Lime application in sugar industry
World, sugar industry developed from the 16th century. The sugar production all over the world increased according to the demand. In the early of industry evolution (1750-1830), the sugar production is about 820.000 tons per year, before the First World War (1914-1918) is about 18 million tons per year,and up to now it reaches 180 million tons per year.
Roles of lime in sugarcane juice purification process
Using lime to purify sugarcane juice has been applied for a long time. After being separated from the plants, sugarcane juice has the acidity with pH = 4.0 – 5.5 and contains other unsweetened impurities. These impurities could be divided into 3 groups: Crude insoluble compounds exist in the form of suspensions opalizing cane juice; colorants such as carotene, antoxian, clorofin, etc making cane juice dark; and the solute unsweetened impurities. Most of these impurities cause negative impact to the production process. So the main purposes of sugarcane juice purification process are:
- Removing the maximum amount of unsweetened impurities out of the mixture, especially the surface active impurities and colloids.
- Neutralizing sugarcane juice mixture
- Removing suspended solids out of sugarcane juice
In sugar factory, the following methods are often used to purify sugarcane juice:
1. Lime method
This is the simplest method, which has used for a long time. The sugarcane juice is just purified under the effect of heat and lime, In order to obtain raw sugar. The lime method is used to produce molasses, yellow sugar, etc. This method could be divided into 3 types as below:
1.1. Putting lime into cool sugarcane juice
Firstly, sugarcane juice is filtered through the mesh to remove cane scraps. After that it will be weighed and pumped to neutralizing tank and lime is put into (about 0.5 – 0.9kg lime for 1 ton cane). Mixture will be stirred, heated up to 105oC before put into deposition equipment. The obtained products are deposited juice and muddy juice. Clarifying muddy juice we will obtain clarified juice. The last step is condensing the mixture of deposited juice and clarified juice.
1.2. Putting lime into hot sugarcane juice
Same as putting lime into cool sugarcane juice, yet the heating stage is performed before putting lime into mixture.
Before neutralizing, some of colloids (such as albumin, silica hydro oxide…) have condensed under the effect of temperature and pH. Therefore, sedimentation process occurs faster, amount of used lime to neutralize cane juice are reduced (about 15 – 20%)…
1.3. Segment putting lime
This is optimal method of purification cane juice by lime. Firstly, lime is put into cane juice mixture to achieve pH = 6.4; heat the mixture (t = 105 – 110oC), then lime is put into the second times to achieve pH = 7.6; heat the mixture again (t = 105– 110oC)…
Segment method could save 35% quantity of lime compare with putting lime into cool sugarcane juice method, reduce deposition time, make more pure cane juice, and increase purifying performance.
2. Sulfite method
Sulfite method also known as SO2 method as we use sulfur in form of SO2 to purify sugarcane juice. Sulfite methods could be divided to 3 methods:
2.1. Acid sulfite method (Aerate SO2 into cane juice to achieve acidic pH) is commonly used in producing white sugar
2.2. Slight alkaline sulfite method (Only aerate SO2 into cane juice, not into molasses and raw sugar)
This method is used to produce raw sugar. However, this method could compare with lime method, more effectively remove non-sugar impurities; but equipments are too complex, and amount of chemicals used are much. So this method is not common currently.
2.3. Strong alkaline sulfite method
This method use 2 pH point (neutral pH and strong alkaline pH) which could remove P2O5, SiO2, Al2O3, MgO, … However, technique condition of this method is not stable.
3. Carbonate method
This method use CO2 to aerate into sugarcane juice to remove unsweetened substances. This method could be divided to 3 methods
3.1. Aerate CO2 once time
In this method, lime cream will be put all into cane juice and CO2 is aerated once time to achieve appropriate alkalinity. Therefore, cane juice only passed a isoelectric point, and remove less unsweetened impurities. In the other hand, CO2 is aerated after putting lime, created sugar-lime complex, which will reduce CO2 absorbility performance and create more foam
3.2. Common method CO2 aeration
Also known as the method with CO2 aeration twice and SO2 aeration twice. This method is used in produce high quality white sugar. Diagram of process as below:
3.3. Aerate CO2 into medial molasses
After cane juice is heated up to 100oC and evaporated to achieve molasses concentration 35-40oBx, it is treated according to common CO2 method.
When cane juice is concentrated to high concentration, unsweetened impurities could be more converged and radically reacted. Therefore chemical could be saved, impurities could be removed more than other methods, and equipment is scaled less than the others. However, sugar lost in mud is more than the others.
All of the above sugarcane juice clarifying methods needs quick lime. To ensure technical requirements of the sugar products, used lime need to achieve below requirements:
CaO content ≥ 90%.
MgO content ≤ 2%
Other impurities ≤ 1%.