Calcium hypochlorite
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Calcium hypochlorite

Calcium hypochlorite is also called bleaching powder concentrated and is the downstream chlorine consuming product of the chlor-alkali industry. The main component is Ca(ClO)2.
Ca(ClO)2 can react with CO2 and H2O in the air and generate 2HClO which is highly oxidative.

Ca(ClO)2+CO2+H2O=2HClO+CaCO3

Its strong oxidability is used for sterilization, disinfection, purification and bleaching and is widely used in wastewater treatment, industrial and domestic bleaching, hygiene and disease control, disinfection in breeding industry, sterilization of swimming pools, etc.

Application of calcium hypochlorite

  • Wastewater treatment
  • Sterilization and disinfection of swimming pool
  • Industrial and domestic bleaching
  • Hygiene and disease control
  • Disinfection in breeding industry

 

Calcium hypochlorite production method

 

Calcium method

Ca(OH)2 is made to slurry and Cl2 is introduced. Calcium hypochlorite is produced through crystallization, separation and drying.

 

2Ca(OH)2+2Cl2=Ca(ClO)2+CaCl2+H2O

 

Normally, calcium hypochlorite exists in the form of compound salt 3Ca(ClO)2•2Ca(OH)2,

 

Advantage:

The process is simple, less equipment and investment are required for the project.

 

Disadvantage:

The highest effective chlorine of the product is 65%, solubility is low. It is prone to moisture absorption and decomposition and safety is not satisfactory (Historically, production and storage accidents mostly occurred in calcium method), large amount of three wastes will be generated and the production cost is high.

 

The product quality and stability of this method is not satisfactory and its application is limited. The sale region radius is small. The product lacks competitiveness in the market. So the technology will be eliminated.

Sodium method

Caustic soda is used in the production process to convert calcium chloride into calcium hypochlorite in view of the disadvantage of the calcium method production technology.

 

2Ca(OH)2+2Cl2=Ca(ClO)2+CaCl2+H2O

2NaOH+Cl2=NaClO+NaCl+H2O

2NaClO+CaCl2=Ca(ClO)2+2NaCl

 

The combination of the 3 formulae above is the principle of sodium method production:

Ca(OH)2+2NaOH+2Cl2=Ca(ClO)2+2NaCl+2H2O

 

Advantages:

  1. Production process safety is satisfactory;
  2. Effective chlorine in the product is improved to over 70% and it is competitive in the market;
  3. CaCl2 is converted into NaCl without moisture absorption and the product storage stability is significantly improved;
  4. The content of Ca(OH)2 is significantly reduced and the product solubility is improved, which can meet the requirements in various industries
  5. Less raw materials are used, product cost is reduced and less three wastes are generated.

 

Disadvantages:

  1. The preparation of NaClO and CaCl2 are added, therefore the production process is longer than that of the calcium method;
  2. Comparing with the calcium method, raw material NaOH is added.

 

Cl2O method

Na2CO3 reacts with Cl2 to prepare Cl2O, which is absorbed with water to become HClO water solution. Then it reacts with Ca(OH)2 and becomes Ca(ClO)2.

Na2CO3+2Cl2=Cl2O+2NaCl+CO2 Cl2O+H2O=2HClO Ca(OH)2+2HClO=Ca(ClO)2+2H2O

 

Advantages:

  1. The product contains no CaCl2 and is very stable;
  2. Effective chlorine is approximately 75%.

 

Disadvantages:

  1. Production of Cl2O is difficult and production operation is complicated;
  2. The requirements on equipment materials are high;
  3. The cost of raw materials is high.

 

We have used the sodium method production technology.  we have optimized and innovated the sodium method 3-step reaction technology. The product quality is stable, the raw materials and auxiliary materials and power consumption are low with lower three wastes and the production process is safe and reliable.

We have developed the bleaching liquid recirculation technology based on the sodium method, which has helped improve the resource utilization rate and is a clean production technology and has expanded the production technology and market share of calcium hypochlorite product.

Calcium hydroxide, chlorine and caustic soda are used as the initial raw materials for the calcium hypochlorite production process and 3-step intermittent sodium method technology is used.

 

Calcium hypochlorite process

 

The production process mainly consists of the following sections.

  • Preparation of sodium hypochlorite
  • Preparation of dibasic salts
  • Preparation of calcium hypochlorite
  • Finish products drying & shaping
  • Secondary mother liquid treatment

 

Preparation of NaClO

NaOH reacts with Cl2 intermittently in the reactor. NaCl crystal and NaClO solution are separated through the centrifuge.

 

2NaOH+Cl2=NaClO+NaCl↓ +H2O

 

The primary mother liquid generated from calcium hypochlorite is used in the preparation of NaClO, which can effectively recover Ca(ClO)2 from the mother liquid and separate the byproduct wet salt. The raw material consumption can be reduced and the byproduct utilization value can be improved.

 

Preparation of dibasic salts

Calcium hypochlorite in the primary mother liquid reacts with calcium hydroxide and generates dibasic salt-calcium hypochlorite. The calcium hypochlorite in the mother liquid is recovered.

 

Ca(ClO)2+2Ca(OH)2= Ca(ClO)2•2Ca(OH)2↓ 2Ca(OH)2+2Cl2=Ca(ClO)2+CaCl2+2H2O

 

The primary mother liquid generated in the production of calcium hypochlorite is used in the preparation of dibasic salt. Ca(ClO)2 in the mother liquid can be recovered effectively. The mother liquid generated by Ca(ClO)2 can be fully used in the production of dibasic salt and NaClO.

 

Preparation of calcium hypochlorite

NaClO, dibasic salt and Cl2 react intermittently in the reactor and generate Ca(ClO)2.

 

Ca(ClO)2•2Ca(OH)2+2Cl2=2Ca(ClO)2+CaCl2+H2O

2NaClO+CaCl2=Ca(ClO)2+2NaCl

 

Multiple process parameters are used for the control of the reaction end point. The product crystal form is good and material handling is convenient. The centrifugal mother liquid can be used and the product unit consumption is low.

 

Finished product drying & shaping

Continuous production process is used for the drying and shaping. Wet material is dried by cyclone separators and then pelletized and packaged automatically.

 

Secondary mother liquid treatment

The secondary mother liquid includes approximately 3% Ca(ClO)2, 5% CaCl2 and 12%NaCl, which can be directly sold according to the local market demand (Normally, they are sold to the paper making, printing and dyeing industries etc in China. The production is prone to restriction by the market).

HCl is used for the treatment of the secondary mother liquid. Cl2 is separated and recovered through the chlorine removal tower. Chlorine removal liquid is precipitated with NaClO. Chlorine is further removed from Na2SO3 and then is returned to the Chlor-alkali system as diluted saline water .

 

Ca(ClO)2+4HCl=CaCl2+2H2O+2Cl2

CaCl2+Na2CO3=2NaCl+CaCO3

Cl2+Na2SO3+H2O=Na2SO4+2HCl

 

Equipment for the unit

Totally, Calcium Hypochlorite Unit has about 160 sets of process equipment (mainly reactors, towers, storage tanks, pumps etc), including about 120 sets non-standard equipment, about 40 standard equipment. Highly oxidative hypochlorites are involved in the production of calcium hypochlorite.

 

The equipment mainly includes about 35 sets of titanium (TA2), about 12 sets of SS (SS304), about 80 sets CS (Q235B, Q345R and other packaged equipment), about 30 sets of non-metal equipment (PVC and FRP). Critical equipment includes 10 sets of chlorination reactors (titanium) and 2 flash dryers (titanium).

 

Special equipment in this unit mainly include pressure vessels (Highest Category 2 pressure vessels), pressure piping (highest GC1 pressure piping) and lifting equipment

Automatic control and electrical

Distributed Control System (DCS) will be used for Calcium Hypochlorite Unit to detect, control and record the critical parameters.

Cl2 detectors will be provided for locations with potential Cl2 leakage. CCTV is provided for reaction building and drying building.

Non-explosion-proof design will be used for E&I.

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