When the NaOH melt is cooled and formed, both the solidification point of NaOH (322°C) and that of the monohydrate, NaOH · H2O (62°C), must be passed through quickly to prevent caking due to unsolidified monohydrate.Solid sodium hydroxide is supplied in the form of flakes, prills, cast blocks, and less commonly as tablets, briquettes, or granules. Flakes have a bulk density of ca. 0.9 kg/dm3, a thickness of ca. 1.5 mm, and a diameter of 5–20 mm, and are easily broken so that some dust is always formed on handling. Prills have a bulk density of >1 kg/dm3 and consist of spheres with a diameter of 0.1–0.8 mm (microprills) or 0.5–2.5 mm.Flakes. To produce flakes, molten NaOH is fed into a trough in which an internally cooled drum rotates. The molten material solidifies on the surface of the partially immersed drum and is scraped off by a knife, which breaks up the sheet of NaOH into flakes. These are taken to a small intermediate storage silo and packed as soon as possible in sacks or steel drums. The material tends to bridge due to its particle shape, so that it cannot be stored in large silos or be transported by compressed air.

he continuous caustic fusion plant for making caustic soda flakes (100%c concentrated

NaOH).The feed solution containing not less than 30% NaOH is evaporated to 50% in two

evaporators & 9kg/cm2 steam. The 50% caustic soda by2 is conc. To 98 to 99% in evaporator

No. 3 using heat transfer slat heated in a furnace with furnace oil & hydrogen gas. The

heating medium contains 7% NaNO3, 50% KNO3 & 43% NaNO2. H2 used for heating is to

minimize oil consumption. The flakes then formed from the flakes is sent to packing section.

            General description of the plant

The plant fulfils the following main requirements

 The plant is of most advanced technology

 The plant is suitable to produce export grade caustic soda flakes from caustic soda soln. Generated in ion exchange membrane cells and / or liquid NaOH soln. Of 50% concentration

 The plant is designed for Optimum low energy consumption.

 The equipment & the plant are of proven design & safe in operation.

The plant consists of the following components

 First stage falling film evaporator using the vapor heat ex second stage & possibly the falling film concentrator. Second stage falling film evaporator using live steam.

 Heat transfer slat heated falling film concentrator, consisting of individual concentrator elements of special design

 High efficiency heat transfer salt heating system by the combustion of fuel oil & H2 gas.

 One specially designed flaking machine to process high concentrated caustic soda melt into flakes at low temp.

 Two semi – automatic bagging scales to fill hygroscopic NaOH flakes into open type bags at 50kg each (one is stand – by to the other)

 Instrument & control systems designed to operate the process plant automatically

            Specification of feed solution

 Feed source ex ion exchange membrane cells

 Analysis (as is basis)

NaOH min 30% w/w

NaCl max 0.005% wt

Na2Co3 max………. ppm wt

NaClO3 max………. ppm wt to be specified

Fe2O3 max………. ppm wt

 Temp 600C

 Pressure at grade 2.5 barg

            Specification of caustic soda solution

At outlet of caustic cooler HE – 4

 NaOH 50% w/w

 Temp 400C

Pressure at grade 2-5 barg

            Specification of flakes

 NaOH 99% w/w total solids

 Temp at outlet of bagging scale 55-600C max

 Thickness approx 0.8 – 1.2 mm

 Bulk density approx 0.7 – 0.9 kg/dm3

 Volume approx 0.3 – 1cm2

            Specification of chemicals

            Sugar (As corrosion Inhibitor)

o   Quality Food grade, white, granulated

(in bags, near area of usage)            Heat transfer salt

o   Composition

KNO3 : 50% w/w

NaNO2 : 43% w/w

NaNO3 : 7% w/w

Sulphur : max. 0.025% w/

Effluents / emissions

            Liquid effluents

            Continuous waste water stream

Waste water coming form various continuous users & washing produces is collected

in the floor trench system & led to the main sever system.

            Discontinuous waste water stream

Under normal circumstances, the flaking machine (F-1) must be washed out 2 times

per month. The flaking machine has to be cleaned with about 2m3 of cooling water.

The resulting NaOH soln, is led via flake chute (D-9) bagging scale (w-1) into the

floor drainage system. 

            Solid effluent

1) Caustic melt from emptying the flaking machine vat (washing procedure) will be

collected in drums

 2) Damaged bags during filling on the bagging scale

 3) Safety overflow from flaking machine & seal pot (T-10)

            Operating flexibilities

The flexibility of the plant is determined by the capacity ranger of the individual plant

components. Operation at nominal capacity has to be considered as 100%

corresponding to be specified nominal capacity of each unit. 

1.  Evaporation unit

The minimum capacity of the evaporation unit lies at 30% of nominal load, ie., at

30TPD (NaOH 100%)

Capacity range of each individual falling film evaporator is substantially smaller. In

order to nevertheless operate the desired partial load, caustic must be re-circulated via

bypass through the individual stages at a nominal load below 65% ie., feeding of

falling film evaporators is kept artificially high.

1.           65 - 100% of nominal load = 65 - 100 TPD (NaOH - 100%). In this range,

capacity can be varied continuously, without re-circulating the caustic via the falling

film evaporators.

2.           30 – 65 % of nominal load = 30 – 65 TPD (NaOH – 100%). In this range, the

capacity must be re-circulated via the falling film evaporators, by opening the by –

pass valves related.

2    Salt heating unit

The fuel oil / H2 burner system operates in a range of 25 to 100%. Corresponding to

the required capacity range of conc. unit. Below 25% a continuous control is not

possible any mere. Smaller capacities can only be done in the on / off mode.

3    Concentration unit

The falling film concentrator operates at a range of 50 to 100% TPD. Exceptionally,

in case of emergency, the capacity of the falling film concentrator can be reduced to

50 TPD. Special procedures are required to achieve smaller capacity than this

minimum capacity.

4    Flaking unit

The flaking machine operates in a range from 50 to 100 TPD. The max. capacity of

the flaking machined is depending on the cooling water temp.

5    Bagging unit

The bagging unit operates in the range of 0 to 100 TPD.

  Process design

            Dehydration

It consists of the following stages

* Evaporation in two effects

Concentration in one effect

Plant is designed to product

100 TPD of liquid caustic soda solution at 50% conc.

100 TPD of flakes

50 TPD of liquid caustic soda solution at 50% conc.

50 TPD of flakes

            Caustic soda solution production

Caustic soda feed is fed to the falling film evaporator EV-1. During single pass

through this evaporator, Operating on product side under or Vacuum of 75 for (0.100

bar), the caustic soda feed is concentrated from 30% up to approx 36.9%. The

generated vapors are condensed in a water – cooled surface condenser (C-1) the

resulting vapor condensate together with the condensate ex falling film evaporator

(EV-1) is collected in a tank (T-4) & discharged from the plant by means of a pump

(P-5). The falling film evaporator (EV-1) is heated by the vapors generated in the

second effect falling film evaporator (EV-2)

The vacuum on product side of the falling film evaporator (EV-1) & on the vapor side

of the surface condenser (C-1) is generated by a watering vacuum pump (P-6).

36.9% caustic soda lye is discharged from the falling film evaporator (EV-1) by

means of a pump (P-2) & fed via H.E (HE-2) & (HE-3) to the falling film evaporator

(EV-2). On passing this heat exchanger, 36.9% caustic soda lye is warmed up from

approx. 66.70C to some 1400C

During a single pass through the falling film evaporator (EV-2), Operating on product

side at atmospheric pressure, the caustic soda lye is con. Up to 0% caustic soda soln.

This stage is heated by steam.

50% caustic soda soln. is discharged from the falling film evaporator (EV-2) by

means of a pump (P-3) & leaves the plant at a temp of 400C, after having passed HE –

2 & HE – 4.

            Flake Production

The caustic soda feed is fed to the falling film evaporator (EV-1). During a single pass

through this evaporator, operating on product side under a vacuum of 94 torr (0.125

bar), the caustic soda feed is conc. From 30% up to approx 45.3%, the generated

vapors are condensed in a water – cooled surface condenser (C-1) & the resulting

vapor condensate together with the condensate ex falling film evaporator (EV-1) is

collected in a tank (T-4) & discharged from the plant by means of a pump (P-5). The

falling film evaporator (EV-1) is heated by the vapor generated in the II effect

evaporator (EV-2) & the vapor from the falling film concentrator (EV-3).

The vacuum on product side of the falling film evaporator (EV-1) & as the vapor side

of the surface condenser (C-1) is generated by a watering vacuum pump (P-6)

45.3% caustic soda lye is discharged from the falling film evaporator (EV-1) by

means of a pump (P-2) & fed via H.E (HE-2) & (HE-3) to the falling film evaporator

(EV-2). On passing these HE, 45.3%. Caustic soda lye is warmed up from approx.

820C to some 1030C.

During a single pass through the falling film evaporator (EV-2), Operating on product

side at atmospheric pressure, the caustic soda low is con. Up to 50% caustic soda soln.

This stage is heated by steam.

The 50% caustic soda soln. is discharged from the falling film evaporator (EV-2) by

means of a pump (P-3) & is fed to the falling film conc. (EV-3).

During a single pass through the specially designed falling film concentrator EV-3,

the caustic soda soln. is dehydrated from 50% up to approx. 99% caustic soda melt.

The falling film concentrator (EV-3) operates on product side under atmosphere

pressure & the generated vapors are used to heat the first effect falling film evaporator

(EV-1). The heat required for the conc. From 50% up to 99% is transferred by heat

transfer salt. It is circulated by a pump (P-7) through a fuel oil / H2 fired forced flow

salt heater (H-1), where it is warmed up to approx 4300C. The flue gases pass through

a combustion air preheater (HE-1), where the combustion air is warmed up from

ambient temp. to approx 2820C. The flue gases are vented through the stack (D-8) into

the atmosphere.

The fuel oil/H2 burner (B-2), elected on top of the salt heater (H-1), is designed for

operation with preheated combustion air.

In order to protect the falling film concentrator (EV-3) from heavy corrosion, by high

concentrated caustic soda, sugar in form of aqueous solution is added to the

50%caustic soda solution. The sugar solution is prepared in dissolving tank (T-3) &

dosed to the process by a metering pump (P-4).

The high concentrated caustic soda melt is fed via a seal pot (T-10) by gravity flow to

one specially designed flaking machine (F-1), where it is processed into flakes at low

temp. the flakes are fed by gravity via flake chute (D-9) to the bagging scales (W-1).

The bagging scales are of airtight design, to avoid moisture pick – up of the

hygroscopic flakes.

The fine caustic particles escaping from the filling nozzle of the bagging scales (W-1)

are sucked – off by means of caustic dust scrubber (S-1). The filled bags are fed by

belt conveyors (W-1) to the bag closing machines (N-2), where they are closed air

tight.

The plant is controlled by instruments to ensure fully automatic operation of the

process. Operating personnel is required only at the bagging & bag closing station. 

   Product flow

Flow controller controls the plant capacity. Flow recorder continuously registers the

actual flow. Minimum plant capacity is 50% of nominal load, ie., at 50 TPD NaOH

100% or at 2.95 m3/hr caustic soda solution 50% NaOH.

Caustic soda piping

A.         Seal pot

The caustic soda melt outlet beneath the falling film evaporator (EV-3) is equipped

with a seal pot (T-10), which prevents the entry of vapors into the piping & flaking

machine (F-1). The seal pot is provided both with heat transfer salt as well as high

pressure steam tracing.

B.         Pipe for caustic soda melt

The piping between seal pot (T-10) & flaking machine (F-1) is provided both with

heat transfer salt as well as with high pressure steam tracing & arranged on a slope

preventing freezing

The caustic soda melt pipe dips into the distributing channel of the flaking machine to

guarantee barometric sealing to prevent O2 entering into the pipe

CONCENTRATION / FLAKING

  Flaking

The caustic soda melt flows from the falling film concentrator (EV-3) via caustic soda

melt piping, heated by heat transfer salt, into the dipping vat of the flaking machine

(F-1). The principle of the flaking machine is based on a rotating, water – cooled

cylinder, dipped 10 – 20mm into a dipping vat which is constantly fed by caustic soda

melt.

The film of 0.8 to 1.3 mm thickness which forms on the surface of the cooling

cylinder, crystallizes & is cooled down to a temp of about 55-600C during one rotation

of the cylinder.

The caustic soda melt layer is scrapped off the cooling cylinder by scraping knives.

During the scrapping off, the layer on the cooling surface breaks into flakes.

Water is used for the cooling of the product. This cooling water is tangentially

sprayed to the inner surface of the cooling cylinder by means of a tube equipped with

nozzles. The cooling cylinder works under over pressure, so the cooling water

continually leaves the cooling cylinder through an outlet tube.

Thus the cooling chamber is emptied during operation & the cooling water flows as a

water film from top to bottom of the cooling cylinder surface.

The flakes reach the vibrating conveyor of the bagging scale (W-1) via a vertical, air

tight flake chute (D-9)

The flaking machine is covered with a closed casing, thus avoiding caustic soda dust,

which forms during scrapping off of the caustic soda melt film, to penetrate into the

ambient air. The dipping vat filled with product is equipped with an overflow leading

into a drum placed under the flaking machine.

In case of an over flow during operation, the caustic soda melt flow must immediately

be reduced, or the rotating speed must be increased.

In case the flaking machine is turned off due to breakdown or product interruption,

the dipping vat must be emptied & washed out. Emptying the dipping vat is done by

tilting it by means of an electric motor & year thus the caustic soda melt flows into

the overflow drum.

  Flake Weighting / Bagging/ Palletizing

The flakes from the flaking machine are closed via vibrating conveyor to the bagging

scale (W-1),which are controlled by a mechanical weighing system. The flakes are

filled into open bags of 50Kg each. The caustic soda dust produced during the

bagging process is sucked off by the caustic soda dust scrubber, thus keeping the

operating area free of caustic soda dust.

The bagging process :

The empty bag is turned upside down on the bagging stud & fixed with the bagging

clamp. By means of a hand lever, the clamping device of the squeezing valves

situated between vibrating conveyor & weighing device is opened.

By means of a magnetic contact at the hand lever. The vibrating conveyor is switched

on, & conveying of the flakes is started. The vibrating conveyor switches off

automatically when the weight is reached.

The squeezing valve must be shut down with the hand lever again, the bagging clamp

is opened & the filled bag is removed from the bagging stud. The filled bag is now

switched by means of the manual bag-closing machine (N-2).

In order to facilitate handling of the filled bags, a belt conveyor (N-1) is installed

beneath the bagging scale (W-1). The filled & closed flake bags are then palletized by

hand.