Phosphotation

PHOSPHATATION The mechanism of separation in phosphatation is different from carbonation. In phosphatation the principal mechanism is flocculation whereas carbonatation acts mainly by inclusion of impurities within the calcium carbonate crystals. Phosphatation relies on the formation of flocs formed by the addition of flocculants, captures the fine colloidal matter and is separated by flotation with dissolved air. The reaction in phosphatation process is given by: 8 𝐶𝑎 (𝑂𝐻) 2 + 6 𝐻3𝑃𝑂4 → 𝐶𝑎8𝐻2(𝑃𝑂4) 6 + 16 𝐻2O

Talo-Phosphatation

The “Talofloc” Phosphatation was developed by Tate and Lyle of England in the late 70’s. It is a combination of traditional Phosphatation process, modified equipment and newly developed synthetic processing chemicals.

Phosphatation 8 𝐶𝑎 (𝑂𝐻) 2 + 6 𝐻3𝑃𝑂4 → 𝐶𝑎8𝐻2(𝑃𝑂4) 6 + 16 𝐻2O

The octacalcium phosphate forms flocs which entraps most of the impurities and color bodies. The primary calcium phosphate flocs formed are very fine and takes time to form. Retention times in the Clarifiers takes as long as 45 minutes. The flocculants act as “bridges” to link the phosphate flocs and form bigger secondary flocs. Flocculants The flocculants act as “bridges” to link the phosphate flocs and form bigger secondary flocs. The flocculants are synthetic polyacrylamide of high molecular weight (up to 26 million). Their negative charge binds with the positive sites of the phosphate flocs. The secondary flocs formed are 1,000 x bigger than the primary flocs. The flocculant is added after the acid and lime are mixed. The use of flocculants resulted to: • Reduced operating temp. to 82°C • Increased brix to > 65°Bx • Reduced retention time from 45 to 25 minutes • Reduced sugar losses • More brilliant and clear liquor Surfactants The positively charged calcium phosphate removes the negatively charged color and impurities by attracting them. Surfactants or color precipitants are synthetic chemicals which are a lot stronger than calcium phosphate, thus it attracts more impurities and color.

Surfactants The formed flocs although numerous are very, very fine and takes time to settle. The addition of flocculant greatly increases the size of the flocs and reduces the flocculation time. When used as an adjunct to Phosphatation, the surfactants increases the % color removal up to 60%. Talo-Phosphatation The “Talofloc” Phosphatation is composed of the following operations: • Primary flocculation with lime and phosphoric acid. • Aeration of the primary floc • Secondary flocculation with the flocculant “Talofloc” • Precipitation of the color bodies and impurities with the surfactant “Taloflote” • Filtration of the clear liquor through a Deep Bed

Filter Talo-Phosphatation The “Talofloc” Phosphatation consists of three main processes: • Clarification of the melt liquor • Filtration of the clarified liquor • Recovery of sugar from the scum through de-sweetening (DBF)

Melt liquor heated in shell and tube HE to 87°C is feed to the stirred reaction tank , where phosphoric acid are automatically mixed in proportion to the liquor flow rate, and lime sucrate is also added to adjust the pH to 7. Lime sucrate is prepared by adding melt liquor to 7°Be milk of lime in a ratio of 7:1. At this pH, the resultant calcium phosphate is in its most insoluble form and flocculates the anionic impurities (color, turbidity, suspended solids). This complex is known as the primary floc.

From the reaction tank, the liquor is gravity feed into the aeration tank where the microscopic particles of air dissolve into the liquor, and physico-chemically attach themselves to the primary floc. At this point a trace amount of flocculant TALOFLOTE is dosed, and this coagulates the primary flocs to form large, gelatinous, aerated secondary flocs. This secondary flocculation takes place in the circular TALO clarifier where the flocs rapidly rise to the surface to leave a sparkling, brilliant clarified liquor underneath.

The clear liquor is drawn-off over a weir with adjustable plate which control the clarifier liquid level and sent to the Deep Bed Filter for filtration. The scum is constantly removed by a rotating scraper blade and sent to the Scum De-sweetening Process.

Talo-Phosphatation Controlled parameters: • Melt liquor flow rate – x m3/hr. *Melt liquor brix – 65 max. • Phosphoric acid dosage – 300 ppm P2O5 on liquors solids *Phosphoric acid assay – 85% • Lime sucrate dosage – pH 6.8-7.2 (Prepared by mixing 100 kg CaO with 1 m3 cold water and 1 m3 raw melt liquor) • Talofloc dosage – 300 ppm on solids (Prepared by mixing 200 kgs of “Talofloc” with 6 m3 of raw melt liquor) • Taloflote dosage – 10 ppm on solids (Prepared by mixing 1 kg of “Taloflote” with 1 m3 of hot water) • Weir box setting – controls the retention time in the Clarifier (25 min. max) and the scum layer thickness (6”-8” deep). • Speed of the scum rake – 1-2 rpm. Deep-Bed Filter It is a column packed with layers of different sized particles decreasing in size from top to bottom of the filter. The layers of filter medium are carefully chosen both in terms of particle size and density. The clarified liquor is pumped via an orifice plate and percolates down thru the filter media from the top to bottom. The suspended solids are trapped in the interstitial spacing of the media particles.

When the concentration of the suspended particles reach a critical value (ΔP = 15 psi, detected by the pressure sensor), back-flush sequence is automatically initiated. The filter is back-flushed in an up-flow direction, with the largest particles having the lowest density, so that after backwashing the gradation of the filter medium size is preserved. During backwash air sparging is initiated to agitate the filter media and release the entrapped suspended solids. Deep-Bed Filter Backwashing is done with the filtered liquor, so that no additional sweet water is generated. The Deep Bed Filter is fully automated. Operation is controlled by series of automatic valves, pumps, & blowers regulated by a central programmable controller, hence requires minimal supervision. The back-washings are returned to the process buffer tank where they are eventually removed by flotation in the main clarifier. Deep-Bed Filter Talo-Phosphatation comes with a specially designed filter called DBF. It replaced the pressure filters which use filter aid. It is constructed like a sand filter but uses carefully sized filter media. There are usually 5-6 layers of gravel, sand and bone char. The arrangement of the filter media ensures complete removal of suspended particulates from the liquor. Uses no filter cloths but the media is replaced periodically (every 2 years) Can be operated singly or in a battery of several units. Service cycle of up to 24 hours.

Scum De-sweetening The Scums are skimmed off the surface of the liquor and are sent to the Scum De-sweetening section. About 3-6% of incoming feed liquor is removed with the scum. After treatment, the scum going to waste should contain <1% of sugar. The efficiency of the process rests on the following: • Constant scum feed rate • Constant dilution feed rate (usually 5:1 or as sweetwater is needed). • Uniform flocculant dosage (1.5 ppm on liquor solids). • Efficient aeration • Correct scum withdrawal (scum thickness and rake speed) 3-stage Scum De-sweetening It is a counter-current sucrose recovery process. Hot condensate or hot pure water is metered via a positive displacement pump to the scum trough of the 2nd clarifier. The quantity of water is proportional to the ratio of water to scum (dilution ratio). This water transfer the scum from the 2nd clarifier to the mixing tank where it is thoroughly mixed by a mechanical stirrer and gravityfeed to the 3rd clarifier where a trace of flocculant TALOFLOTE is added. A stream of aerated water is added to this which results in a flotation of scum and Sweetwater.

This scum is discharged to drain while the low-brix Sweetwater to the trough of the 1st clarifier into the mixing tank of the 2nd clarifier. A mechanical stirrer thoroughly mixes the scum and the Sweetwater and a trace of flocculant TALOFLOTE is added, simultaneously an aerated Sweetwater stream is added & the mixture flows by gravity into the 2nd stage clarifier where separation takes place by flotation to produce scum on the surface and Sweetwater underneath.

The Sweetwater is pumped to the mixing tank of the 1st where it is mixed with the scum from the TALOFLOC phosphatation process. Separation takes place in the 1st stage clarifier, and the Sweetwater if transferred to the melter of the affination station.

Comparison Carbonatation is technically the best choice for decolorization.

Destruction of Invert • High pH in the first Carbonator ,eliminates most invert • Phosphatation due low pH creates more invert

Buffer Capacity/Sucrose Loss • Carbonated Liquor pH of 7.8 – 8.2 • Phosphate Liquor pH of 6.8 – 7.1

Stability of Process • Carbonatation - excellent process stability • Phosphatation – temperamental or unforgiving

Quality of Sugar Product • Carbonatation ✓ Sparkling ✓ Much less sediment in final product • Phosphatation – lesser quality ✓ Secondary floc ✓ Some particulates are not removed by Deep Bed Filter

Additional Benefits of the Carbonatation: • Products contamination by pan scale due to sulfates will be practically eliminated. • Acid washing of vacuum pan scale will be avoided. • The operation of a Carbonation process generally is more consistent than other decolorization process.

Phosphatation: • Higher sucrose loss due to low pH • Higher energy consumption due to process instability • Lower initial capital cost • Flexibility if capacity expansion • Low degree of color removal