as deposits in a filter may be contributing towards the sealing of plates against leakage into the operating area, removal of those deposits may lead to some such leakage.  Where feasible, it is desirable to drape the filter with plastic during the period that acid is being circulated through the unit.  It is also desirable to have water hoses available to flush down any concrete or tile surfaces on which acid might leak or spray.

If it is not feasible to circulate acid through a filter requiring cleaning, it will be necessary to use the fill and soak method.  Disadvantages of this approach are, obviously, the converse of the advantages previously for the recirculation method:  the requirement for higher acid strength, requirement for longer washing time, potential interference with cleaning by foam, increased potential for filter port pluggage, increased potential for gas pocketing and lack of any means for effectively monitoring the cleaning operation.

With the fill and soak method, the acid is simply pumped into the filter unit it is full and this is left in the unit, normally for periods of 4-8 hours.  It is then drained out and the filter inspected.  If the desired cleaning effect has not been achieved the filter will have to be refilled and the above repeated.

Regardless of whether the recirculation method or the fill and soak method has been used, it is necessary that the filter be totally filled with acid during the cleaning period.  With filters set up for pre-coating or pre filling, a “horse collar” of piping is normally installed at the head of the filter which in conjunction with a valve located in the bottom filtrate piping is used in normal operation to cause filling of the filter with water (filtrate) at the beginning of each operating cycle.

On filters so equipped, this requirement for filling with acid can be accomplished by simply closing off the valve.  On filters not so equipped, it will be necessary to take whatever action is required to accomplish this end including blanking off of piping.

Because metal surfaces in a filter will be impregnated with hydrogen and therefore subject to corrosion after acid has been drained from the unit, this filter should immediately be flushed out with clean water.  This flushing should continue until the flushing water emerging from the filter is free from both acidity and iron.  The reason for concern over soluble iron, at this point, is that iron salts in the under drain area will react with high pH filtrate causing gelatinous deposits of ferric hydroxide in the under drain.

The simplest test for acidity is addition of a few drops of methyl red indicator to a sample of the effluent flushing waster as long. As acid is present, the sample so treated will turn red.

The simplest test for residual soluble iron is addition of a pinch of soda ash to a sample.  If such iron is present, a greenish precipitate will form.

Immediately after a filter has been flushed it should be opened and inspected.  At this time, there will generally be a layer of deposit on the face of the filter cloth and this should be flushed off with a water hose.  Filter media should then be pulled back from the bottom area of the filter plates and any significant deposits between the media and the web of the plate which could cause subsequent plugging of the filtrate ports of the plate should be removed.  The ports should be inspected for deposits and any such deposits should be poked out with a piece of wire, welding rod, etc.

Assuming that sludge supplied to the filter is conditioned with lime, or is other at a high pH, the filter can now be re-closed and returned to service.  If it is not to be returned to service immediately, or if conditioned sludge supplied to the unit is not at a high pH, the unit should be filled at the point with high pH water to insure that any residual surface acidity has been neutralized.

It must be recognized that acid washing of a filer even using inhibited acid, will removed some metal from the unit each time that such washing occurs.  For this reason, and because acid washing does present a certain amount of hazard to personnel, it is recommended that this not be performed as a regular procedure.  High pressure water washing is a preferred procedure for routine washing with acid washing reserved for such special requirements as cleaning of sever fouling of under drain surfaces, etc.