This application claims the benefit of U.S. Provisional Application

No. 60/021,889, filed July 17, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is relates to the removal of scale from metal surfaces, and more

particularly, to the removal of scales containing fluorides from metal surfaces.

2. Description of the Prior Art

When coal or other ash-containing organic materials are gasified in a high-

pressure, high-temperature partial oxidation quench gasification system, the ash material

commonly becomes partitioned between coarse slag, finely divided slag particles, and water-

soluble ash components. Water is used in the system to slurry the feed coal, to quench the hot

synthesis gas, also referred to as "syngas" and to quench the hot slag byproduct. Water is

also used to scrub particulate matter from the syngas, and to assist in conveying the slag

byproduct out of the gasifier.

Calcium fluoride and magnesium fluoride scale which forms on evaporator

tubes is usually chemically removed by inorganic acids such as sulfuric, hydrochloric, or

nitric acids. When sulfuric acid is used for scale removal, CaSO is sometimes precipitated.

During acid cleaning of fluoride scale, corrosive hydrofluoric acid is formed in the cleaning

solution and certain metals and metal alloys, such as titanium, nickel, and stainless steel can

become subject to severe corrosion from the hydrofluoric acid. The presence of fluoride ion

(F ^" ) in the solution interferes with the protective oxide films that form on these metals and

allows for dissolution of the titanium, iron, and nickel ions in an acidic solution. Therefore,

chemical cleaning of fluoride scale by the use of acids alone in process equipment is not

practical. It is also noted that calcium scale can be chemically removed by use of ethylene

diamine tetracetic acid.

Scale can also be removed by mechanical means such as by scraping or by

impact with a hammer or by hydroblasting. However, chemical cleaning is preferred and is usually more thorough because scale can be dissolved and removed in places where a

hydroblasting nozzle cannot reach. It is therefore desirable to chemically dissolve fluoride

scale from equipment constructed of titanium or stainless steel. Titanium and stainless steels

are commonly used in the wastewater treatment industry, especially in the construction of

wastewater evaporators.

The literature has also addressed the problem of hydrofluoric acid corrosion in

process equipment made of stainless steels, nickel alloys and titanium alloys. Koch, G. H.,

"Localized Corrosion in Halides Other Than Chlorides," Environment Effects. June 1993

discloses that ferric or aluminum ions can inhibit corrosion.

The effect of water solutions and their corrosiveness in flue gas desulfurization

process scrubbers has also been studied. These solutions contain chlorides, fluorides and

sulfates at low pH, for example, 4800 mg/kg fluoride at a pH of 1. The addition of flyash

minerals which contain significant amounts of silicon, iron, and aluminum can inhibit

corrosion of titanium in otherwise aggressive fluoride containing solutions. It was also found

that if 10,000 mg aluminum/kg (added as aluminum sulfate) were added to a corrosive acidic

solution containing 10,000 mg kg chloride and 1,000 mg/kg fluoride, the solution is no longer

corrosive to titanium.

SUMMARY OF THE INVENTION

Fluoride-containing scale can be removed from metal surfaces such as

titanium, titanium alloys, nickel alloys, and stainless steel by contacting the metal surfaces

with an aqueous salt solution of an inorganic acid, including its hydrates. The cationic

portion of the salt can be aluminum, iron and mixtures thereof. The anionic portion of the

salt can be a chloride, a nitrate, a sulfate, and mixtures thereof. The contracting occurs in the

absence of the addition of an acid, such as hydrochloric, nitric, or sulfuric acid. The presence

of the aqueous salt solution with the dissolved fluoride scale does not accelerate or increase

the normal rate of metal corrosion that can occur in the absence of the aqueous salt solution or

any acidic cleaning agent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to conserve water, gasification system operating units seek to

recirculate the process water, usually after a purification treatment, such as removal of the

finely divided particulate slag or "slag fines" in a solids settler. Since the gasification

reaction consumes water by producing hydrogen in the synthesis gas, there is generally no

need to remove water from the system to prevent accumulation. Nevertheless, a portion of the process wastewater, also referred to as the aqueous effluent, grey water, or blowdown

water, is usually removed from the system as a purge wastewater stream to prevent excessive

buildup of corrosive salts, particularly chloride salts.

As shown in Table 1, which follows, with data from the gasification of high-

chloride Eastern U.S. coal, the composition of the wastewater blowdown from the

gasification system is fairly complex. For a feedstock with relatively high levels of chloride,

the principal wastewater component is ammonium chloride.

TABLE 1 ASH CONTENT OF HIGH-CHLORIDE EASTERN COAL

Gasifier Feed Coal Blowdown Water Percentage (Flo =71 ,950 kg hr) (Flow=33,208 liters/hr) of Coal

Ash Mass Flow Mass Flow Material In Species Concentration (grams/hr) Concentration (grams/hr) Water

Ammonia N 1.4 % 1007300 1500 mg/L 49812 4.95

Sodium 590 micrograms/gram 42450.5 32 mg/L 1063 2.50

Potassium 1200 micrograms/gram 86340 12 mg/L 398 0.46

Aluminum 10000 micrograms/gram 719500 2.3 mg L 76 0.01

Calcium 2600 micrograms/gram 187070 20 mg/L 664 0.36

Magnesium 700 micrograms/gram 50365 4.3 mg/L 143 0.28

Boron 54 micrograms/gram 3885.3 37 mg/L 1229 31.62

Chloride 0.2 % 86340 2600 mg L 86341 100.0

Fluoride 0.019 % 13670.5 63 mg/L 2092 15.30

Formate 0 770 mg/L 25570

Silicon 19000 micrograms/gram 1367050 60 mg/L 1992 0.15

Some materials found in the ash are partially water soluble, that is, a portion of

the material remains in the solid slag or ash fines and a portion dissolves in the water. For

example, sodium and potassium compounds dissolve in water as their ions, and remain in

solids as sodium minerals. Boron compounds dissolve in water as boric acid and borate ions,

and remain in solids as oxidized boron minerals. Aluminum, silicon, calcium and magnesium

compounds are primarily insoluble, and fluoride compounds are also primarily insoluble.

Since wastewater blowdown from the gasification system contains salts and

other potentially environmentally harmful constituents, treatment is necessary before the

water can be discharged. Wastewater treatment for a variety of contaminants can be

somewhat elaborate and expensive, therefore, other more economic means for treating the

wastewater are desirable.

Distillation of the wastewater or brine under certain conditions is an effective

and economical means for recovering relatively pure water from the wastewater. Suitable

means for distilling gasification wastewater include falling film evaporation and forced

circulation evaporation. This invention provides a means of removing fluoride scale which

forms on the metal surfaces of these evaporators, and on any other equipment.

In falling film evaporation, the main system heat exchanger is vertical. The

brine to be evaporated is introduced to the top of the heat exchanger tubes and withdrawn

from the bottom. The brine is pumped to the top of the tubes from a brine sump located

below the heat exchanger tubes. The brine falls downwardly through the tubes as a film on

the interior tube walls, receiving heat so that the water contained therein evaporates and forms

steam as the brine descends. A mixture of brine and steam exits the bottom of the heat

exchanger tubes and enters the brine sump, wherein the water vapor and concentrated liquid

brine separate. The steam exits from the top of the brine sump, and the residual concentrated

liquid brine collects in the brine sump where it is recirculated by a pump to the top of the heat

exchanger tubes. The steam can then be condensed to form a water distillate which can be

recycled to the gasification system. Feed water, such as effluent wastewater from the

gasification system can be continuously added to the brine sump, and a portion of the

concentrated brine is continuously withdrawn for the crystallization and recovery of the

concentrated salts contained therein.

In forced circulation evaporation, the main system heat exchanger is

horizontal, with liquid brine pumped through the tubes and steam introduced on the shell side

of the exchanger to heat the brine. The brine does not boil as it travels through the tubes

because there is sufficient pressure therein to prevent boiling. The hot brine exiting the

exchanger tubes is then transferred upwardly to a brine sump located above the heat

exchanger. As the brine travels upwardly, the pressure drops and the hot brine boils to form a

two-phase mixture of concentrated brine and water vapor. When the two-phase mixture

enters the brine sump, the water vapor separates from the brine, and exits the sump to a

condenser where the water vapor is condensed to form distillate water. The brine is recycled

to the evaporator by means of a recirculation pump, with a portion removed as a brine

blowdown stream for further salt crystallization and recovery. Also as with the falling film

evaporator, feed water is added to the brine sump or to the brine recirculation line.

Although both falling film and forced circulation evaporators are commonly

used for water distillation applications, their usability depends on the rate of scale formation

and accumulation on the evaporator heat exchanger surfaces. The removal of scale from the

evaporator heat exchanger and sump surfaces is very important because scale formation on

the equipment surfaces acts as an insulator and must be removed periodically in order to

operate the evaporator unit effectively.

The composition of the scale shown in Table 2, which follows, was formed

from evaporation of gasification grey water wherein a falling film and a forced circulation

evaporator were used in series. The primary scale components are silica (Si0 ₂ ), calcium

fluoride (CaF ₂ ), and magnesium fluoride (MgF ₂ ).

TABLE 2

COMPOSITION OF TUBE SCALE AND SUMP SCALE

FROM BLOWDOWN WATER EVAPORATION

Magnesium Silicon Phosphorus Sulfur Calcium Iron

(weight (weight (weight (weight (weight (weight

%) %) %) %) %) %)

Forced Circulation 91 2 2 0 3 2 Evaporator Tube Scale

Forced Circulation 1 80 0 7 8 4 Evaporator Sump Scale

Falling Film 3 55 0 2 40 0 Evaporator Tube Scale

Falling Film 3 43 1 0 49 4 Evaporator Sump Scale

In accordance with the present invention, fluoride scale can be removed from

titanium, titanium alloys, nickel alloys, and stainless steel by using an aqueous salt solution

of an inorganic acid, including its hydrates. The cationic portion of the salt can be aluminum,

iron or mixtures thereof. The anionic portion of the salt can be a chloride, a nitrate, a sulfate,

and mixtures thereof. The contacting occurs in the absence of the addition of an acid, such as

hydrochloric, nitric, or sulfuric acid. The presence of the aqueous salt solution with the

dissolved fluoride scale does not accelerate or increase the normal rate of metal corrosion that

can occur in the absence of the aqueous salt solution or any acidic cleaning agent.

Preferred salts are aluminum salt solutions made from aluminum chloride,

aluminum sulfate, aluminum nitrate, and their hydrates, and mixtures thereof. Aluminum

nitrate is the preferred aluminum salt where the equipment being treated is part of a partial

oxidation gasification system, because the spent solution can be returned to the gasification system, and has the least impact on the gasifier feed. The nitrate components of the

aluminum nitrate salt become part of the synthesis gas, such as N ₂ , NH, or CO. In contrast,

aluminum chloride adds chloride to the feed in the form of ammonium chloride, and

aluminum sulfate adds sulfur and calcium sulfate precipitate in the evaporator

Although iron salts of inorganic acids can also be used to dissolve fluoride

scale, iron salts are generally not as effective as aluminum salts on a molar comparison basis for dissolving fluoride scale and inhibiting fluoride corrosion of titanium in acidic solutions.

The aqueous salt solution of the inorganic acid should have a concentration of

about 1% to about 40%, preferably about 15% to about 20% and a temperature of about 32°F

to about 212°F. The salt solution is more effective in dissolving fluoride scale with respect to

rate and quantity dissolved if the solution is heated to a temperature of about 100°F to about

212°F and preferably to about 175°F to about 212°r In a comparison test, scale that

dissolved in 90 minutes at 100°F, was able to dissolve in one minute at 175°F.

The aqueous inorganic salt solution is contacted with the scale surface for a

time sufficient to effect removal or dissolution of the fluoride scale, which is generally from

about 30 minutes to about 24 hours, and preferably from about 1 hour to about 3 hours. A

combination of inorganic salt solutions, including solutions of their hydrates can also be used.

The initial pH of the aqueous salt solution is generally at least about 1.5

Before or after the treatment of the metal surface with the aqueous aluminum

salt solution of the inorganic acid, a solution of an alkali metal hydroxide such as sodium

hydroxide (NaOH) or potassium hydroxide (KOH) can be used to contact and treat the metal

surface to remove any silica-containing scale, or πon cyanide scale

The alkali metal hydroxide treatment, particularly the NaOH treatment, is

generally chosen as the first scale cleaning solution, primarily because the caustic solution is

less expensive than the aluminum salt solution, particularly the aluminum nitrate solution.

The alkali metal hydroxide solution should have a concentration of about 1%

to about 25%, and preferably about 2% to about 6%, and should be heated to a temperature of

about 170°F to about 212°F, or to the boiling point of the solution at atmospheric pressure.

The alkali metal hydroxide solution should be contacted with the scale surface for a time

sufficient to effect removal of the silica or iron cyanide scale, which is generally from about

30 minutes to about 24 hours, and preferably about 2 hours to about 6 hours. A mixture of

sodium hydroxide and potassium hydroxide can also be used. A sodium nitrate inhibitor is

generally used with the caustic when scale is removed from titanium.

After the caustic cleaning operation has been completed, the caustic solution

should be removed from the equipment, such as by draining it therefrom, before introducing

the aqueous inorganic salt solution, and vice-versa. No special cleansing is necessary after

removal of each cleaning solution. Thus, the next cleaning solution, that is, the aqueous

inorganic salt solution can be introduced into the equipment and removed in similar fashion.

The combined spent neutralized solutions of the sodium hydroxide and the

aqueous inorganic salt solution can be combined, diluted with water to a concentration of

about 95% water and neutralized to a pH of about 7 using additional sodium hydroxide, if

necessary.

The neutralized spent cleaning solution can then be used to slurry a feedstock,

such as coal, for a partial oxidation reaction. Thus, for example, fluoride, sodium, aluminum

and silicon constituents become components of the byproduct slag. If the spent alkali

solution is recycled to the gasifier, the recycled solution should be added in small quantities

to the feedstock so as not to increase sodium or potassium feed concentrations significantly

which can have an adverse effect on the refractory lining of the gasifier. An unneutralized

spent aluminum salt solution can be recycled to the gasifier feed as long as it is blended with

the feedstock at a low enough rate so that the pH of the feedstock is not reduced below 6.0.

It is noted that by use of the aqueous salt solution without an acid, instead of

using an inorganic acid cleaning solution with an added aluminum salt, the cleaning process

does not accelerate corrosion or increase the corrosion rate, whereas with an acid, care must

be used to add enough aluminum inhibitor to reduce or halt the acceleration of corrosion.

Since, the amount of scale in the equipment is not exactly known prior to cleaning and there

is an economic need to conserve chemical cleaning solutions, this is a significant

consideration.

The means for determining whether more cleaning solution needs to be added

to the equipment can be determined by a total dissolved solids analysis in which a filtered

cleaning solution is taken from the equipment being treated and dried at 105°C and the

residue weight measured.

The total dissolved solids concentration of the initial cleaning solution and the

cleaning solution in contact with the scale can be used to determine if the cleaning solution is

saturated with scale compounds. A molar ratio of 0.5 silica to alkali hydroxide and a molar

ratio of 0.65 calcium fluoride to aluminum salt solution should be used in determining the

saturation point of the cleaning solution. In this way, the amount of cleaning solution used

can be minimized.

In the examples, and throughout the specification, all concentrations are in

weight percent, unless otherwise specified.

EXAMPLES 1 - 6

Blowdown water of the composition in Table 1 is evaporated in a falling film

evaporator to produce a mixture of water vapor and brine. This mixture is fed to the brine

sump of a falling film evaporator where the water vapor is separated from the brine and fed

to a condenser to recover the water distillate. After operation of the evaporator for about

42 days, scale develops on the titanium surface inside the evaporator tubes and on the surface

of the Hastelloy™ C-276 (Haynes Metals Co.) high nickel alloy that forms the sump.

The scale is mechanically removed from the metal surface of the brine sump

by peeling flakes from the surface and from the evaporator tubes by impacting the outside of

the titanium tubes with a hammer. The composition of the scale is approximately 50%

amorphous silica and 50% calcium fluoride. Separate 6 gram samples of the scale are

initially contacted with 100 grams of a sodium hydroxide solution having a concentration of

6% or 10% at a temperature of 170°F for at least 2 hours. After the treatment period the

caustic solution is analyzed by the Inductively Coupled Plasma (ICP) Instrument Method for

metals and ion chromatography for fluoride, and the weight of Si, Ca and F dissolved by the

caustic solution is determined.

The scale sample is then contacted with a solution of aluminum nitrate

(1 1.2%, 12% or 16%) at a pH of 1 -2 and a temperature of 100°F or 170°F for at least 2 hours.

In EXAMPLES 4-6, the aluminum nitrate solution also contains 0.5 or 1% sodium nitrate

(NaN0 ₃ ) which is used to inhibit hydride phase formation in titanium. After the treatment

period the aluminum nitrate solution is analyzed by ICP Methods for metal and ion

chromatography for fluoride and the weight of Si, Ca and F dissolved by the aluminum

nitrate solution is determined The examples show that a fluoride containing scale is

effectively removed using aluminum nitrate solutions, with over 90% scale removal

accomplished in Examples 1 , 4 and 6. The results are recorded in Table 3, which follows.

TABLE 3 FALLING FILM EVAPORATOR SUMP SCALE REMOVAL

CAUSTIC TREATMENT

Time Temp Si Ca F Molar (hour) (°F) Dissolved Dissolved Dissolved Ratio

(% of (% of (% of of Si initial initial initial dissolved scale scale scale to weight) weight) weight) NaOH in

Example Solution cleanmg solution

1 6% NaOH - 11 2% 2 170 30 0 3 043 A1(N0 ₃ ) ₃

2 6% NaOH - 1 1 2% 2 5 170 20 0 1 5 0 29 A1(N0 ₃ )

-> 10% NaOH (l% NaNO ₃ ) - 4 170 7 7 0 3 7 0.064 1 1 2% A1(N0 ₃ ) ₃

4 10% NaOH (l% NaNO ₃ ) - 5 3 170 10 0 5 5 0 089 16% A 1(N0 ₃ ) ₃

5 10% NaOH (0 5% NaN0 ₃ ) - 5 8 170 9 1 0 3 7 0 097 12% A1(N0 ₃ ) ₃

6 10% NaOH (0 5% NaN0 ₃ ) - 5 5 170 7 6 0 3 6 0.086 16°/o Al(N0 ₃ ) ₃

NOTE Maximum capacity of NaOH solution is to dissolve 0 5 moles of Si for every mole of NaOH (2 moles of NaOH are required to form 1 mole of sodium silicate) Solution is completely utilized when ratio of Si to NaOH is 0 5

Maximum capacity of A1(N0 ₃ ) ₃ solution at 100°F is to dissolve approximately 1 3 moles of fluoride (0 65 moles CaF-,) for every mole of aluminum (previously determined in CaF ₂ dissolution tests) Solution is completely utilized when ratio of fluoride to aluminum is 1 3 or ratio of fluoride to N0 ₃ is

0 43 At 174°F 1 6 moles of fluoride (0 8 moles Cal ,) is dissolved per mole of aluminum

TABLE 3 (Continued) FALLING FILM EVAPORATOR SUMP SCALE REMOVAL

NITRATE TREATMENT

Time Temp Si Ca F Molar (hour) (°F) Dissoiv Dissolved Dissolved Ratio ed (% of (% of ofF

(% of initial initial dissolved initial scale scale to scale weight) weight) N0 ₃ in weight)

Example Solution cleaning solution

1 6% NaOH- 11.2% 2 100 0.4 15 15 0.28 A1(N0 ₃ ) ₃

2 6% NaOH- 11.2% 6.3 100 0.1 21 14 0.26 A1(N0 ₃ ) ₃

— 10%NaOH(l%NaNO ₃ )- 4 100 0.3 22 17 0.32 11.2%A1(N0 ₃ ) ₃

4 10%NaOH(l%NaNO ₃ )- 6 100 0 25 27 0.33

5 10% NaOH (0.5% NaN0 ₃ ) - 3.5 170 0.2 21 22 0.28 12%A1(N0 ₃ ) ₃

6 10% NaOH (0.5% NaN0 ₃ ) - 1 170 0.2 21 18 0.26 16%A1(N0 ₃ ) ₃

TABLE 3 (Continued) FALLING FILM EVAPORATOR SUMP SCALE REMOVAL

RESIDUE COMPOSITION

Example Description Residue after Residue after

Caustic Acid

Cleaning as a Cleaning as a SI O Ca F Al

% of Initial % of Initial

Scale Weight Scale Weight

1 6% NaOH - 1 1.2% 51 8 37 51 4 0 — A1(N0 ₃ ) ₃

2 6% NaOH - 1 1.2% 55 22* 35 53 6 0 — A1(N0 ₃ ) ₃

3 10% NaOH (l% NaNO ₃ ) - — 20** 8 0 50 23 — 1 1.2% A1(N0 ₃ ) ₃

4 10% NaOH (l% NaNO ₃ ) - 73 6 31 46 1 0 — 16% A1(N0 ₃ ) ₃

5 10%, NaOH (0.5% NaN0 ₃ ) - 71 2| *** 14 30 I 22 29 12% A1(N0 ₃ ) ₃

6 10% NaOH (0.5% NaN0 ₃ ) - 74 ₇ *** 6 30 4 26 26 16% A10 IO ₃ ) ₃

The residue from Ex. 2 was subjected to further successive cleanings using fresh solutions of A10MO ₃ ) ₃ and NaOH until all the scale was completely dissolved. The following results were obtained and are presented in order of succession with the solution concentration, time, temperature, and percent residue after cleaning. 3rd Cleaning - 1 1.2% AI(N0 ₃ ) ₃ - 3 hrs - 14%; 4th Cleaning - 1 1.2% A1(N0 ₃ ) ₃ - 6 hrs - 13%; 5th Cleaning - 2% NaOH - 2 hrs - 6%; 6th Cleaning - 6% NaOH - 1.5 hrs completely dissolved the scale.

The residue from Ex. 3 was subjected to 3.2 g of 10% NaOH - 1% NaN0 ₃ at 170°F for 5.5 hrs. and the residue was reduced to 12% (the primary component of this reside was CaF ₂ ).

X-ray diffraction analyses showed this residue to predominantly contain A l,(OH) ₃ F ₃ .

EXAMPLE 9

Two aqueous solutions, designated "A" and "B" are prepared containing 1%

fluoride from calcium fluoride powder, and 4% aluminum chloride added as a corrosion

inhibitor. A 1% concentration of hydrochloric acid is also added to solution A. Both

solutions are heated to 100°F and contacted with grade 2 titanium for 24 hours. The corrosion

rates and other data are recorded in Table 4.

TABLE 4

Titanium

HC1 Solution Solution pH corrosion rate concentration pH (initial) (final) (mils/year)

Solution A 1% 0.3 0.4 636.6

Solution B — 2.7 3.3 0.8

An acceptable corrosion rate would be less than about 10 mils/year, and

preferably less than about 5 mils/year. The solution A corrosion rate is very high and would

result in substantial metal loss. It is evident that the use of an acid solution to dissolve

fluoride scale, even with corrosion inhibitor, can result in disastrous corrosion when cleaning

fluoride scale from titanium using an acid.

The problem with using an acid cleaner is that the amount of fluoride scale in

the unit is not known ahead of time. Therefore, the amount of aluminum corrosion inhibitor

would have to be extremely overdosed as a precautionary measure. By use of the aluminum

salt solution without an acid, the fluoride scale is dissolved and the titanium corrosion rates

are acceptably low.