Dehua Xu,Benhe Zhong,Xinlong Wang,Xue Li,Yanjun Zhong,Zhengjuan Yan,Jingxu Yang,Xiaobin Li,Yumei Wang,Xiaohou Zhou

Ministry of Education Research Center for Comprehensive Utilization and Clean Processing Engineering of Phosphorus Resources,College of Chemical Engineering,Sichuan University,Chengdu 610065,China

Keywords:Slurry monoammonium phosphate (MAP)Diammonium phosphate (DAP)Industrial grade MAP Water soluble MAP Water soluble ammonium polyphosphate(APP)

ABSTRACT Ammonium phosphate fertilizer is the compounds containing nitrogen and phosphorus that are usually produced through the neutralization reaction of phosphoric acid and ammonia.At present,there are a variety of products,such as slurry monoammonium phosphate (MAP),diammonium phosphate (DAP),industrial grade MAP,water soluble MAP,water soluble ammonium polyphosphate (APP) and so on.After more than 60 years of development,China’s ammonium phosphate fertilizer industry has experienced the road of from scratch and from weak to strong.The successful development of the slurry MAP technology ended the history that the high concentration phosphate fertilizer cannot be produced by using the medium and low grade phosphate ore.The continuous,stable and large-scale production of DAP plant provides sufficient guarantee for DAP products in China.The development of new ammonium phosphate fertilizer products,such as industrial grade MAP,water soluble MAP,water soluble APP,provides technical support for the transformation and upgrading of phosphorus chemical enterprises.In this paper,the production methods,the development history and the latest research progress of ammonium phosphate fertilizers were reviewed.

1.Introduction

Ammonium phosphate fertilizer is the compounds containing nitrogen and phosphorus that are usually produced through the neutralization reaction of phosphoric acid and ammonia.This is one of the most productive and popular types of fertilizers that is applied to almost all soils and crops with high concentrations of active ingredients and less hygroscopicity.In developed countries,such as the United Kingdom,the United States,France,Italy,Germany,Japan,etc.,the rapid development of phosphorus compound fertilizer industry started in the early 1950s and it entered the mature period in the 1970s,which is also the highly developed period of these countries.In the subsequent 20 years of maturity,the phosphate compound fertilizer industry in these developed countries began to decline gradually in the late 1980s and early 1990s,under the combined influence of environmental protection,resource supply,soil quality requirements,modern agricultural development needs,ecological protection and other factors.In China,after more than 60 years of development since the Founding of New China,the phosphate fertilizer industry has gone through a road of development from imported products to imported technologies and then to exported products,and realized the historical leap from a big importer to a big manufacturing country and then to an exporting country.It has become the world’s largest phosphate fertilizer production country with the remarkable achievements.The quantity,quality and production technology of phosphate fertilizer products,as well as the equipment manufacturing,enterprise management,marketing and profit level,have realized the leap from quantity to quality change.Besides,the phosphorus fertilizer varieties have developed from single nutrient,low concentration,to high concentration,compound,multivariety.Remarkably,2007 was a landmark year that China became a net exporter of phosphate fertilizer for the first time,and 2007 also became a milestone year in the development of China’s phosphate fertilizer industry.

The development course of ammonium phosphate fertilizers in China is different from that in foreign countries,due to the high proportion of middle and low grade and poor quality of the phosphate ore in China [1].Ammonium phosphate fertilizers include slurry monoammonium phosphate (MAP),diammonium phosphate (DAP),industrial MAP,water soluble MAP,water soluble ammonium polyphosphate (APP),etc [2–4].In this paper,the production methods,development history and the latest research progress of ammonium phosphate fertilizers were reviewed.

2.Development Course of Ammonium Phosphate

2.1.Slurry MAP

In the early 1980s,the proportion of high-concentration phosphate compound fertilizer was less than 1%,and it was mainly dependent on the import of ammonium phosphate to meet agricultural needs in China.At that time,the developed countries generally used phosphoric acid concentration process(APPAC process)to produce ammonium phosphate,which was suitable for high quality phosphate rock with high grade.However,the phosphate ore in China is mainly middle and low grade refractory phosphate ore (nearly 90.8%).When using this APPAC process to produce ammonium phosphate with the medium and low grade phosphate ore as raw material in China,there are some problems,including the hard and dense scaling in the inner wall of the phosphoric acid concentration heating tube that is not easy to remove,and serious corrosion of the equipment caused by phosphoric acid concentration,which lead to the unsustainable production[5].Based on China’s basic national conditions and resource endowment,the research team led by Benhe Zhong from Sichuan University carried out a series of basic research on the phase,composition,physicochemical properties of phosphate ore and its products,scale layer and scaling mechanism,etc.[6],and revealed the fundamental reason why the traditional APPAC process was not suitable for the production of ammonium phosphate from domestic medium and low grade ore,and further put forward the “new neutralizing slurry concentration process for ammonium phosphate”(APPSC process) [7,8].The APPSC process changed from “phosphoric acid concentration” to “ammonium phosphate slurry concentration”,thus avoiding the problem of “phosphoric acid concentration” and has shown significant advantages [9,10].

The flow chart of APPSC process is shown in Fig.1.Slurry MAP(powder MAP or granular MAP) can be obtained from dilute acid through the steps of neutralization,evaporation,concentration and drying.The key of this process is to neutralize dilute phosphoric acid first,and then concentrate ammonium phosphate slurry.It adopts external circulation ammoniation reactor and the “integration of neutralization and concentration” technology in the neutralization and concentration of slurry process [11–13].The conveying and spraying do not need a pump and more than 60%of the rotating equipment in the system is reduced.Moreover,the process is simplified,and the operation capacity of the equipment is improved,and the material and energy consumption is reduced as well[14].The APPSC process has broken the monopoly of foreign technology and ended China’s history of not being able to produce high concentration of ammonium phosphate using medium and low grade phosphate ore.It has realized the localization of “ammonium phosphate preparation by slurry concentration” technology,large-scale of the devices and the technology integration,and formed an international leading level of ammonium phosphate production technology,and solved the key problem of self-sufficient production of phosphate fertilizer in China[15–17].The related achievements have won a series of important awards such as the first prize and second prize of national science and technology progress,and have been listed as one of the five major achievements in China’s science and technology since the sixth five-year plan by the former state planning commission.More than 100 sets of industry installations have been built based on the APPSC process,and the production capacity of a single set of installation has reached 200–300 kt·a-1.In 2019,the domestic capacity was about 16 million t·a-1.Although the process is very mature and has made a great contribution to the country,the fluorine in wet-process phosphoric acid is not utilized and enters the product.

Fig.1.The flow chart of APPSC process.

2.2.Diammonium phosphate (DAP)

DAP is the main ammonium phosphate fertilizer abroad.In the 1980s,China introduced more than ten sets of large and mediumsized ammonium phosphate plants,all of which adopt the traditional “phosphoric acid concentration process” to produce granular DAP,including a variety of specific technological routes.These devices have been run in for many years and the production technology of DAP has been mastered by most enterprises in China[18–20].The principle process of DAP production is shown in Fig.2,including dilute acid evaporation and concentration,preneutralization,drum ammoniation granulation,drying and other steps.According to the different neutralization reactors,DAP production processes can be divided into pre-neutralization and ammonification granulation process,tubular reaction granulation process,dry spray granulation process,etc [21,22].China is one of the countries that use the most types of DAP process in the world with mature production technology and large output.In 2018,the physical output of DAP was about 16.2 million tons[23].At present,the production capacity of a single set of DAP plant is given priority to with 600 kt·a-1and it adopts the tubular reaction–ammoniation granulation process.This process has the advantages of low energy consumption,good product appearance,and easy scale-up.The improvement of DAP process is mainly reflected in the in-depth treatment of exhaust gas,such as the increase of electric defogging,the reduction of humidity,and the reduction of dust emission concentration.The main problem with the DAP production process is that the tubular reaction would formulate scaling during operation and cannot run continuously for a long period.Besides,since DAP produced from different phosphorus ores may have different colors,each manufacturer has a process of dyeing it into coffee color and it is worth exploring whether the dyeing process is necessary actually.

Fig.2.The flow chart of DAP process.

2.3.Industrial grade MAP

Industrial grade MAP is mainly used as fire extinguishing agent and water soluble fertilizer for drip irrigation and the demand is increasing at present [24].Especially with the popularization and promotion of fertigation technology,water soluble fertilizer has been developed rapidly in recent years,which has led to the development and growth of industrial grade MAP.The production processes of MAP at home and abroad mainly include:furnaceprocess phosphoric acid neutralization method,wet-process phosphoric acid organic extraction purification method and wet-process phosphoric acid chemical purification method.The main advantages and disadvantages of various production processes are shown in Table 1.The furnace-process phosphoric acid neutralization method is simple and the product is of high quality.However,furnace process phosphoric acid is limited by resources,environment,energy and other factors,and the production process is of high energy consumption,high cost and high technical requirements,which leads to the high price of industrial grade MAP produced by this method.The route using wet-process phosphoric acid as raw material to produce industrial grade MAP were very mature as early as in the 1980s overseas and Italy,the United States,Russia,Japan and other developed countries all had mastered mature technology [25].The development of the technical route of preparing industrial grade MAP from wet-process phosphoric acid was twists and the furnace-process phosphoric acid neutralization method was used as the main route,accounting for more than 90% of total industrial MAP production capacity in China.Since 2003,the price of yellow phosphorus had risen sharply due to high power price and other reasons,which made it was difficult to maintain industrial grade MAP production by furnace process phosphoric acid neutralization method with low profit,and many companies had stopped production gradually.Although it was still technically difficult to produce an industrial grade MAP from wet-process phosphoric acid at that time,the low production cost had attracted the interest of many researchers and enterprises [26–28].

Table 1 Comparison of industrial grade MAP production process

After the development in recent years,the production method has gradually shifted from furnace-process phosphoric acid neutralization method to wet-process phosphoric acid organic extraction purification method and wet-process phosphoric acid chemical purification method.Production of industrial MAP by extraction phosphoric acid requires a large investment,and it is mainly used in downstream manufacturers with a series of fine phosphates.The as-obtained industrial grade MAP particles are not uniformly,and the product appearance is poor.At the same time,it is difficult to reuse the raffinate acid.With the depletion of phosphate rock,the competitiveness of this method graduallydeclines.Regarding the production of industrial grade MAP technology by wet process phosphoric acid chemical purification,Sichuan University developed and applied it in the original Shifang Chemical Plant (a part of Hongda Group,Now) in the late 1980s and early 1990s.However,this technology was shelved and was not promoted due to market and other factors.In 2003,Sichuan University conducted an industrial expansion test using this technology in Sichuan Blue Sword Chemical (Group) Co.,Ltd.,and further improved this technology and reduced the cost of the product.In 2004,an industrial MAP production plant of 25 kt·a-1was completed and produced qualified industrial grade MAP product.In the following 10 years,Sichuan Mianyang Chuanyin Chemical,Yunnan Yuntianhua Co.,Ltd.(Yunfeng Branch),Sichuan Mianzhu Sanjia Feed Co.,Ltd.,and Yunnan Ciangfeng Group built 10–50 kt·a-1industrial grade MAP plants by using above method,respectively,and the product index has reached all the indexes of industrial grade MAP produced by furnace process phosphoric acid neutralization method.Through the continuous efforts of Sichuan University,the technology has many characteristics,such as full-process DCS process control,three-stages ammoniation and purification,three effect concentration of refined ammonium phosphate solution,high efficiency steady state continuous crystallization,fully closed mother liquor cycle,and so on [28–33].It is in a leading position in technology among similar products at home and abroad.Since Sichuan University successfully developed and put into industrial production in 2003,the chemical purification method has attracted much attention in the industry.Especially in recent years,many companies have successively adopted chemical purification method to produce industrial grade MAP.At present,the output of industrial grad MAP in the domestic market is about 2 million t·a-1.The basic process flow of industrial grade MAP production is shown in Fig.3.Industrial grade MAP is obtained though ammoniating purification,filtration,evaporation and concentration,crystallization,drying and other steps,meanwhile,white phosphate fertilizer is a by-product.The key to this process is whether there is a supporting device for resource utilization of mother liquor and white phosphate fertilizer.

2.4.Water soluble MAP

As mentioned above,industrial grade MAP is mainly used as water soluble fertilizer and fire extinguishing agent,but its cost is relatively high.Therefore,on the basis of meeting the application conditions of water soluble fertilizer or fire extinguishing agent,appropriately reducing the degree of ammonia purification can produce the water-soluble MAP (N+P2O5:66%–72%) with relatively low production cost and high yield of phosphorus.The differences between this technology and the industrial MAP production technology are as follows:first,the degree of ammonia purification is reduced,and the yield of phosphorus is improved with less white fertilizer.Second,industrial grade MAP is obtained by cooling and crystallization,while water soluble MAP is obtained by spray drying in a drying tower.At present,Chengdu Wintrue Holding Co.,Ltd.,Sichuan Chanhen Holding Group Co.,Ltd.,Sichuan Hongda Co.,Ltd.,etc.use this technology to produce water soluble MAP and the production is hundreds of thousands of tons each year.

Fig.3.The process flow chart of industrial grade MAP.

It is worth mentioning that either the solvent extraction method or the chemical purification method is used to produce industrial or water-soluble MAP,the metal ion impurities in the phosphoric acid must be removed first.In order to prepare the fertilizer with medium and trace element,it is necessary to add a variety of chelated medium and trace elements to the basic fertilizer.The removing firstly and then adding medium and trace elements not only wastes the resources,but also increases the production cost.At the same time,it easily leads to pipe plugging during fertigation due to uneven mixing and the existence of insoluble matter.Therefore,on the basis of ammoniation and neutralization of wet-process phosphoric acid to produce MAP,Sichuan University proposed an in-situ chelation technology to produce water soluble MAP fertilizer with medium and trace elements by using wet-process phosphoric acid in recent years.The Chelating properties of metal cations in wet-process phosphoric acid by chelating agents such as citric acid were systematically explored,and water soluble MAP fertilizer containing medium and trace elements was prepared.A variety of chelating formulations were developed and a complete set of technology and system integration was formed [34–38].The technical route is shown in Fig.4 and the water soluble MAP containing medium and trace elements is manufactured by the neutralization,filtration,chelation reaction,evaporation concentration and drying.The products not only retain the original middle and trace elements in the wet-process phosphoric acid,but also increase the phosphorus yield,and enhance the product competitiveness.This technology is the first in China,and the products obtained have also shown good results in farmland trials,which can replace industrial MAP.Jiashili(Jingzhou) Fertilizer Co.,Ltd.Jiashili (Yicheng) Fertilizer Co.,Ltd.,etc.,have built ten thousand tons of industrial production plant.At present,the output of the water soluble MAP containing medium and trace elements is still small and a large amount of fertilizer efficiency verification tests are needed to further verify the product reliability.

Fig.4.The process flow chart of the water soluble MAP containing medium and trace elements.

2.5.Water soluble APP

Ammonium polyphosphates(APP)can be grouped into an inorganic polymer with P-O-P alternating structure.As a downstream product of the phosphate industry,APP with different crystal forms contribute in various fields[39].The form I or amorphous APP with low polymerization degree(n,defined as the number of monomer unit in polyphosphate chains)has become a new compound fertilizer containing nitrogen and phosphorus because of its desirable water solubility,slow release of nutrients,chelation of medium and trace elements,and improvement of soil [3,40,41].The n of agricultural APP is generally less than 10 for smoothly dissolved as the base of fluid fertilizer formula.European and American countries started the APP study earlier [42–44].In the 1960s and 1970s,Tennessee Valley Authority (TVA) developed a water soluble APP with crop-suitable distribution of polymerization degree and successfully industrialized it.Its commonly used specifications were defined 11-37-0 (N-P2O5-K2O) and 10-34-0.While in China,the agricultural APP emerged as late as 1990s.Lu [45] in Sichuan University prepared a lab-scale water soluble APP via polycondensation of monoammonium phosphate.Then the water soluble APP got slowly developed.Until the last two decades,the technologies and involved mechanisms has entered a rapidly advanced period in China.Institutions or corporations such as Sichuan University,YunTianHua Group,ChanHen Co.,Ltd.,and Sinochem Holdings etc.have made certain progress in the industrialization of water soluble APP [3,46–50].

The current mainstream technical routes of water-soluble APP are shown in Fig.5,mainly includes superphosphoric acid process and phosphoric acid/ammonium phosphate–urea process.The former route (Fig.5a) uses TVA tubular reactor to produce watersoluble APP,with the main fluid product of 11-37-0,in which phosphorous species coverSuperphosphoric acid process can realize a reliable and continuous operation,but the raw superphosphoric acid (105% or 115% H3PO4) used is costly especially.The typical manufacturers include Sino-Linchem Group and Sinochem Jilin Changshan Chemical Co.,Ltd.[51].The later route (Fig.5b) is the mainstream for the production of watersoluble APP in China.The current industrial producing is usually applied in a kneading machine platform[52,53].This batch process is simple and has relatively low production cost,but the product quality is not stable and the main product is ammonium pyrophosphate.The representative enterprises in China are Changfeng Chemical Co.,Ltd,Tianyao Chemical Co.,Ltd.,Chanhen Group,and Shindoo Chemi-Industry Co.,Ltd.,etc.[3]Current APP specifications include 18-58-0 and 14-65-0,and the polymerization degree distribution is justandin general.Although the watersoluble APP in China got through a more than 10 years of growth,the industrialization is still in the initial stage.Therefore,the urgent issue to be solved is how to achieve low-cost,continuous and large-scale production of wide-distribution APP.In recent years,Sichuan University has focused on the breakthrough of the controllable preparation of APP and realized the running of a 10,000-ton scale plant in Yunnan Yuntianhua Co.,Ltd.(Yunfeng Branch).The resulting product was 14-65-0 and the distribution has coveredtowhich is similar to the distribution of the 11-37-0 APP from TVA technology.The product quality is excellent and has a good application prospect.

Fig.5.Two typical processes of water soluble APP.

3.Conclusions

In this paper,the production methods,development history and the latest research progress of ammonium phosphate fertilizer products,including slurry MAP,DAP,industrial MAP,water soluble MAP,water soluble APP,etc.,were reviewed.The current mainstream technical means of them are APPSC process,tubular reaction-ammoniation granulation process,wet-process phosphoric acid chemical purification process,ammonia purificationspray drying process and ammonium phosphate-urea process respectively.In general,China’s ammonium phosphate series fertilizers have formed the situation of overcapacity,and it is urgent to transform and upgrade the industrial phosphates via developing new and highly effective products.As the traditional phosphate fertilizers(i.e.,slurry MAP and DAP)productive units are supported by stable equipment,mature technology,and large output and their development space is limited in the future.While industrial and water soluble MAPs can be used as the raw material of sprinkling or drip irrigation,due to their water solubility,which can greatly save water and improve the nutrient utilization efficiency,thus exhibiting a desirable prospect.Water soluble APP may become one direction to evoke the modern agriculture,but at present,the domestic technology and product quality need to be further improved.To this end,the following suggestions are put forward:

(1) It should promote industrial integration and eliminate stagnated production capacity.China’s traditional ammonium phosphate capacity is seriously excess.We should promote the deep integration of the industry,and proceed with capacity elimination to guide a healthy and long-term development of the phosphate industry,to ensure the long-term supply of phosphorus resources in China.

(2) It should exploit actively the new ammonium phosphate products,such as water-soluble MAP containing medium and trace nutrients,and water soluble APP etc.,to promote the transformation and upgrading of phosphate manufacturers and improve phosphorus utilization efficiency.

(3) It should build a green phosphorus chemical process.The full life-cycle assessment of the phosphate process during enterprise operation should be put on the agenda.The recycling and utilization of associated resources such as fluorine,iodine,calcium,magnesium and iron from phosphate ore should become a consideration in judging a manufacturing license.In addition,the proper disposal and comprehensive utilization of waste resources like phosphogypsum should be also strengthened.Through these measures,the green processing of phosphate ore will be realized,the utilization efficiency of phosphorus resource will be improved,and the strategic supply of national phosphorus resource will be guaranteed continuously.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.