Method for regenerating scale solvent

Barium sulfate scale is dissolved by contacting the scale with an aqueous solvent having a pH of about 10 to about 14, preferably about 12, and comprising a chelating agent comprising a polyaminopolycarboxylic acid or salt of such an acid, and a synergist anion. The solvent containing the dissolved scale is regenerated by acidifying the spent solvent with an acidic chelating agent such as EDTA or DTPA, preferably DTPA, that release alkaline earth cations which react with sulfate ions present in the solvent to form a precipitate of an insoluble salt of the alkaline earth metal. The precipitate is then removed from the solvent by settling/decanting, filtering, or centrifuging. Thereafter, caustic is added to increase the pH of the solvent to a pH of about 10 to about 14, preferably about 12, to recover a regenerated solvent for reuse, in dissolving scale. The amount of chelating agent added to acidify the spent solvent balances the total amount of dilution caused by adding the caustic thereby maintaining the activity of the chelating agent in the regenerated solvent at least equivalent to or greater than the activity in the spent solvent.

CROSS REFERENCE TO RELATED APPLICATION 
This application is related to prior co-pending application Ser. No. 
07/593,136, filed Oct. 5, 1990, of J. M. Paul and R. L. Morris. 
1. Field of the Invention 
This invention relates to solvent compositions which are effective for 
solubilizing and removing scale, particularly strontium and barium sulfate 
scale, and naturally occurring radioactive material (NORM), usually 
containing mainly radium 226 and radium 228, from surfaces with scale 
deposits on them, the improvement residing in a novel treatment of the 
solvent to remove the dissolved scale so that the solvent can be 
regenerated for reuse. 
2. Background of the Invention 
Many waters contain alkaline earth metal cations, such as barium, 
strontium, calcium and magnesium, and anions, such as sulfate, 
bicarbonate, carbonate, phosphate and fluoride. When combinations of these 
anions and cations are present in concentrations which exceed the 
solubility product of the various species which may be formed, 
precipitates form until the respective solubility products are no longer 
exceeded. For example, when the concentrations of the barium and sulfate 
ions exceed the solubility product of barium sulfate, a solid phase of 
barium sulfate will form as a precipitate. Solubility products are 
exceeded for various reasons, such as evaporation of the water phase, 
change the pH, pressure or temperature and the introduction of additional 
ions which can form insoluble compounds with the ions already present in 
the solution. 
As these reaction products precipitate on the surfaces of the 
water-carrying or water-containing system, they form adherent deposits or 
scale. Scale may prevent effective heat transfer, interfere with fluid 
flow, facilitate corrosive processes, or harbor bacteria. Scale is an 
expense problem in any industrial water systems, in production systems for 
oil and gas, in pulp and paper mill systems, and in other systems, causing 
delays and shutdowns for cleaning and removal. 
In the co-pending application referenced above, there is disclosed a method 
for removing barium sulfate and other sulfate scales by a solvent 
comprising a combination of a chelating agent comprising a catalyst or 
synergist comprising polyaminopolycarboxylic acid such as EDTA or DTPA 
together with anions of (1) a monocarboxylic acid such as acetic acid, 
hydroxyacetic acid, mercaptoacetic acid or salicylic acid; (2) oxalates, 
or (3) thiosulfates. The scale is removed under alkaline conditions, 
preferably at pH values of at least 10, usually 10-14, with best results 
being achieved at about pH 12. When the solvent becomes saturated with 
scale metal cations, the spent solvent containing dissolved scale is 
regenerated by acidifying the solvent in the presence of anion which forms 
a precipitate of an insoluble salt of the alkaline earth metal of the 
dissolved scale. The precipitate is then removed from the solvent. 
Thereafter, the pH of the solvent is increased to a pH of about 10 to 
about 14, to recover a regenerated solvent for reuse, in dissolving scale. 
This invention provides an improved method to regenerate the spent solvent 
by chemically removing the dissolved scale from the spent solvent through 
precipitation of the dissolved metal ions without decreasing the activity 
of the regenerated solvent. 
SUMMARY OF THE INVENTION 
In the method for removing alkaline earth sulfate scale by contacting the 
scale with an aqueous solvent having a pH of about 10 to about 14, 
preferably about 12, and comprising a chelating agent comprising a 
polyaminopolycarboxylic acid or salt of such an acid and an anion 
synergist comprising an oxalate, thiosulfate, nitriloacetate or 
monocarboxylic acid; the improvement comprising removing alkaline earth 
sulfate scale dissolved in said solvent by acidifying the solvent with an 
acidic chelating agent that releases alkaline earth cations which react 
with sulfate ions present in the solvent to form a precipitate of an 
insoluble salt of the alkaline earth metal, separating the precipitate 
from the solvent and increasing the pH of the solvent to a pH of about 10 
to about 14, preferably about 12, by the addition of potassium hydroxide 
to recover a regenerated solvent for reuse to dissolve scale. 
The acidification is carried out using a solid acidic chelating agent such 
as diethylenetriaminepentaacetic acid (DTPA) or 
diethylenediaminetetraacetic acid (EDTA), preferably DTPA, until the pH of 
the solvent has been reduced to either pH 6 or pH 7. At a pH of 6 or 7, 
the chelating agent originally in the solvent is destabilized by reacting 
with hydrogen ions and releases alkaline earth cations which react with 
sulfate anions present in the spent solvent to form a precipitate of an 
insoluble salt of the alkaline earth metal. Removal of the residual 
sulfate ions from the solvent is beneficial because these ions are known 
to inhibit dissolution of alkaline earth sulfate scale by the solvent 
after the solvent is regenerated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Alkaline earth metal scales, especially barium sulfate scale deposits are 
removed from oil field and other tubular goods with a scale-removing 
solvent comprising a chelating agent and a catalyst or synergist to speed 
up the dissolution of the scale. The pH of the solvent is maintained at pH 
values of about 10 to about 14, preferably at about 12, with the addition 
of potassium hydroxide (caustic potash). Suitable chelating agents 
comprise polyaminopolycarboxylic acid such as EDTA or DTPA or salts of 
such acids. The catalyst or synergist comprise anions of at least one 
monocarboxylic acid such as mercaptoacetic acid, aminoacetic acid and 
hydroxyacetic acid; oxalates, and/or thiosulfates. This invention 
describes a method for regenerating the solvent containing scale through 
precipitation of the dissolved metal ions and subsequent upward adjustment 
of pH to a pH of about 10 to about 14. 
The scale solvent effectively removes all the common oil field scales, 
including sulfates of barium, calcium and strontium, calcium and iron 
carbonates and naturally occurring radioactive material (NORM), mainly 
radium 226 and radium 228. The chelating agent in the solvent is capable 
of binding alkaline earth metal ions tightly at high pH levels. If certain 
anions that form very insoluble alkaline earth metals are present in the 
solvent solution after the pH is lowered, then alkaline earth metals may 
be displaced from the chelating agent originally in the solvent and 
precipitated. Thus, if the pH of the spent solvent is lowered to either a 
pH of 6 or 7 with addition of an acidic chelating agent such as 
diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic 
acid (EDTA), preferably DTPA, the chelating agent originally in the 
solvent is destabilized by reacting with hydrogen ions and releases 
alkaline earth cations which react with residual sulfate ions originally 
present in the scale dissolved in the solvent to form an insoluble 
alkaline earth metal sulfate. Removal of the residual sulfate ions from 
the solvent is beneficial because these ions are known to inhibit 
dissolution of alkaline earth sulfate scale by the solvent after the 
solvent is regenerated for reuse. Anions as sulfate in solution at the 
time of lowering the pH are suitable for forming precipitates with the 
released metal ions. The following equations illustrate precipitation of 
barium ions using DTPA: 
##STR1## 
The amount of solid acidic chelating agent used is that amount needed to 
reduce the pH of the spent solvent to a value of about 6 to 7. 
The solvent is then separated from the barium sulfate precipitate by 
centrifuging, decantation, or filtration. Thereafter, the pH of the 
solvent is increased to a pH of about 10 to about 14, preferably about 12, 
by the addition of potassium hydroxide to recover a regenerated solvent 
for reuse to dissolve scale. 
The advantage of the present method of regenerating the spent solvent is 
the fact that the chelating agent itself is used to lower the solvent pH 
during the first step of regeneration, which balances the total amount of 
dilution caused by adding the caustic required to adjust the pH to 12 
during the second step of regeneration, thereby maintaining the activity 
of the chelating agent in the regenerated solvent at least equivalent to 
or greater than the activity in the spent solvent. Therefore, even though 
the solvent volume increases with each successive regeneration cycle, the 
activity of the chelating agent in the regenerated solvent remains at 
least equivalent to or greater than the activity in the spent solvent. 
The following example shows the method of regenerating scale solvents 
through precipitation of the dissolved metal ions and subsequent upward 
adjustment of pH. 
EXAMPLE 
Regeneration of Scale Solvent Using pH Adjustments 
A sample of spent scale solvent containing dissolved barium sulfate was 
treated with solid DTPA (diethylenetriaminepentaacetic acid) until the pH 
of the solvent was reduced from 12 to 6. At a reduced pH of 6, a 
precipitate of barium sulfate was formed when the chelating agent (DTPA) 
was destabilized by reacting with hydrogen ions and releasing barium ions 
which react with residual sulfate ions to form an insoluble barium sulfate 
salt. These ions, in addition to strontium, and calcium, are found in oil 
field scales and could be readily dissolved by the scale solvent. The 
residual sulfate ions present in the spent solvent are therefore used to 
advantage to remove the released barium ions as a sulfate precipitate and 
reduce sulfate concentration in the solvent. Since sulfate ions must be 
removed in order to further dissolve more alkaline earth sulfate scales, 
because of the common ion effect, this step has a dual purpose removal of 
both objectionable cation and anion. 
In the second step of the process, the barium sulfate precipitate was 
removed from the solvent by centrifuging, filtering or decanting the 
solvent from the precipitate. 
In the third step of the process, the pH of the solvent was re-adjusted to 
12 with the addition of solid potassium hydroxide (caustic potash) to 
produce a fully regenerated solvent. 
An example of this procedure is shown in the drawing. In the drawing, three 
cycles of regeneration are illustrated. Cycle zero is the initial amount 
of barium sulfate dissolved in fresh solvent (61.8 g/l). Cycle #1 is the 
result of one pH adjustment, precipitation, and re-adjustment of pH. In 
the drawing, for a pH reduction to 6, a least square line was drawn 
through the cumulative barium sulfate dissolved in each cycle and was 
extrapolated through 5 cycles using the right most Y axis. After three 
cycles, about six times the initial amount of scale is dissolved using 
this regeneration scheme. It was determined independently of these results 
that neither the chelating agent (DTPA) nor the synergist (Oxalate) are 
precipitated during the pH lowering processes. Based on bulk chemical 
prices (Chemical Marketing, Aug. 20, 1990), the regeneration process is an 
economical alternative to makeup of fresh scale solvent since the cost of 
regeneration is only a small fraction of the cost of fresh scale solvent.