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topic 12373, page 2

Zinc oxide production by French process

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A discussion started in 2002 but continuing through 2018


Dear sir,

we are manufacturer of Zinc Oxide by French process for last 20 years.

We are facing problem with Zinc Dross having aluminium content more then 2%.

When ever we have to use this kind of dross it doesn't melt in the furnace,and starts evaporating instantly,and we are not able to clean the Dross of impurities.the result is that the ZnO formed is blackish in colour.

Can anybody suggest any chemical,which can be mixed with dross ,while charging it in to crucibles,so that the Dross gets cleaned,and the resulting Zinc Oxide is white in colour free of impurities.

Hope to receive an answer,and early date.




Mr. Ankur Agrawal,
I have experience in processing dross up to 4.5% aluminum. In the same process we cant skim liquid metal. So we can go with aluminum in the liquid metal. Try to skim liquid metal with ammonium chloride and Keep @8 inches space below crucible cover. Take care of splashing liquid metal. If you will not skim non-metallic impurities with ammonium chloride it blocks the hole for vapors in cover, and the crucible cover can throw away by zinc vapors. Because of this liquid metal got flashed and can cause hazards in the plant.
For high aluminum containing dross better to go for high capacity crucibles furnace.

Umesh Charki
MetalliKon Consultancy Services - Thane, India


Dear Readers:

(1)I came across the thread very recently and amazed that people have taken lots of interest. I shall be pleased to know how many persons have been benefited with this discussion.

(2)I am with Zinc Oxide industry for last more than 30 years serving all area of working of the industry. During course of my marketing I faced lots of problems faced by the consumers and I had to interact with our Technical people for the proper quality products.I have been able to pursue and solve the problems faced by the manufacturers of Zinc Oxide who are investing huge amount of money.

(3)I have understood that main impurities like Iron, Lead, Alu. etc are contained in Zinc Dross are most in form of compounds (oxide, sulfate, carbonate etc) depending on the process adopted by the Galvanisers (there are several different processes) that is why the material purchased by the ZnO Mfgrs are not found uniform and they face end product problems. It is understood that at about 800 deg. C. Temp. most of the Iron, Lead etc compounds are decomposed to metallic particles and if it is allowed to stand for few hours after stirring at the same temp. metallic particles will settle down leaving zinc to be free from these impurities. Use of Fluxes will add some other impurities such as chloride etc which are not desirable in certain grades of ZnO.

(4)For economy of the fuel, I shall seek advise from my friends, while only surface area of Zinc in the crucible is reacted to form zinc vapour, what is the necessity to keep the whole 500 to 700 Kgs of Zinc in one crucible boiling for hours consuming valuable fuel?
I think instead of more depth more wide with less depth crucible should be used and continuous molten zinc should be fed. Such device could be done. I feel since standard sizes of crucibles are available (which are used for melting of iron and other metals and alloy and are more in demand they are being produced) other suitable for zinc oxide is not desired/demanded by the ZnO mfgrs.

(5)I can say that ratio of Zinc HG should be Min. 100:123 and for Zinc Dross (from Steel Plant continuous process) it could be 100:115. This result is regularly achieved.

(6)For Furnace Oil consumption, Burner with auto-control with proper insulation with latest refractories materials, etc will improve on fuel consumption.

(7)I had tried wet process for making Zinc Oxide from Zinc Ash, but due to pollution problems could not proceed but achieved the quality.

Trilok Shah
designer, consultant - Kolkata, West Bengal, India


We are producing zinc oxide by French process. I have come through some material that is used to clean residue of iron from bottom, which is a major headache.

unfortunately I do not know the name of the material but it is like crystals of sugar and bluish in colour. it gives us good results in cleaning bottom iron. can any one throw some light on it?

2. when we use dross with aluminium it is difficult for us to clean ash and other impurities from top and we get inferior quality zinc oxide in off white colour. can any one solve the problem?

Sunil Aaghara
director - Morbi, India


Dear Ranashayer,
Lot of thanks for your suggestion. Already we are doing same exercise. Thanks,

Ravi Rajput
Q. A. Manager - Jammu, India


Dear Mr. Shah, Kolkata

we are at present producing zinc oxide white seal from Dross.

we mainly supply to ceramic industry. when we use dross with Al we find zinc oxide which is not as white compare to bottom dross of structure galvanising. but in terms of % both are more than 99.5%. can you solve our problem?

we are also interested in producing oxide from ash.

can you help me in this way?

Sunil Aghara
industrialist - Morbi, Gujarat, India









Pls. clarify how to calculate the capacity of the zinc oxide plant keeping in view the size of the crucible,Two No. of crucibles/furnace and output of zinc oxide /hour viz a viz weight gain and type of raw materials employed by zinc oxide industry.




Dear Rahul Sharma,
I am answering your queries step by step,
1) Capacity of ZnO furnace- two crucibles per furnace is most suitable combination of ZnO furnace. And Not only crucible size but burner capacity also have very important role in capacity calculation of ZnO furnace. Vapourising rate of zinc metal is important factor in productivity of ZnO furnace. Holding is defined by crucible size but it does not affect zn vapourisation rate.
2) Output of zinc oxide furnace is not easy to determine by per hour basis. as vapourisation rate does not remain fix over time because this is not continuous process but batch process. Crucible cleaning also interrupt production. ZnO furnace capacity is defined by vapourisation rate per day after deducting downtime like charging time, heating time etc.
3) Weight gain is weight of oxygen added to zinc while formation of zinc oxide. It is theoretically 24% weight gain. But in actual there is 22% of weight gain, When we minimize by products generation, and loses in the processing. Weight gain for dross and other scrap is defined by, {wt of zinc oxide =%metallic zinc content * 1.2}
4)Raw materials:- French process is concern only to metallic zinc content in raw material and raw material is deciding factor for quality of zinc oxide formed. So in ZnO industry we use raw material with more metallic zinc content above 88% metallic zinc.
Galvanisers dross is major source of RM. zinc roofings, scrap, turnings, scores, wires,blowings,etc.

Umesh Charki
MetalliKon Consultancy Services - Thane, India


Respected sir,

Mr. Charki,

I am very impressed to your all answers in Sir I know you are a expert in about zinc oxide plant.

I am working in zinc oxide plant As a officer production from last three years,Now I understand all the process parameters in the plant but I want to know from you about French process from beginning to End. I want to know process in between zinc Dross to Zinc Oxide.

sir, I hope you will give me reply.

Ramashare Yadav
employee - India


Mr. Ramashare,
Zinc Dross to Zinc oxide is very simple gas phase reaction between zinc vapours and oxygen. Metallic zinc from zinc dross vapourises and due to high temp ( 950 degree C) it starts spontaneous oxidizing with atmospheric oxygen. This is as simple to burning of carbon in air, as it gives CO2. Zinc combustion gives zinc oxide.
This reaction happens in fraction of seconds, and after zinc oxide formation we can not purify it. So it is very important to set parameters proactively to form desirably pure form. Controlling factors for this process are flow rates of zinc vapours and air, which gives different partial pressures of reactants; as well while and after formation temp. control gives desired physical form means particle size distribution for ZnO. And dispersion of zinc vapours in air while oxidizing is important for complete combustion of zinc, as zinc oxide forms impermeable and hard coating over unreacted zinc particle surface.which gives free metallic zinc in zinc oxide.

I hope you have cleared most of your queries,

I request to you all fellow ZnO enthusiasts to be environmentally careful while manufacturing zinc oxide. It is very much important as French process is energy intensive process and Major problem in this industry is air pollution.

Thanking you,

Umesh Charki
MetalliKon Consultancy Services - Thane, India


Dear all,
I am an engineer from a non-ferrous metal powder company. In French process how zinc vapour is collected from furnace and react with air (O2).what are the precautions to made avoid explosion?after the reaction completed how you cool and collect the ZnO powder?

Engineer - Madurai,Tamil Nadu, India


Dear muthupandi,

Good inquiry but its very difficult to explain so that you must be visit any ZnO plant for your query.

Ramashare Yadav
- India


Dear Ramashare yadav,

thanks for your kind reply.

Engineer - Madurai


Dear Muthupandi,
I think you have gone through the total forum.I am explaining you the process,
1)In French process, crucibles are used to boil zinc metal, Crucibles keep covered by refractory tile with 2-3 inches hole for vapour outlet. When zinc metal starts boiling, vapours come out with velocity of 0.1 mach ( 30 m/s) and enters in hood above the furnace. continues air flow maintained in hood with help of blower, this air used for oxidation; carrying away zinc oxide to collection system and sudden cooling of zinc oxide.
2)As explosive limit of zinc dust in air is 0.6 kg zinc dust in one cubic meter air at 600 °C. but in this process zinc vapours starts burning spontaneously as they comes out of crucibles and contacted with air. This process is exothermic so temp. of flame suddenly increases to 1500 °C. When it comes in contact with air flow at 30 °C formed zinc oxide quenched to below 450 °C. This quenching define zinc oxide particle properties. how fine it should be?
3)This formed zinc oxide at 400-450 °C carried through long duct, where it gets cooled further by natural convection. It passed through settlers to separate heavy particles. after cooling below 100 °C air filtered to separate zinc oxide.

Umesh Charki
MetalliKon Consultancy Services - Thane, India


Dear all,

I have learn a lot from your discussions. I am an Engineer for refractory-Crucible and have one customer to manufacture zinc oxide. This customer use 2 kinds of different crucible furnaces for French process, one is for high pure zinc ingot and one is for zinc dross, both try to use the fuel efficient. For the high purity zinc ingot, the crucible can be used for over 3 months. For zinc dross, the average lifetime for crucible is about 40 days; There are much dross left in this crucible, the dross will be powdered after a few days when they shut down the furnace. Analysis shows that there are much Fe/Al/Pb/Sb in the dross, I think these comes from galvaniser and these impurities result in the short life of crucible. does anyone find this problem before? how is the crucible lifetime in your plant?
I have searched the internet, all the zinc oxide plant I found use a similar furnace as shown in Dr. Shahrom Mahmud's paper. What I have seen in my customer is totally different, I believe my customer's experience can use the fuel more efficient.

Jiyang Chen
- Suzhou, China
  ^-- this reader rates this thread: winner

I making zinc oxide with French process since two years .I want to know how I can make active zinc oxide

Anurag Jain
- Ganga Nagar, Rajesthan, India

January 3, 2008

Dear all,

We are one of the Zinc Oxide production in Thailand using French process. As we know, the Fuel oil price is very high now. Could anybody suggest us for replacing FO to Natural gas?

Also, we have problem with contamination from FO combustion (carbon from FO flame) in our Zinc Oxide product. We think Natural gas will help us for this problem.

Last question, we set the target for fuel oil per Mt ZnO at 200. Would it be correct for 200 Lit/mt ZnO?

Best Regards,

Thanat K.
Production manager - Bkk, Thailand

February 23, 2008

Zinc Oxide Yield using PWG Grade Zinc

Dear Readers,

I am running 4 furnaces and using both SHG and PWG grade zinc.
I am mixing PWG grade zinc in a 10-15% but have a notices a drop in pure oxide
yield from 1.20% down to 1.17% . I was expecting a drop in yield of 1% only.

Can anybody help & explain on how to control this Yield ?

thank you,

Vinay Dhawan
- Bangkok -Thailand

March 4, 2008

Dear Mr. Vinay Dhawan,
PWG zinc( prime western) contains up to 1.5% lead. Which does not converted in oxide. only zinc is converted in Oxide with additional oxygen weight. Reduction of yield could be because of loss of zinc in bottom of crucible with lead, increase of by-product like ZnO pipe on vapour opening. There are many reasons for reduction of yield.
Also mixing of PWG zinc gives increased lead content in zinc oxide. As lead comes with zinc vapours at 0.005 times percentage of lead in liquid metal in crucible.


Umesh Charki
MetalliKon Consultancy Services - Thane, India

May 24, 2008

Dear All,
I am in this field for last 7 years. I have installed 24 ZnO furnaces till date. Now While thinking over rising oil prices. I think electrical furnace will be better option. Oil 150 ltrs/ MT is std. consumption which we are getting. but for 1 MT ZnO production electrical furnace consumes 450 KWH power. which is most cheap in some part of world. This furnace is producing 3 MT/ day ZnO. This furnace cost is also same compare to oil fired furnace(with combustion control system).
I think this will be most profitable furnace in these days for rising oil prices.

Any input regarding this are most welcome.

Umesh Charki
MetalliKon Consultancy Services - Thane, India

May 28, 2008

I am into Hot Dip Galvanising Business. I am planning to venture into Zinc Oxide and Zinc Sulphate production business. I have gone through the posting in the forum. They are invaluable.

As first information, I would appreciate if you could provide me information on
1) what amount of Zinc Dross would produce what amount of Zinc Oxide.

2) cost of machinery.

Keeping in mind, the plant would target a production of 150 MT/month of ZnO.

I would appreciate if anyone is aware of the same regarding production of Zinc Sulfate.

Thanks in Advance.

VN Reddy
- Hyderabad, India

June 3, 2008

Dear Umesh,

Thank you for the explanation on reasons for drop in Yield with use of HG.
as I understand if I begin with 95% pure Zinc Ingot, then effectively,
I should be able to get only 1: 1.15% Oxide Yield depending on the Pb & Fe impurities ?

also, I wanted to further enquire on process conditions which could help
reduce the Grit & Ash and improve the Yield ? should I reduce the charge/time/furnace ?
regards, Vinay

Vinay Dhawan
manufacturer - BKK -Thailand

June 11, 2008

Dear Mr. Vinay Dhawan,
using dross in zinc oxide process is commercial decision. You can utilize most of metallic zinc from dross by avoiding skimming. for grit and heavy particles removal use gravity settlers effectively. You can get 99.7% zinc oxide from dross. in this process dross of 90% minimum can be used efficiently.
Process controls, variable settling systems will give good quality zinc oxide from dross.

Umesh Charki
MetalliKon Consultancy Services - Thane, India

  ^-- this reader rates this thread: winner
June 16, 2008

Dear Umesh,

I agree to your views regarding Use of Electric furnace for ZnO production.However following issues are vital.First,Can All types of Zinc Dross be efficiently used in E-Furnace ? Or only Molten Zinc is to be transferred in the Furnace.

Secondly the Life of the Furnace and skilled manpower to run these e-furnaces,as mostly in India 99% furnaces are still Oil fired ones.

Lastly the availability of Continuous power of min 100 kW to run one furnace ?

Awaiting your views.


- Alwar, Rajasthan, India

June 17, 2008

Dear Ankurji,
I am also working on electrical furnace. The power consumption is 800-900 KWH per Mt zinc oxide. This is single crucible furnace. Using heating elements. This resistance heating type furnace. You can operate as oil fired furnace. no special skilled workers are required. care is taken to deal with liquid zinc spillage, dust, crucible breakage detection etc. issues.
In some part of India electricity is no problem. Like in South India, you can install wind mill and get power from Electricity board. There are so many ways to go for non-conventional energy sources.
This is time to go for such things never thought in zinc oxide field.

Inputs are welcome,

Umesh Charki
MetalliKon Consultancy Services - Thane, India

  ^-- this reader rates this thread: winner
September 14, 2008

we are manufacturing zinc oxide white seal from galvanising bottom dross. we are getting yield as 1 : 1.12 generally. we use furnace oil as a fuel and it generally takes 220 liters per ton of zinc dross. initially when crucible is new fuel consumption is lower but after using nearly 20 MT dross it takes more time as well as consumes more fuel. we use LAP 1 type burner and 3 HP 2800 RPM air blower.

can I do any modification in burner or air blower to reduce fuel consumption?

Sunil Aghara
Director - Morbi, Gujarat, India

September 29, 2008

Dear Mr. Umesh Charki,

Thanks for clarifications on HG & Dross Yield.
now, we are looking to expand/add more 4 furnaces
& want to completely redesign our plant .

best regards, Vinay

Vinay Dhawan
Manufacturer - Thailand

October 22, 2008

Dear Friends,

Nice to meet you all..

We are Zinc Oxide manufacturer in Indonesia..
We want to know new information, innovation, research and maybe new application of Zinc Oxide..

I hope will get something important from this discuss for me to develop my factory in Indonesia.

Thank you for your kind attention..

Best regards

Hendra Gunawan
manufacturer of zinc oxide - Indonesia

November 3, 2008

Dear Mr Umesh Charki our company is a leading manufacturer in ceramic field. Currently we have a plan to produce zinc oxide from zinc with the total capacity of at least 1000 tons/months. If you are interested in cooperating with us, please contact me as soon as possible. I hope with your expertise we can save a great deal of time for not testing all the configurations.

Khanh Nguyen
- Vietnam

Ed. note: People have offered great info on this page, especially Umesh Charki. Thanks! This site made possible by supporting advertisers for camaraderie and public exchange of technical tips. Our advertisers are pleased to see that camaraderie and the exchange of tips, but they obviously don't want their money spent putting potential customers into private contact with their competitors, nor steering people towards someone else's proprietary technology. Share what non-proprietary public tips you can, but if you wish to get commercial benefit from this site, you need to be a supporting advertiser. Thanks.

November 12, 2008

Greetings Readers - For your information - The second International ZnO conference (organised by the International Zinc Association, Brussels) will be held at Rancho las Palmas, Palm Springs, California, USA on February 24th and 25th, 2009. We hope to build on the success of last year's meeting and we look forward to meeting readers again next year.
Best wishes,

Peter Robinson
- Fergus, Ontario, Canada

November 13, 2008

Dear Mr. Nguyen,
Installing plant with capacity 1000 MT/Month is good job. Before installing we should check for the quality required. So finalisation of plant setup and raw material can be specified.

Umesh Charki
MetalliKon Consultancy Services - Thane, India

December 12, 2008

Dear readers,

I'm just new into ZnO and we are putting up a plant in Haryana for Zinc oxide by French process and only source with virgin Zinc ingots. I have some Q regarding my project

1. How to get smaller particle size at least to introduce in pharma sector ? And does the diameter of cooling duct effect the particle size?

2. What are the other factors involved in particle size like , size of hood and duct, suction speed of blower , furnace temperature , and handling or packing ?

3. By which test I can get the surface area in M sqr / gram for zinc oxide?

I would appreciate for any answer.

Thanks .

Suvneet Kalsi (Project manager)
- Delhi, India

December 15, 2008

Dear Suvneetji,
Zinc oxide particle size is depend upon cooling pattern of ZnO after formation. This is gas phase reaction with product in solid form.
French process limitation is to produce ZnO with surface area from 2 m2/ gm to 10 m2/gm with wide particle size distribution.
Hood design is also important as it gives such flow pattern to disperse and mix zinc vapour and atmospheric oxygen, just like combustion of fuel.
As this is gas phase reaction, partial pressures of reactants are also important.
You can measure surface area with BET method of nitrogen condensation.

Umesh Charki
MetalliKon Consultancy Services - Thane, India

December 17, 2008

Dear Mr.Umesh Charki ji,
Thanks for your reply. Sir, as you said we get surface area between 2-10 with wide distribution, but our requirement is for between 7-10, So Can you tell that what should be the proper separation to get maximum output of 7-10 and in what percentage we can expect out of total production.

On the other hand I want to ask is there any defined cooling system for cooling duct or it is naturally done. Presently we are getting 5-7 surface area.

One more thing what is partial pressure of reactants actually?

Hope to see you again.

Suvneet Singh
- New Delhi, India

December 28, 2008

Dear Suvneetji,
French process can produce 2-10 Sq mt./ gm. But it requires different set of parameters. Generally this setup produces 4-7 For higher surface area we need to design reaction in proper manner.
We are using natural cooling for this process as it is cheapest method available. And surface area is depend upon cooling rate to 450 °C; below this temp. surface area does not changes effectively.

Partial pressure is percentage of every reactant in reaction mixture for gas phase reaction.

Umesh Charki
MetalliKon Consultancy Services - Thane, India

December 30, 2008

Dear Umesh ji,
Thanks for your mail again,
we are putting up a new furnace of one crucible of capacity 800 kgs to get the better results.

I have a query regarding the distance between the top of the crucible and lower end of the hood. Does it help me to get the better results regarding surface area?
I want to increase the distance because to get the more air intake. presently we have 10-12 inches of gap.

If I put insulation on hood to keep it hot does it help me to get better results ?

Hope you have answers for that.

Thanks and regards

Suvneet Kalsi
- New Delhi, India

January 1, 2009

Dear Suvneetji,
We install two crucible furnace to increase holding capacity of the furnace. Due to high holding capacity zinc dilution is possible for reducing lead content in product. In this case we can increase vapourisation time to increase productivity. Because more zinc we vapourise in 24 hrs will be add more production capacity with good fuel efficiency.
Distance between crucible top and lower end of hood defines velocity of the air at entry point. here high velocity results in better mixing of air and zinc vapours. the distance you are having is sufficient. For more surface area we need to cool down zinc oxide formed. More cold air will result in more quenching of zinc oxide formed. so you concentrate on the air volume to be suck in the system.
Putting insulation will damage the hood earlier, as it is made of MS and need to operate below 300-400 °C. And cooling of the zinc oxide is done by the air entering in the hood from bottom.
I hope most of the queries cleared,

Umesh Charki
MetalliKon Consultancy Services - Thane, India

February 20, 2009

Dear Sunil,

About energy consumption for French process of Zinc Oxide production, it ranges in a wide scope, from 120 L to over 250 L heavy oil per ton Zinc Oxide. Basically, the energy consumption is quite dependent on the furnace design.
During zinc oxide production, since the crucible wall is porous, the zinc vapor is trying to penetrating the crucible wall. This penetration accelerates especially when the furnace operates at very high temperature. And the penetrated zinc will oxide and deposit on the outside of the crucible wall. Once this deposit happens, the crucible will start to lower its thermal conductivity and fuel consumption will go up. So if you monitor the fuel consumption, you will find the fuel consumption is lower when the new crucible is used and after a few weeks the energy consumption will go up. In this case, an Isostatic pressed crucible with higher density/lower porosity will be better for French process.

For higher surface(higher BET) Zinc oxide:
zinc oxide produced from French process usually is lower surface area below 10 m2/g. However I believe it is possible to produce a higher surface area zinc oxide by French process. As said by Mr. Umesh Charki, French process for zinc oxide is a gas reaction, so it is quite possible to produce a higher Surface area ZnO by adjusting the parameters in the following reaction:
Zn(l) =>Zn(g)
Zn(g)+O2(g) =>ZnO(g)
ZnO(g) =>ZnO(s)
Such parameters as temperatures, pressure of Zn(g),O2 would be very important.
Normally these parameters are not controlled by the zinc oxide producers.

Hope this helps you.

Jiyang Chen
- SuZhou, China

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