CN115232891A

CN115232891A - Leather-making ammonium-free deliming agent with grain surface protection function and deliming method

Info

Publication number: CN115232891A
Application number: CN202210901037.6A
Authority: CN
Inventors: 张春晓; 魏世伟; 彭必雨; 张旭
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2022-07-28
Filing date: 2022-07-28
Publication date: 2022-10-25
Anticipated expiration: 2042-07-28
Also published as: CN115232891B

Abstract

The invention discloses a tanning ammonium-free deliming agent with grain surface protection function and a deliming method, wherein the deliming agent comprises the following components in parts by weight: 5-30 parts of aromatic sulfonic acid and 1-20 parts of phosphate; the deliming method comprises the following steps: step 1: cleaning the tannery liming leather, adding water, and keeping constant temperature; step 2: adding a deliming agent, wherein the deliming agent accounts for 1.0-3.5 wt% of the weight of the ash dipping skin, and discharging liquid after deliming is finished; the aromatic sulfonic acid and dihydric phosphate synergistic effect of the deliming agent has good permeability and buffering performance, and meets the deliming requirement; crust leather produced by deliming with the deliming agent meets the requirements on the sense and mechanical properties, and the grain surface of the crust leather is protected, so that the damage caused by enzyme in the softening process can be avoided; the total nitrogen concentration of the wastewater and ammonia nitrogen in the process of deashing is respectively reduced by 99 percent and 94 percent; provides a new idea for developing a clean deliming technology and improving the quality of finished leather.

Description

Ammonium-free leather-making deliming agent with grain surface protection function and deliming method

Technical Field

The invention relates to the technical field of leather deliming agents, in particular to a leather-making ammonium-free deliming agent with a grain surface protection function and a deliming method.

Background

The tanning is used as an auxiliary industry of the livestock breeding industry, solves the problem of resource utilization of about 700 million tons of animal skins every year, and is a sustainable development processIt is an important link in economy. Leather making is also an industry with large water resource demand and serious environmental pollution. According to different leather varieties and raw leather sources, about 45-70 m of fresh water is consumed for producing 1 ton of raw leather ³ . Meanwhile, 145-231 kg of COD, 12-18 kg of nitrogen, 4-9 kg of sulfide and other pollutants are generated. Therefore, tanning is considered as one of the most serious industries in environmental pollution and is a main factor limiting the sustainable development of the tanning. In order to reduce the pollution of the tanning industry to the environment, each tannery in China must be equipped with an independent sewage treatment plant to enhance the terminal treatment of the tanning sewage, and the concentration of pollutants such as carbon, nitrogen, phosphorus and the like in the treated wastewater must reach the relevant standard before the pollutants can be discharged.

Deliming-softening is a key process of leather preparation work, plays an important role in neutralizing alkaline substances such as lime, sodium hydroxide and sodium sulfide added in the liming process, eliminating the alkali swelling state of pelts, and treating the pelts with an enzyme preparation at pH 7.0-9.0 to meet the style and performance requirements of different leather varieties. The ammonium salt has the advantages of good buffer performance, high permeation speed, low price, capability of stabilizing the enzyme activity performance and the like, and is the most widely applied deliming material. However, NH in ammonium salt deashing waste liquor ₃ the-N concentration is as high as 2000mg/L to 4000mg/L, and occupies NH in the tanning process ₃ More than 80% of N emission is one of the main factors causing overlarge N content in the wastewater, causing unbalance of C/N and N/P and influencing the comprehensive sewage treatment effect and recycling efficiency. In order to discharge the nitrogen content of the treated comprehensive wastewater up to the standard, auxiliaries such as magnesium ammonium phosphate and the like are generally required to be added to promote anaerobic and aerobic treatment and enhance denitrification effect, so that the cost and time for wastewater treatment are increased.

The supercritical carbon dioxide can achieve a good deliming effect, can avoid the use of water in the deliming process, is one of effective measures for controlling ammonia nitrogen and total nitrogen emission, but has higher equipment requirement and is difficult to effectively popularize. Boric acid is one of the best deliming materials to replace ammonium salts due to its superior buffering and permeability properties. But its use in tanning is increasingly limited due to its greater reproductive toxicity. Glycine instead of ammonium salt also gave good deliming effect and reduced ammonia nitrogen emissions, but the total nitrogen control effect was not ideal. Magnesium precipitates with hydroxide to form magnesium hydroxide, and attempts have been made to use magnesium salts with a certain buffering property at a ph of about 10.0 for deliming. However, deliming with magnesium salt alone shows a phenomenon of being impervious to dealkalization, i.e., it is difficult to lower the pH to 10.0 or less. Organic weak acids such as formic acid, glycol acid, carbonate and the like can also achieve the deliming effect, but the buffering performance is poor, so that pelts are easy to generate acid swelling, and the organic weak acids are mainly matched with ammonium salt or boric acid for use.

The deliming material at least needs to meet 3 conditions and has certain acidity, so that alkaline substances inside and outside the pelts can be effectively neutralized; 2) Has better permeability in the naked skin, but can not cause the naked skin to be acid and swollen; 3) The buffer performance is better at about 7.0-9.0, and necessary conditions can be provided for softening. Aromatic sulfonic acid is a medium strong acid, has good permeability in the pelts, can prevent the pelts from swelling in the range of pH3.0 or less, and has been applied to the development of salt-free pickling technology; however, most aromatic sulfonic acids do not have buffering properties and the pH at the end of deliming is difficult to control to meet softening requirements, so deliming is not generally performed separately.

And the prior deliming material and the prior deliming technology do not have the function of protecting the grain surface, and the excessive action of enzyme on the grain surface is easy to cause the grain surface damage in the softening process.

Disclosure of Invention

The invention provides a tanning ammonium-free deliming agent with a grain surface protection function and a deliming method aiming at the problems in the prior art.

The technical scheme adopted by the invention is as follows: a tanning ammonium-free deliming agent with grain surface protection function comprises the following components in parts by weight: 5 to 30 portions of aromatic sulfonic acid and 1 to 20 portions of phosphate.

Further, the aromatic sulfonic acid is one or two or more of sulfosalicylic acid, phenolsulfone sulfonic acid, p-hydroxybenzene sulfonic acid, naphthalene disulfonic acid, naphthalene formic acid sulfonic acid and naphthol sulfonic acid which are mixed in any proportion.

Further, the phosphate is one or more of potassium phosphate, sodium phosphate, potassium hydrogen phosphate, sodium hydrogen phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, metaphosphate, phosphite and polyphosphate which are mixed according to any proportion.

A deliming method of a tanning ammonium-free deliming agent with grain surface protection function comprises the following steps:

step 1: cleaning the tannery liming leather, adding water, and keeping constant temperature;

and 2, step: adding a deliming agent, wherein the deliming agent accounts for 1.0-3.5 wt% of the weight of the ash dipping skin, and discharging liquid after deliming is finished.

Furthermore, the amount of water in the step 1 accounts for 50% of the mass of the ash dipping skin, and the temperature is kept at 25-40 ℃.

Further, the ash removal time in the step 2 is 40-240 min, and the pH value in the ash removal process is 7.0-9.0.

The beneficial effects of the invention are:

(1) The aromatic sulfonic acid and dihydric phosphate synergistic effect of the deliming agent has good permeability and buffering performance, and meets the deliming requirement;

(2) The crust leather produced by deliming with the deliming agent meets the requirements on the sense and mechanical properties, and the grain surface of the crust leather is protected, so that the damage caused by enzyme in the softening process can be avoided;

(3) The total nitrogen concentration of the wastewater and ammonia nitrogen in the deashing process of the deashing agent obtained by the invention is respectively reduced by 99 percent and 94 percent;

(4) In the deliming agent obtained by the invention, dihydric phosphate forms a precipitate on the surface of a naked skin so as to protect the grain surface and avoid the excessive action of enzyme on the surface; on the other hand, the aromatic sulfonic acid permeates into the skin, effectively worsens the alkali in the skin and promotes the loosening effect of the enzyme on the collagen fibers.

Drawings

FIG. 1 is a graph showing the effect of the deliming agent on the buffering capacity in examples 4 to 6 of the present invention and comparative example.

FIG. 2 is a schematic diagram showing the effect of the addition amount of the deliming agent on the deliming effect in the present invention.

FIG. 3 is a graph showing the concentration of soluble protein in the softening bath during the leather softening process according to the different softening methods of the present invention.

FIG. 4 is SEM images of the scanning electron micrographs of leather obtained by different treatment methods of the invention, wherein a is the grain surface morphology, b is the section of the nipple layer, and c is the reticular layer fiber.

Detailed Description

The invention is further described with reference to the following figures and specific examples.

An ammonium-free leather-making deliming agent with a grain surface protection function can protect the grain surface of a leather-making bare leather and avoid the damage caused by the excessive action of an enzyme preparation in the softening process. Comprises the following components in parts by weight: 5 to 30 portions of aromatic sulfonic acid and 1 to 20 portions of phosphate. The aromatic sulfonic acid is one or more of sulfosalicylic acid, phenolsulfone sulfonic acid, p-hydroxybenzene sulfonic acid, naphthalene disulfonic acid, naphthalene formic sulfonic acid and naphthol sulfonic acid which are mixed in any proportion. Preferably p-hydroxybenzenesulfonic acid. The phosphate is one or more of potassium phosphate, sodium phosphate, potassium hydrogen phosphate, sodium hydrogen phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, metaphosphate, phosphite and polyphosphate which are mixed in any proportion. Sodium dihydrogen phosphate is preferred.

A deliming method of a tanning ammonium-free deliming agent with grain surface protection function can be used for deliming any tanning raw leather, and comprises the following steps:

step 1: cleaning the tannery liming leather, adding water, and keeping constant temperature; the water accounts for 50 percent of the mass of the ash soaking skin, and the temperature is kept between 25 and 40 ℃.

And 2, step: adding a deliming agent, wherein the deliming agent accounts for 1.0-3.5 wt% of the weight of the ash dipping skin, and discharging liquid after deliming is finished. The deashing time is 40-240 min, and the pH value in the deashing process is 7.0-9.0.

The calfs used in the following examples were limitted calfs head layers (approximately 3.0mm thick) and other reagents were as follows:

ammonium sulfate (AR, chengdu jinshan chemical reagent factory); sodium dihydrogen phosphate (SPD, AR, chengdu jinshan chemical reagent factory); sulfosalicylic acid (SSA, AR, chengdu jinshan chemical reagent works); phenolsulfone sulfonic acid (PSSA), p-hydroxybenzenesulfonic acid (PHBA), naphthalene Sulfonic Acid (NSA), laboratory synthesis; alkaline protease (Elastase substrate Activity 210,000u/g (pH8.0); novoxin); trypsin (collagen substrate activity 360,000u/g (pH 8.0); novoxin); chrome tanning agent (Chromosal B, langerhans); other chemicals were analytical reagents.

Example 1

A deliming method of a tanning ammonium-free deliming agent with a grain surface protection function comprises the following steps:

step 1: taking cowhide from the symmetrical part of the cowhide head layer after the secondary liming along the dorsal ridge line, marking and weighing the cowhide head layer to be used as a material reference. Washing with tap water 200 wt% twice at room temperature for 30min each time. The mass was placed in a drum, the water was adjusted to 50% of the mass in the ash, and the temperature was maintained at 33 ℃.

Step 2: adding a proper amount of sulfosalicylic acid SSA, wherein the concentration of hydrogen ions is controlled to be the same, and the weight percentage of the deliming agent in the cowhide is 1.6 wt%; rotating for 60min, adding appropriate amount of sulfosalicylic acid, rotating for 120min, measuring bath lotion every other time, and performing phenolphthalein detection on the nude skin incision.

Example 2

A deliming method of a tanning ammonium-free deliming agent with grain surface protection function, the other steps are as in example 1, except that the deliming agent adopts p-hydroxybenzenesulfonic acid PHBA.

Example 3

The other steps of the deliming method of the leather-making ammonium-free deliming agent with the grain surface protection function are as example 1, except that the deliming agent adopts phenol sulfone sulfonic acid PSSA.

Example 4

The other steps of the deliming method of the ammonium-free deliming agent for leather making with the grain surface protection function are as in example 1, except that the deliming agent adopts naphthalenesulfonic acid NSA.

After deliming, the influence of the molecular structure of the aromatic sulfonic acid on the deliming effect is illustrated by testing the pH value of the bath solution, the color development of the nude skin incision phenolphthalein and the thickness change condition. The results are shown in Table 1.

TABLE 1 deashing Effect of aromatic sulfonic acid alone

* : the nude incised phenolphthalein indicator is colorless.

Leather-making deliming is a heterogeneous neutralization reaction process, and a deliming material needs to effectively permeate into skins to achieve a good deliming effect. As can be seen from the thickness change rate of the pelts after deliming, the expansion state of the pelts can be effectively eliminated by the four aromatic sulfonic acids, and the pelts are not swollen by acid. SSA and PHBA with a single-ring structure can effectively neutralize alkali in the nude skin after rotating for 120min, and the nude skin incision phenolphthalein is colorless when detected. After the PSSA and NSA with the double-ring structure rotate for 150min, alkali in the skin is still difficult to neutralize, the incision center still shows red color during phenolphthalein inspection, and the pH value of bath liquid is relatively lower. Therefore, the monocyclic aromatic sulfonic acid is easier to quickly permeate in the skin than the bicyclic aromatic sulfonic acid, so that the alkali inside and outside the naked skin is effectively neutralized, and the deliming effect is achieved. However, SSA easily reacts with trace iron to form a pink complex, so that the surfaces of the naked leather and the wet blue leather are red, and PHBA is preferred.

The swelling reducing rate of the pelts is calculated according to the following method:

marking 10 points on the pelts, measuring the thickness of each point before and after deliming, and then taking the average value of the thickness change rate of the 10 points to represent the deliming degree.

The peltry detumescence rate (%) = (thickness of washing ash bark-thickness of washing ash bark) × 100%/thickness of washing ash bark.

Example 4

step 1: and (4) taking cowhide from the symmetrical adjacent position of the head layer of the re-liming cowhide along the dorsal ridge line, marking and weighing the cowhide, and taking the cowhide as a material reference. The mixture was washed twice with 200% by weight tap water at room temperature for 30min each time. The skin pieces were placed in a drum, the water was adjusted to 50% of the grey skin, and the temperature was maintained at 33 ℃.

And 2, step: adding a deliming agent, wherein the deliming agent accounts for 2.0wt.% of the weight of the cowhide; deliming for 120min, measuring bath liquid at one end of interval, and performing phenolphthalein detection on the nude skin incision. Wherein the mass ratio of PHBA to SPD in the deliming agent is 1.

Example 5

step 1: taking cowhide from the symmetrical part of the cowhide head layer after the secondary liming along the dorsal ridge line, marking and weighing the cowhide head layer to be used as a material reference. The mixture was washed twice with 200% by weight tap water at room temperature for 30min each time. The mass was placed in a drum, the water was adjusted to 50% of the mass in the ash, and the temperature was maintained at 33 ℃.

Step 2: adding a deliming agent, wherein the deliming agent accounts for 2.0wt.% of the weight of the cowhide; deliming for 120min, measuring bath liquid at one end of interval, and performing phenolphthalein detection on the nude skin incision. Wherein the mass ratio of PHBA and SPD in the deliming agent is 2.

Example 6

step 1: and (4) taking cowhide from the symmetrical adjacent position of the head layer of the re-liming cowhide along the dorsal ridge line, marking and weighing the cowhide, and taking the cowhide as a material reference. The mixture was washed twice with 200% by weight tap water at room temperature for 30min each time. The mass was placed in a drum, the water was adjusted to 50% of the mass in the ash, and the temperature was maintained at 33 ℃.

Step 2: adding a deliming agent, wherein the weight percentage of the deliming agent to the cow leather is 2.0wt.%; deashing for 120min, measuring bath lotion every other time, and performing phenolphthalein detection on the nude skin incision. Wherein the mass ratio of PHBA and SPD in the deliming agent is 2.5.

Comparative example 1

A deliming method of a deliming agent comprises the following steps:

step 1: and (4) taking cowhide from the symmetrical adjacent position of the head layer of the re-liming cowhide along the dorsal ridge line, marking and weighing the cowhide, and taking the cowhide as a material reference. Washing with tap water 200 wt% twice at room temperature for 30min each time. The skin pieces were placed in a drum, the water was adjusted to 50% of the grey skin, and the temperature was maintained at 33 ℃.

Step 2: adding a deliming agent, wherein the deliming agent accounts for 2.0wt.% of the weight of the cowhide; deashing for 120min, measuring bath lotion every other time, and performing phenolphthalein detection on the nude skin incision. Wherein the mass ratio of PHBA and SPD in the deliming agent is 0.

PHBA used alone for deliming can effectively neutralize the alkaline substance in the skin, but the buffering performance is poor, and the pH value is easy to drop suddenly or the end point pH value is difficult to control in the deliming process. Therefore, there is a need to further improve the buffering performance of PHBA deliming systems. Sodium dihydrogen phosphate SPD is a gradually ionized acid salt, has good buffer performance in the pH value range of 6.0-8.0, and is commonly used for preparing buffer solution. And a proper amount of SPD is added to improve the buffer performance in the deliming process of the PHBA, so that a novel ammonium-free deliming system can be constructed. The results of examples 4 to 6 and comparative example are shown in FIG. 1.

As can be seen from the figure, after being mixed with SPD, the buffering performance of PHBA when being singly deashed is improved, the initial pH value of the bath liquid can be kept above 5.0, and the pH value of the bath liquid can be quickly increased back to 7.5-8.0 after being rotated for about 80min, which shows that the PHBA-SPD mixture has better intradermal permeability. When the mixing ratio of PHBA and SPD is about 2.0-2.5 (w/w), the alkali in the skin and the core can be effectively neutralized within about 80min, the peltry incision phenolphthalein is colorless, SPD can be prevented from forming precipitates with calcium ions in bath liquid, the deliming bath liquid is in a clear state, and the deliming requirement can be met. The results are shown in Table 2.

TABLE 2 influence of PHBA on deliming Performance

Example 7

And 2, step: adding a deliming agent, wherein the deliming agent accounts for 1.2wt.% of the weight of the cowhide; deashing for 120min, measuring bath lotion every other time, and performing phenolphthalein detection on the nude skin incision. Wherein the mass ratio of PHBA and SPD in the deliming agent is 2.

Example 8

Step 2: adding a deliming agent, wherein the deliming agent accounts for 1.6wt.% of the weight of the cowhide; deashing for 120min, measuring bath lotion every other time, and performing phenolphthalein detection on the nude skin incision. Wherein the mass ratio of PHBA and SPD in the deliming agent is 2.

Example 9

step 1: and (4) taking cowhide from the symmetrical adjacent position of the head layer of the re-liming cowhide along the dorsal ridge line, marking and weighing the cowhide, and taking the cowhide as a material reference. Washing with tap water 200 wt% twice at room temperature for 30min each time. The mass was placed in a drum, the water was adjusted to 50% of the mass in the ash, and the temperature was maintained at 33 ℃.

Step 2: adding a deliming agent, wherein the weight percentage of the deliming agent to the cow leather is 2.5wt.%; deashing for 120min, measuring bath lotion every other time, and performing phenolphthalein detection on the nude skin incision. Wherein the mass ratio of PHBA and SPD in the deliming agent is 2.

One of the main purposes of deliming is to adjust the pH of the pelts to 7.0-9.0 in preparation for subsequent enzymatic softening, examples 7, 7-9 being the mass ratio of PHBA and SPD in the deliming agent 2:1, the influence of the dosage of the deliming agent on the change rule of the pH value in the deliming process. As can be seen from figure 2, when the dosage of the PHBA-SPD mixture is increased from 1.2 percent to 2.5 percent, the initial pH value of the bath liquid is reduced from 5.8 to about 4.9, and can be increased back to about 7.0 within 25-50 min, the pelts are white and soft, and no swelling phenomenon occurs; when the total deliming time is about 120min, the naked skin cuts are uniformly milky, and when the dosage is more than 1.6 percent, the delimed naked skin cuts are colorless after being checked by phenolphthalein, so that a good deliming effect can be achieved; moreover, when the dosage of the PHBA-SPD mixture is increased from 1.6 percent to 2.5 percent, the pH value of the end point is only reduced to 7.8 from 8.4, and the PHBA-SPD deashing has good process stability, can provide stable conditions for a softening process and avoids the influence on the subsequent softening process.

From examples 5 and 7 to 9, it can be seen that PHBA and SPD are mixed according to the proportion of 2 (w/w), the dosage of the PHBA and SPD is about 1.6 percent of the weight of the ash bark, and the excellent deliming effect can be achieved, and the necessary conditions can be provided for the softening process, so that the purposes of ammonium-free and boron-free deliming can be achieved.

Comparative example 2

A deliming method of an ammonium sulfate deliming agent comprises the following steps:

Step 2: adding a deliming agent, wherein the deliming agent accounts for 3.0wt.% of the weight of the cowhide; deashing for 120min, controlling pH value to 8.0-9.0, draining liquid, measuring bath liquid at one end of each interval, and performing phenolphthalein detection on the nude cut. Wherein the deliming agent is ammonium sulfate.

Comparative example 3

A deliming method of ammonium dihydrogen phosphate deliming agent comprises the following steps:

Step 2: adding a deliming agent, wherein the deliming agent accounts for 1.5wt.% of the weight of the cowhide; deashing for 120min, controlling the pH value to be 8.0-9.0, and discharging liquid. And measuring the bath lotion at one end of each interval, and performing phenolphthalein detection on the nude skin incision. Wherein the deliming agent is sodium dihydrogen phosphate.

The results after deashing for example 8, comparative example 2 and comparative example 3 are shown in table 3.

FIG. 3 results after deliming

As can be seen from the table, when the SPD is used alone for deliming, the rotating is carried out for 120min, the peltry incision still has a yellow center, the incision phenolphthalein examination has a 1/3 red center, the thickness change rate is only about 25.7 percent, which indicates that the SPD alone is not delimed to effectively neutralize the alkali in the skin, and the inside of the peltry is still in an expansion state; when the PHBA-SPD is delimed, the initial pH value is about 5.68, but after rotating for 120min, the pH value of the bath solution rises back to about 8.24, which can provide necessary conditions for softening; moreover, the pelts are soft, the thickness change rate is about 32.5%, the cut is milky white, phenolphthalein is colorless when being checked, and the integral state of the pelts is equivalent to that of the pelts delimed by ammonium sulfate; this again shows that the PHBA-SPD mixed system has good deliming effect and can meet the requirement of leather making.

The delimed ashed cow leather of example 8, comparative example 2 and comparative example 3 was washed with 200% tap water having a temperature of about 33 c for 30min each time 2 times. The water-liquid ratio is readjusted to be 50% of the weight of the ash bark. Adding 1.0% diluted alkaline protease and trypsin (activity diluted to 1000U/mL) at 33 deg.C, softening for 240min, draining, and washing with water. The synergistic deliming effect of PHBA-SPD is demonstrated by softening the properties of total protein, desmosine, hydroxyproline and fat-liquoring crust leather in bath liquid.

The method for measuring soluble protein in softening bath liquid is as follows:

centrifuging the softened bath lotion at 3500rpm for 5min every 60min, diluting the supernatant, and measuring the soluble protein content of the bath lotion by Folin-phenol reagent method.

The method for measuring hydroxyproline and desmosine in softening bath solution comprises the following steps:

centrifuging 1.50mL of softened bath solution at 8000rpm for 6min, collecting 1.00mL of supernatant, adding 1.0mL of 12mol/L concentrated hydrochloric acid, digesting at 120 deg.C for 12 hr, deacidifying, concentrating, diluting, and measuring hydroxyproline and desmosine concentration in the bath solution with A3000 amino acid analyzer.

Softening is a key process immediately after deliming, and plays an important role in supplementing and removing leather-making useless substances, supplementing loose collagen fibers and moderately hydrolyzing collagen to meet the performance requirements of different leather varieties; the softening and adjusting of the necessary conditions are one of the important purposes of the deliming process; whether or not appropriate conditions can be provided for softening is a prerequisite for evaluating the practicability of the deliming technology. Therefore, after the pelts are delimed and washed by water, the liquid ratio and the temperature are respectively adjusted, a softening enzyme preparation is added, and the changes of soluble protein in bath liquid and the concentrations of hydroxyproline and desmosine in the softening process are examined to represent the influence of different deliming methods on the softening effect. The results are shown in table 4 and fig. 3.

TABLE 4 concentrations of hydroxyproline and desmosine in the softening baths

As can be seen from fig. 3, the content of soluble protein in the softening bath gradually increased with the increase of the softening time. The concentrations and the growth rates of soluble protein in the ammonium sulfate and PHBA-SPD deliming pelt softening bath liquid are not obviously different, and the concentrations of hydroxyproline and desmosine are not greatly different; when the SPD is used alone for deliming, the soluble protein concentration, the hydroxyproline concentration and the desmosine concentration in the bath solution are all lower.

This is because SPD alone, when delimed, does not effectively neutralize the alkali inside the pelt, affecting the enzymatic activity performance. On the other hand, SPD permeates into the skin and forms a mineralized layer on the surface of fibrin in the naked skin together with calcium ion insoluble calcium salt, and the action of enzyme on collagen and elastin is not beneficial. The PHBA-SPD is used for cooperatively deliming, so that the alkali inside and outside the pelts can be effectively neutralized, the whole state of the delimed pelts is equivalent to that of ammonium sulfate delimed pelts (Table 3), the SPD content in the PHBA-SPD is low, only a mineralized layer is formed on the surface of the pelts to avoid the excessive action of enzyme on the surface of the pelts, and the penetration of the enzyme into the skin and the action on an intracutaneous protein substrate are not influenced, so that the softening effect is not influenced, the grain surface damage degree can be reduced, and the high-grade rate of finished leather is favorably improved.

After the delimed cowhide of example 8, comparative example 2 and comparative example 3 was softened, the conventional cowhide sofa leather process was then subjected to the following operations of pickling, chrome tanning, squeezing, shaving, backwater, neutralization, fatliquoring, etc., and after drying and softening, the sample was taken from a portion near the symmetrical portion along the ridge line, and the morphology of the cut papilla layer, the network layer fiber and the grain surface was observed by a scanning electron microscope, and the result is shown in fig. 4. After being dried and flaked, the retanning-fatliquoring crust leather is symmetrically sampled at the adjacent part along the ridge line, and the mechanical performance is analyzed according to the national standard method (QB/T2707-2005 QB/T2710-2005 QB/T2711-2005.

As can be seen from FIG. 4, the damage of the grain surface of the ammonium sulfate delimed crust leather is serious, and the grain surfaces of the PHBA-SPD and SPD delimed crust leather are complete; but the ammonium sulfate and PHBA-SPD deliming crust leather collagen fiber is higher in loose degree and is softer and plump. The reason is that the SPD reacts with calcium ions of the grey skin to generate insoluble calcium salt, and a mineralized layer is formed on the surface of the fibrin, so that the effect of enzyme on the surface of the naked skin is reduced to a certain extent, and the effect of protecting the grain surface of the naked skin is achieved; however, the SPD can not uniformly permeate into the pelts, thereby affecting deliming-softening effect, effectively loosening intracutaneous collagen fibers in the enzyme softening process, and obviously affecting the overall sensory performance of crust leather. When the PHBA-SPD is delimed, on one hand, the SPD forms a precipitate on the surface of a naked skin to protect the grain surface and avoid the excessive action of enzyme on the surface, and on the other hand, the PHBA permeates into the skin to effectively neutralize the alkali in the skin and promote the loosening effect of the enzyme on collagen fibers. This shows that PHBA-SPD deliming can achieve the ammonium sulfate deliming effect, does not affect the treatment effects of softening and other processes, does not affect the sensory property and the mechanical property of crust leather, can protect grain surfaces to a certain extent, and improves the safety and the controllability of the softening process.

TABLE 6 pollution load of ammonium sulfate and PHBA-SPD deliming technology

As can be seen from Table 6, the deashing of PHBA-SPD reduced the ammonia nitrogen and the total nitrogen in the waste liquid by 99.5% and 94.1%, respectively, which mainly resulted from the dissolution and decomposition of partial protein hydrolysate. Because the PHBA has higher carbon content and SPD is added, the TOC content in the deliming wastewater and the total phosphorus content in the deliming, softening and pickling wastewater are also higher than those in the ammonium sulfate deliming process. In the conventional deliming-tanning process, the carbon-nitrogen ratio of the wastewater is generally too low, the nitrogen-phosphorus ratio is generally too high, and the NH is difficult to effectively remove by biological treatment of the comprehensive wastewater ₃ Pollution of-N and TN, therefore, the introduction of TOC and TP by PHBA-SPD can enhance the biological treatment effect of the tannery wastewater.

Therefore, the deliming method can improve the treatment efficiency of the tanning comprehensive wastewater and has good environmental benefit and economic benefit. Can be used as the guide of the design of the cleaning production engineering of the tannery, and particularly provides the basis for the improvement of the terminal treatment scheme of the tannery wastewater.

Aromatic sulfonic acid is a medium strong acid, has good permeability in the pelts, can prevent the pelts from swelling in the range of pH3.0 or less, and has been applied to the development of salt-free pickling technology; however, most aromatic sulfonic acids do not have buffering properties and the pH at the end of deliming is difficult to control to meet softening requirements, so deliming is not generally performed separately. Sodium dihydrogen phosphate (SPD) is a gradually ionized acid salt, has good buffering performance in the pH range of 6.0-8.0, and is commonly used for preparing buffer solutions, however, the dihydrogen phosphate easily forms an insoluble salt with calcium ions, resulting in poor intradermal permeability, and the over-use thereof causes nakedfastoma. The aromatic sulfonic acid and the dihydric phosphate are simultaneously used for deliming, so that the defect of deliming by independently using the aromatic sulfonic acid and the dihydric phosphate can be overcome, the improvement of C/N/P in the leather-making wastewater can be facilitated, and the treatment efficiency of the comprehensive wastewater can be improved.

The deliming agent disclosed by the invention can achieve a deliming effect equivalent to ammonium sulfate, can form a mineralized layer on the surface of collagen by utilizing phosphate and calcium ions, and avoids grain surface damage caused by excessive action of enzyme on the surface of animal skin in a softening process, so that the grain surface of naked skin is protected, and the grain surface is prevented from being damaged by excessive action of the enzyme in the softening process. The PHBA-SPD is combined for deliming, so that the discharge of ammonia nitrogen in the deliming process can be almost completely avoided, and the ammonia nitrogen and total nitrogen content in the leather-making sewage are obviously reduced. Although TOC and P in the tanning wastewater are increased, the ratio of C to N to P of the tanning wastewater is closer to the optimal ratio for aerobic biological treatment by the deashing of PHBA-SPD, which is beneficial to improving the treatment efficiency of the tanning comprehensive sewage. Provides a new idea for developing a clean deliming technology and improving the quality of finished leather.

Claims

1. A tanning ammonium-free deliming agent with a grain surface protection function is characterized by comprising the following components in parts by weight: 5 to 30 parts of aromatic sulfonic acid and 1 to 20 parts of phosphate.

2. The ammonium-free leather deliming agent with the grain surface protection function according to claim 1, wherein the aromatic sulfonic acid is one or a mixture of two or more of sulfonic salicylic acid, phenolsulfone sulfonic acid, p-hydroxybenzene sulfonic acid, naphthalene disulfonic acid, naphthalene formic sulfonic acid and naphthol sulfonic acid in any proportion.

3. The ammonium-free leather deliming agent with the grain surface protection function as claimed in claim 1, wherein the phosphate is one or more of potassium phosphate, sodium phosphate, potassium hydrogen phosphate, sodium hydrogen phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, metaphosphate, phosphite and polyphosphate which are mixed in any proportion.

4. A method for deliming leather making ammonium-free deliming agent with grain surface protection function according to any one of claims 1 to 3, characterized by comprising the following steps:

step 2: adding a deliming agent, wherein the deliming agent accounts for 1.0-3.5 wt% of the weight of the ash dipping skin, and discharging liquid after deliming is finished.

5. The deliming method of the ammonium-free leather deliming agent with the grain surface protection function as claimed in claim 4, wherein the water is used in an amount accounting for 50% of the mass of the liming leather in the step 1, and the temperature is kept between 25 and 40 ℃.

6. The deliming method of the ammonium-free leather deliming agent with the grain surface protection function as claimed in claim 4, wherein the deliming time in the step 2 is 40-240 min, and the pH value in the deliming process is 7.0-9.0.